Shikher Goyal

Temperate Cyclones or Frontal cyclones or Mid-latitude or Wave cyclones or Extratropical Cyclone 

Temperate or Extratropical cyclones are capable of producing anything from:- Cloudiness and mild showers to heavy gales, thunderstorms, blizzards, and tornadoes. Probably most significant of all atmospheric disturbances are mid latitude or temperate cyclones. Throughout the mid-latitudes, they dominate weather maps, are basically responsible for most day-to-day weather changes, and bring precipitation to much of the populated portions of the planet.

Consisting of large, migratory low-pressure cells, they are usually called depressions in Europe and lows or low pressure systems, wave cyclones, extra-tropical cyclones, or even simply (although not very precisely) as “storms” in the United States.

Mid-latitude or frontal cyclones are associated primarily with air mass convergence in regions between about 30° and 70° of latitude.  Thus, they are found almost entirely within the band of westerly winds. Their general path of movement is toward the east, which explains why weather forecasting in the mid-latitudes is essentially a west-facing vocation.

• Because each mid-latitude cyclone or Temperate cyclones differs from all others in greater or lesser detail, any description must be a general one only. The discussions that follow, then, pertain to “typical” or idealized conditions.
• Moreover, these conditions are presented as Northern Hemisphere phenomena. For the Southern Hemisphere, the patterns of isobars, fronts, and wind flow should be visualized as mirror images of the Northern Hemisphere patterns.

Characteristics of Temperate Cyclones | UPSC IAS

A typical mature mid-latitude cyclone or Temperate cyclones has a diameter of 1600 kilometers (1000 miles) or so. It is essentially a vast cell of low-pressure air, with ground-level pressure in the center typically between 990 and 1000 millibars. The system usually tends toward an oval shape, with the long axis trending northeast–southwest. Usually a clear-cut pressure trough extends southwesterly from the center.

• Formation of Fronts: Mid-latitude cyclones or Temperate cyclones have a converging counterclockwise circulation pattern in the Northern Hemisphere. This wind flow pattern brings together cool air from the north and warm air from the south. The convergence of these unlike air masses characteristically creates two fronts: a cold front that extends to the southwest from the center of the cyclone and runs along the pressure trough extending from the center of the storm, and a warm front extending eastward from the center and running along another, usually weaker, pressure trough.
• Sectors: The two fronts divide the cyclone into a cool sector north and west of the center where the cold air mass is in contact with the ground, and a warm sector to the south and east where the warm air mass is in contact with the ground. At the surface, the cool sector is the larger of the two, but aloft the warm sector is more extensive. This size relationship exists because both fronts “lean” over the cool air. Thus, the cold front slopes upward toward the northwest and the warm front slopes upward toward the northeast.
• Clouds and Precipitation: Clouds and precipitation develop in the zones within a midlatitude cyclone or Temperate cyclones where air is rising and cooling adiabatically. Because warm air rises along both fronts, the typical result is two zones of cloudiness and precipitation that overlap around the center of the storm (where air is rising in the center of the low pressure cell) and extend outward in the general direction of the fronts.

Along and immediately behind the ground-level position of the cold front (the steeper of the two fronts), a band of cumuliform clouds usually yields showery precipitation. The air rising more gently along the more gradual slope of the warm front produces a more extensive expanse of horizontally developed clouds, perhaps with widespread, protracted, low-intensity precipitation. In both cases, most of the precipitation originates in the warm air rising above the fronts and falls down through the front to reach the ground in the cool sector.

This precipitation pattern does not mean that the entire cool sector has unsettled weather and that the warm sector experiences clear conditions throughout. Although most frontal precipitation falls within the cool sector, the general area to the north, northwest, and west of the center of the cyclone is frequently cloudless as soon as the cold front has moved on.

Thus, much of the cool sector is typified by clear, cold, stable air. In contrast, the air of the warm sector is often moist and tending toward instability, and so thermal convection and surface-wind convergence may produce sporadic thunderstorms. Also, sometimes one or more squall lines of intense thunderstorms develop in the warm sector in advance of the cold front.

Movements of Temperate Cyclones | UPSC IAS |  Geography Optional

Midlatitude cyclones or Temperate cyclones are essentially transient features, on the move throughout their existence. Four kinds of movement are involved:

1. The whole storm moves as a major disturbance in the westerlies, traversing the midlatitudes generally from west to east. The rate of movement averages 30 to 45 kilometers (about 20 to 30 miles) per hour, which means that the storm can cross North America in three to four days (often faster in winter than in summer).
2. The route of a cyclone is likely to be undulating and erratic, although it moves generally from west to east, often in association with the path of the jet stream.
3. The system has a cyclonic wind circulation, with wind generally converging counterclockwise (in the Northern Hemisphere) into the center of the storm from all sides.
4. The cold front usually advances faster than the center of the storm (the advancing dense, cold air easily displaces the lighter, warm air ahead of the front).
5. The warm front usually advances more slowly than the center of the storm, causing it to appear to lag behind. (This is only an apparent motion, however. The warm front is actually moving west to east, just like every other part of the system.)

Life Cycle of Temperate Cyclones: Cyclogenesis | Geography Optional

A typical midlatitude cyclone or Temperate cyclones progresses from origin to maturity, and then to dissipation, in about three to ten days. It is believed that the most common cause of cyclogenesis (the birth of cyclones) is upper troposphere conditions in the vicinity of the polar front jet stream. Most midlatitude cyclones begin as “waves” along the polar front.

• Waves are undulations or curves that develop in the paths taken by upper level winds such as a jet stream, and that the polar front is the contact zone between the relatively cold polar easterlies and the relatively warm westerlies.
• The opposing airflows normally have a relatively smooth linear motion on either side of the polar front. On occasion, however, the smooth frontal surface may be distorted into a wave shape.

There appears to be a close relationship between upper level airflow and ground-level disturbances. When the upper airflow is zonal—by which we mean relatively straight from west to east – ground-level cyclonic activity is unlikely. When winds aloft begin to meander north to south in a meridional airflow, large waves of alternating pressure troughs and ridges are formed and cyclonic activity at ground level is intensified. Most mid-latitude cyclones  or Temperate cyclones are centered below the polar front jet stream axis and downstream from an upper-level pressure trough.

A cyclone is unlikely to develop at ground level unless there is divergence above it. In other words, the convergence of air near the ground must be supported by divergence aloft. Such divergence can be related to changes in either speed or direction of the wind flow, but it nearly always involves broad north-to-south meanders in the Rossby waves and the jet stream.

• Various ground factors: such as topographic irregularities, temperature contrasts between sea and land, or the influence of ocean currents – can apparently initiate a wave along the front. For example, cyclogenesis also occurs on the leeward side of mountains. A low-pressure area drifting with the westerlies becomes weaker when it crosses a mountain range. As it ascends the range, the column of air compresses and spreads, slowing down its counterclockwise spin. When descending the leeward side, the air column stretches vertically and contracts horizontally. This change in shape causes it to spin faster and may initiate cyclonic development even if it were not a full-fledged cyclone before. This chain of events happens with some frequency in winter on the eastern flanks of the Rocky Mountains, particularly in Colorado, and with lesser frequency on the eastern side of the Appalachian Mountains, in North Carolina and Virginia. Cyclones formed in this way typically move toward the east and northeast and often bring heavy rain or snowstorms to the northeastern United States and southeastern Canada.
• Occlusion: Ultimately, the storm dissipates because the cold front overtakes the warm front. As the two fronts come closer and closer together, the warm sector at the ground is increasingly displaced, forcing more and more warm air aloft. When the cold front catches up with the warm front, warm air is no longer in contact with Earth’s surface and an occluded front is formed. This occlusion process usually results in a short period of intensified precipitation and wind until eventually all the warm sector is forced aloft and the ground-level low-pressure center is surrounded on all sides by cool air, a stable condition. This sequence of events weakens the pressure gradient and shuts off the storm’s energy and air lifting mechanism – and so its cloud-producing mechanism – and the storm dies out.

Conveyor Belt Model of Mid-latitude Cyclones | UPSC IAS

The description of mid-latitude cyclones we’ve just provided is sometimes called the “Norwegian” model because it was first presented by meteorologists in Norway in the 1920s. Although this explanation of midlatitude cyclones remains useful today, new data has provided a more complete explanation of these storms, especially air flow in the upper troposphere. A modern model, called the conveyor belt model, now offers a better explanation of the three dimensional aspects of these storms.

Weather Changes with the Passing of a Mid-latitude Cyclone | UPSC IAS

Although the exact details vary from storm to storm, basic structure and movements of a midlatitude cyclone or Temperate cyclones we just described can help us understand the often abrupt weather changes we experience on the ground with the passing of one of these storms. This is especially true when the cold front of a mid-latitude cyclone passes through in winter.

For example, imagine we’re in the warm sector of a mid-latitude cyclone or Temperate cyclones – the situation just before the cold front moves through. Remember, the whole storm is moving from west to east and so the cold front is moving closer to us hour by hour. When the cold front passes, all four elements of weather will likely change:

• Temperature: As the cold front passes, temperature drops abruptly because the cold front is the boundary between the cold air mass and the warm air mass of the storm. Pressure: Because the cold front is associated with a trough (a linear band of low pressure) extending south from the heart of the storm, as the front approaches, pressure will be falling, reaching its lowest point at the front. Then, as the cold front passes and the trough moves away, pressure will begin to rise steadily.
• Wind: Because of the overall converging counterclockwise wind pattern (in the Northern Hemisphere), winds in the warm sector come from the south (the situation before the cold front). Once the front passes, wind will tend to shift and come from the west or northwest.
• Clouds and Precipitation: The generally clear skies ahead of the cold front are replaced by cloudiness and precipitation at the front generated by the adiabatic cooling of the warm air as it is lifted along the front—to be replaced again some hours later by clear skies in the cold air mass behind the cold front. Similar changes, although of lesser magnitude, occur with the passage of a warm front.

Occurrence and Distribution of Temperate Cyclones | UPSC IAS

• At any given time, from 5 to 15 mid-latitude cyclones or Temperate cyclones exist in the Northern Hemisphere mid-latitudes, and an equal number in the Southern Hemisphere.
• They occur at scattered but irregular intervals throughout the zone of the westerlies. In part because temperature contrasts are greater during the winter, these migratory disturbances are more numerous, better developed, and faster moving in winter than in summer. They also follow much more equatorward tracks in winter.
• In the Southern Hemisphere, the Antarctic continent provides a prominent year-round source of cold air, and so vigorous cyclones are almost as numerous in summer as in winter. The summer storms are farther poleward than their winter cousins, however, and are mostly over the Southern Ocean. Thus, they have little effect on land areas.

Classification of Air masses and Fronts | Geography Optional | UPSC – IAS

Air Masses

Although the troposphere is a continuous body of mixed gases that surrounds the planet, it is by no means a uniform blanket of air. Instead, it is composed of many large parcels of air that are distinct from one another. Such large parcels are referred to as air masses.

Characteristics Air Masses

To be recognized as a distinct air mass, a parcel of air must meet three requirements:

• It must be large. A typical air mass is more than 1600 kilometers (1000 miles) across and several kilometers deep (from Earth’s surface to the top of the air mass).
• It must have uniform properties in the horizontal dimension. This means that at any given altitude in the air mass, its physical characteristics:- primarily temperature, humidity, and stability; are relatively homogeneous.
• It must travel as a unit. It must be distinct from the surrounding air, and when it moves it must retain its original characteristics and not be torn apart by differences in airflow.

Origin of Air Masses

• An air mass develops its characteristics when it stagnates or remains over a uniform land or sea surface long enough to acquire the temperature/humidity/stability characteristics of the surface below.
• This stagnation needs to last for only a few days if the underlying surface has prominent temperature and moisture characteristics. Stable air is more likely to remain stagnant for a few days than unstable air, so regions with anticyclonic (high pressure) conditions commonly form air masses.

Source Regions of Air Masses

The formation of air masses is usually associated with what are called source regions: regions of Earth’s surface that are particularly well suited to generate air masses. Such regions must be extensive, physically uniform, and associated with air that is stationary or anticyclonic.

• Ideal source regions are – ocean surfaces and extensive flat land areas that have a uniform covering of snow, forest, or desert.
• Air masses rarely form over the irregular terrain of mountain ranges.

Image portrays the principal recognized source regions for air masses that affect North America. Warm air masses can form in any season over the waters of the southern North Atlantic, the Gulf of Mexico/Caribbean Sea, and the southern North Pacific, and in summer, they can form over the deserts of the southwestern United States and northwestern Mexico. Cold air masses develop over the northern portions of the Atlantic and Pacific Oceans and over the snow-covered lands of north-central Canada.

It may well be that the concept of source regions is of more theoretical value than actual value. A broader view, one subscribed to by many atmospheric scientists, holds that air masses can originate almost anywhere in the low or high latitudes but rarely in the midlatitudes due to the prevailing westerlies where persistent wind would prevent air mass formation.

Classification of Air Masses

Air masses are classified on the basis of source region. The latitude of the source region correlates directly with the temperature of the air mass, and the nature of the surface strongly influences the humidity content of the air mass. Thus,

• A low-latitude air mass is warm or hot
• A high-latitude one is cool or cold.

If the air mass develops over a continental surface, it is likely to be dry; if it originates over an ocean, it is usually moist. A one- or two-letter code is generally used to identify air masses. Although some authorities recognize other categories, the basic classification is sixfold are as follows:-

Movement and Modification of Air Masses

Some air masses remain in their source region for long periods, even indefinitely. In such cases, the weather associated with the air mass persists with little variation. Our interest, however, is in masses that leave their source region and move into other regions, particularly into the midlatitudes. When an air mass departs from its source region, its structure begins to change. This change is due in part to thermal modification (warming or cooling from below), in part to dynamic modification (uplift, subsidence, convergence, turbulence), and perhaps also in part to addition or subtraction of moisture.

Once it leaves its source area, an air mass modifies the weather of the regions into which it moves: it takes source-region characteristics into other regions.

A midwinter outburst of continental polar (cP) air from northern Canada sweeps down across the central part of North America. With a source-region temperature of −46°C (−50°F) around Great Slave Lake, the air mass has warmed to −34°C (−30°F) by the time it reaches Winnipeg, Manitoba, and it continues to warm as it moves southward. Throughout its southward course, the air mass becomes warmer, but it also brings some of the coldest weather that each of these places will receive all winter. Thus, the air mass is modified, but it also modifies the weather in all regions it passes through. Temperature, of course, is only one of the characteristics modified by a moving air mass. There are also modifications in humidity and stability.

North American Air Masses

The North American continent is a prominent area of air mass interaction. The lack of mountains trending east to west permits polar air to sweep southward and tropical air to flow northward unhindered by terrain, particularly over the eastern two-thirds of the continent. In the western part of the continent, though, air masses moving off the Pacific are impeded by the prominent north– south trending mountain ranges.

Continental polar (cP) – Air masses develop in central and northern Canada, and Arctic (A) air masses originate farther north and so are colder and drier than cP air masses – both are dominant features in winter with their cold, dry, stable nature. Maritime polar (mP) air from the Pacific in winter can bring cloudiness and heavy precipitation to the mountainous west coastal regions. In summer, cool Pacific mP air produces fog and low stratus clouds along the coast. North Atlantic mP air masses are also cool, moist, and unstable, but except for occasional incursions into the mid-Atlantic coastal region, Atlantic mP air does not affect North America because the prevailing circulation of the atmosphere  is westerly.

Maritime tropical (mT) air from the Atlantic/Caribbean/ Gulf of Mexico is warm, moist, and unstable. It strongly influences weather and climate east of the Rockies in the United States, southern Canada, and much of Mexico, serving as the principal precipitation source in this broad region. It is more prevalent in summer than in winter, bringing periods of uncomfortable humid heat. Pacific mT air originates over water in areas of anticyclonic subsidence, and so it tends to be cooler, drier, and more stable than Atlantic mT air; it is felt only in the southwestern United States and northwestern Mexico, where it may produce coastal fog and moderate orographic rainfall where forced to ascend mountain slopes. It is also the source of some summer rains in the southwestern interior.

Continental tropical (cT) air is relatively unimportant in North America because its source region is not extensive. In summer, hot, very dry, unstable cT air surges into the southern Great Plains area on occasion, bringing heat waves and dry conditions.

Equatorial (E) air affects North America only in association with hurricanes. It is similar to mT air except that E air provides an even more copious source of rain than does mT air because of high humidity and instability.

FRONTS

When unlike air masses meet, they do not mix readily; instead, a boundary zone called a front develops between them. A front is not a simple two-dimensional boundary. A typical front is a narrow three-dimensional transition zone several kilometers or even tens of kilometers wide. Within this zone, the properties of the air change rapidly.

The frontal concept was developed by Norwegian meteorologists during World War I, and the term front was coined because these scientists considered the clash between unlike air masses to be analogous to a confrontation between opposing armies along a battle front. As the more “aggressive” air mass advances at the expense of the other, some mixing of the two occurs within the frontal zone, but for the most part the air masses retain their separate identities as one is displaced by the other.

Types of Fronts

(Cold, Warm, Stationary, and Occluded)

• The most conspicuous difference between air masses is usually temperature. A cold front forms where an advancing cold air mass meets and displaces warmer air (see image), whereas a warm front forms where an advancing warm air mass meets colder air (see image).
• In both cases, there is warm air on one side of the front and cool air on the other, with a fairly abrupt temperature gradient between.
• Air masses may also have different densities, humidity levels, wind patterns, and stability, and so these factors can have a steep gradient through the front as well. In some cases, a front may remain stationary for a few hours or even a few days.
• More commonly, however, a front is in more or less constant motion. Usually one air mass is displacing the other; thus, the front advances in the direction dictated by the movement of the more active air mass. Regardless of which air mass is advancing, it is always the warmer air that rises over the cooler.
• The warmer, lighter air is inevitably forced aloft, and the cooler, denser air mass functions as a wedge over which the lifting occurs. As you can see in both the images fronts “lean” or slope upward from the surface, and it is along this slope that the warmer air rises and cools adiabatically to form clouds and often precipitation.
• Indeed, fronts lean so much that they are much closer to horizontal features than vertical ones. The slope of a typical front averages about 1:150, meaning that 150 kilometers away from the surface position of the front, the height of the front is only 1 kilometer above the ground.
• Because of this very low angle of slope (less than 1°), the steepness shown in most diagrams of fronts is greatly exaggerated. Notice that the “leading edge” of a cold front precedes its higher altitude “trailing edge,” whereas a warm front leans “forward” so that the higher altitude part of the front is ahead of its lower altitude “trailing edge.”

Cold Fronts

Image Explanation: A cold front forms when a cold air mass is actively underriding a warm air mass. As a cold front advances, the warm air ahead of it is forced upward. This displacement often creates cloudiness and relatively heavy precipitation along and immediately behind the groundlevel position of the front. (In this diagram, the vertical scale has been exaggerated.)

Because of friction with the ground, the advance of the lower portion of a cold air mass is slowed relative to the upper portion. As a result, a cold front tends to become steeper as it moves forward and usually develops a protruding “nose” a few hundred meters above the ground.

• The average cold front is twice as steep as the average warm front. Moreover, cold fronts normally move faster than warm fronts because the dense, cold air mass easily displaces the lighter, warm air.

This combination of steeper slope and faster advance leads to rapid lifting and adiabatic cooling of the warm air ahead of the cold front. The rapid lifting often makes the warm air very unstable, and the result is blustery and violent weather along the cold front.

• Vertically developed clouds, such as cumulonimbus clouds, are common, with considerable turbulence and showery precipitation.
• Both clouds and precipitation tend to be concentrated along and immediately behind the ground-level position of the front. Precipitation is usually of higher intensity but shorter duration than that associated with a warm front.
• On a weather map, the ground-level position of a cold front is shown either by a blue line or a solid line studded at intervals with solid triangles that extend in the direction toward which the front is moving.

Warm Fronts

Image Explanation – A warm front forms when a warm air mass is actively overriding a cold air mass. As warm air rises above cooler air, widespread cloudiness and precipitation develop along and in advance of the ground-level position of the front. Higher and less dense clouds are often dozens or hundreds of kilometers ahead of the ground-level position of the front. (In this diagram, the vertical scale has been exaggerated.)

The slope of a typical warm front is more gentle than that of a cold front, averaging about 1:200. As the warm air pushes against and rises over the retreating cold air, it cools adiabatically, usually resulting in clouds and precipitation.

Because the frontal uplift is very gradual, clouds form slowly and turbulence is limited. High-flying cirrus clouds may signal the approaching front many hours before it arrives. As the front comes closer, the clouds become lower, thicker, and more extensive, typically developing into altocumulus or altostratus. Precipitation usually occurs broadly;

• It is likely to be protracted and gentle, without much convective activity. If the rising air is inherently unstable, however, precipitation can be showery and even violent. Most precipitation falls ahead of the ground-level position of the moving front.
• The ground-level position of a warm front is portrayed on a weather map either by a red line or by a solid line along which solid semicircles are located at regular intervals, with the semicircles extending in the direction toward which the front is moving

Stationary Fronts

When neither air mass displaces the other or if a cold front or warm front “stalls” their common boundary is called a stationary front. It is difficult to generalize about the weather along such a front, but often gently rising warm air produces limited precipitation similar to that along a warm front.

• As Image shows, stationary fronts are portrayed on a weather map by a combination of warm and cold front symbols, alternating on opposite sides of the line—cold air is opposite the triangles, and warm air opposite the half circles.

Occluded Fronts

A fourth type of front, called an occluded front, is formed when a cold front overtakes a warm front. Occluded fronts are shown on a weather map by a combination of warm and cold front symbols, alternating on the same side of the line.

Air Masses, Fronts, and Major Atmospheric Disturbances

We will now turn our attention to the major kinds of atmospheric disturbances that occur within the general circulation. Most of these disturbances involve unsettled and sometimes violent atmospheric conditions and are referred to as storms.

Some, however, produce calm, clear, quiet weather that is quite the opposite of stormy. Some of these disturbances involve air mass contrasts or fronts, and many are associated with migrating pressure cells. The following are common characteristics of atmospheric disturbances in general:-

• They are smaller than the components of the general circulation, although they are extremely variable in size.
• They are migratory.
• They have a relatively brief duration, persisting for only a few minutes, a few hours, or a few days.
• They produce characteristic and relatively predictable weather conditions.

Midlatitude Disturbances: The midlatitudes are the principal “battleground” of tropospheric phenomena: where polar and tropical air masses meet, where most fronts occur, and where weather is most dynamic and changeable from season to season and from day to day. Many kinds of atmospheric disturbances are associated with the midlatitudes, but two of these – midlatitude cyclones and mid latitude anticyclones – are much more important than the others because of their size and prevalence.

Tropical Disturbances: The low latitudes are characterized by monotony – the same weather day after day, week after week, month after month. Almost the only breaks are provided by transient atmospheric disturbances, of which by far the most significant are tropical cyclones (locally known as hurricanes when they intensify), but also less dramatic disturbances known as easterly waves.

Localized Severe Weather:  Other localized atmospheric disturbances occur in many parts of the world. Short-lived but sometimes severe atmospheric disturbances such as thunderstorms and tornadoes often develop in conjunction with other kinds of storms.

Global Distribution of Major Soil Types in the World | Geography Optional | UPSC

There are twelve (12) orders of soils, which are distinguished largely on the basis of properties that reflect a major course of development, with considerable emphasis on the presence or absence of notable diagnostic horizons. We consider each of the twelve (12) orders, beginning with those characterized by little profile development and progressing to those with the most highly weathered profiles.

• Alfisols – “al” for aluminum, “f” for iron (chemical symbol Fe), two prominent elements in these soils
• Andisols –  Rock formed from type of magma in Andes Mountains volcanoes; soils high in volcanic ash
• Aridisols –  Dry soils
• Entisols – These are recently formed soils
• Gelisols –  Soils in areas of permafrost
• Histosols – These soils contain mostly organic matter
• Inceptisols – Young soils at the beginning of their “life”
• Mollisols – Soft soils
• Oxisols – Soils with large amounts of oxygen containing compounds
• Spodosols – Ashy soils
• Ultisols – Soils that have had the last of their nutrient bases leached out
• Vertisols – Soils in which material from O and A horizons falls through surface cracks and ends up below deeper horizons.

Image – Theoretical soil order development pathways. Soils evolve along “pathways” in which different factors—such as parent material, climate, local site conditions, or length of time – may dominate.

Entisols (Very Little Profile Development)

The least well developed of all soils, Entisols have experienced little mineral alteration and are virtually without pedogenic horizons. Their undeveloped state is usually a function of time (the very name of the order connotes recency).

• Most Entisols are surface deposits that have not been in place long enough for pedogenetic processes to have had much effect. Some, however, are very old, and in these soils the lack of horizon development is due to a mineral content that does not alter readily, to a very cold climate, or to some other factor totally unrelated to time.
• The distribution of Entisols is therefore very widespread and cannot be specifically correlated with particular moisture or temperature conditions or with certain types of vegetation or parent materials.
• In the United States, Entisols are most prominent in the dry lands of the West but are found in most other parts of the country as well. They are commonly thin and/ or sandy and have limited productivity, although those developed on recent alluvial deposits tend to be quite fertile.

Inceptisols (Few Diagnostic Features)

Another immature order of soils is the Inceptisols. Their distinctive characteristics are relatively faint, not yet prominent enough to produce diagnostic horizons. If the Entisols can be called “youthful,” the Inceptisols might be classified “adolescent.”

• They are primarily eluvial soils and lack illuvial layers. Like Entisols, Inceptisols are widespread over the world in various environments. Also like Entisols, they include a variety of fairly dissimilar soils whose common characteristic is lack of maturity.
• They are most common in tundra and mountain areas but are also notable in older valley floodplains. Their world distribution pattern is very irregular. This is also true in the United States, where they are most typical of the Appalachian Mountains, the Pacific Northwest, and the lower Mississippi Valley.

Andisols (Volcanic Ash Soils)

Having developed from volcanic ash, Andisols have been deposited in relatively recent geological time. They are not highly weathered, therefore, and there has been little downward translocation of their colloids.

• There is minimum profile development, and the upper layers are dark.  Their inherent fertility is relatively high.
• Andisols are found primarily in volcanic regions of Japan, Indonesia, and South America, as well as in the very productive wheat lands of Washington, Oregon, and Idaho.

Gelisols (Cold Soils with Permafrost)

Gelisols are young soils with minimal profile development. They develop only slowly because of cold temperatures and frozen conditions. These soils typically have a permafrost layer that is a defining characteristic. Also commonly found in Gelisols is cryoturbation or frost churning, which is the physical disruption and displacement of soil material by freeze–thaw action in the soil.

• Most of the soil forming processes in Gelisols take place above the permafrost in the active layer that thaws every year or so. Gelisols are the dominant soils of Arctic and subarctic regions. They occur in association with boreal forest and tundra vegetation.
• Thus, they are primarily found in Russia, Canada, and Alaska and are prominent in the Himalaya Mountain country of central Asia. Altogether nearly 9 percent of Earth’s land area has a Gelisol soil cover.

Histosols (Organic Soils on Very Wet Sites)

Least important among the soil orders are the Histosols, which occupy only a small fraction of Earth’s land surface, a much smaller area than any other order. These are organic rather than mineral soils, and they are invariably saturated with water all or most of the time. They may occur in any waterlogged environment but are most characteristic in mid- and high-latitude regions that experienced Pleistocene glaciation.

• In the United States, they are most common around the Great Lakes, but they also occur in southern Florida and Louisiana. Nowhere, however, is their occurrence extensive.
• Some Histosols are composed largely of undecayed or only partly decayed plant material, whereas others consist of a thoroughly decomposed mass of muck.
• The lack of oxygen in the waterlogged soil slows down the rate of bacterial action, and the soil becomes deeper mostly by growing upward, that is, by more organic material being added from above.
• Histosols are usually black, acidic, and fertile only for water-tolerant plants. If drained, they can be very productive agriculturally for a short while. Before long, however, they are likely to dry out, shrink, and oxidize, a series of steps that leads to compaction, susceptibility to wind erosion, and danger of fire.

Aridisols (Soils of Dry Climates)

• Nearly one-eighth of Earth’s land surface is covered with Aridisols, one of the most extensive spreads of any soil order. They are preeminently soils of the dry lands, occupying environments that do not have enough water to remove soluble minerals from the soil. Thus, their distribution pattern is largely correlated with that of desert and semi-desert climates.
• Aridisols are typified by a thin profile that is sandy and lacking in organic matter, characteristics clearly associated with a dry climate and a scarcity of penetrating moisture. The epipedon is almost invariably light in color. There are various kinds of diagnostic subsurface horizons, nearly all distinctly alkaline.
• Most Aridisols are unproductive, particularly because of lack of moisture; if irrigated, however, some display remarkable fertility. The threat of salt accumulation is nonetheless ever present.

Vertisols (Swelling and Cracking Clays)

Vertisols contain a large quantity of clay that becomes a dominant factor in the soil’s development. The clay of Vertisols is described as “swelling” or “cracking” clay. This clay-type soil has an exceptional capacity for absorbing water: when moistened, it swells and expands; as it dries, deep, wide cracks form, sometimes 2.5 centimeters (an inch) wide and as much as 1 meter (3 feet) deep.

• Some surface material falls into the cracks, and more is washed in when it rains. When the soil is wetted again, more swelling takes place and the cracks close. This alternation of wetting and drying and expansion and contraction produces a churning effect that mixes the soil constituents (the name Vertisol connotes an inverted condition), inhibits the development of horizons, and may even cause minor irregularities in the land surface.
• An alternating wet and dry climate is needed for Vertisol formation because the sequence of swelling and contraction is necessary. Thus, the wet–dry climate of tropical and subtropical savannas is ideal, but there must also be the proper parent material to yield the clay minerals. Consequently, Vertisols are widespread in distribution but are very limited in extent.
• The principal occurrences are in eastern Australia, India, and a small part of East Africa. They are uncommon in the United States, although prominent in some parts of Texas and California. The fertility of Vertisols is relatively high, as they tend to be rich in nutrient bases. They are difficult to till, however, because of their sticky plasticity, and so they are often left uncultivated.

Mollisols (Dark, Soft Soils of Grasslands)

The distinctive characteristic of Mollisols is the presence of a mollic epipedon, which is a mineral surface horizon that is dark and thick, contains abundant humus and basic cations, and retains a soft character (rather than becoming hard and crusty) when it dries out.  Mollisols can be thought of as transition soils that evolve in regions not dominated by either humid or arid conditions.

• They are typical of the mid-latitude grasslands and are thus most common in central Eurasia, the North American Great Plains, and the pampas of Argentina.
• The grassland environment generally maintains a rich clay–humus content in a Mollisol soil. The dense, fibrous mass of grass roots permeates uniformly through the epipedon and to a lesser extent into the subsurface layers.
• There is almost continuous decay of plant parts to produce a nutrient-rich humus for the living grass. Mollisols on the whole are probably the most productive soil order.
• They are generally derived from loose parent material rather than from bedrock and tend to have favorable structure and texture for cultivation. Because they are not overly leached, nutrients are generally retained within reach of plant roots. Moreover, Mollisols provide a favored habitat for earthworms, which contribute to softening and mixing of the soil.

Alfisols (Clay-Rich B Horizons, High Base Status)

The most wide ranging of the mature soils, Alfisols occur extensively in low and middle latitudes. They are found in a variety of temperature and moisture conditions and under diverse vegetation associations.

• By and large, they tend to be associated with transitional environments and are less characteristic of regions that are particularly hot or cold or wet or dry.
• Their global distribution is extremely varied. They are also widespread in the United States, with particular concentrations in the Midwest.
• Alfisols are distinguished by a subsurface clay horizon and a medium to generous supply of basic cations, plant nutrients, and water. The epipedon is ochric (light-colored), but beyond that, it has no characteristics that are particularly diagnostic and can be considered an ordinary eluviated horizon.
• The relatively moderate conditions under which Alfisols develop tend to produce balanced soils that are reasonably fertile. Alfisols rank second only to Mollisols in agricultural productivity.

Ultisols (Clay-Rich B Horizons, Low Base Status)

• Ultisols are roughly similar to Alfisols except that Ultisols are more thoroughly weathered and more completely leached of nutrient bases. They have experienced greater mineral alteration than any other soil in the mid-latitudes, although they also occur in the low latitudes.
• Many pedologists believe that the ultimate fate of Alfisols is to degenerate into Ultisols. Typically, Ultisols are reddish as a result of the significant proportion of iron and aluminum in the A horizon. They usually have a fairly distinct layer of subsurface clay accumulation.
• The principal properties of Ultisols have been imparted by a great deal of weathering and leaching. Indeed, the connotation of the name (derived from the Latin ultimos) is that these soils represent the ultimate stage of weathering.
• The result is a fairly deep soil that is acidic, lacks humus, and has a relatively low fertility due to the lack of bases. Ultisols have a fairly simple world distribution pattern.
• They are mostly confined to humid subtropical climates and to some relatively youthful tropical land surfaces.
• In the United States, they are restricted largely to the southeastern quarter of the country and to a narrow strip along the northern Pacific Coast.

Spodosols (Soils of Cool, Forested Zones)

The key diagnostic feature of a Spodosol is a spodic subsurface horizon, an illuvial dark or reddish layer where organic matter, iron, and aluminum accumulate. The upper layers are light-colored and heavily leached.  At the top of the profile is usually an O horizon of organic litter. Such a soil is a typical result of podzolization.

• Spodosols are notoriously infertile. They have been leached of useful nutrients and are acidic throughout. They do not retain moisture well and are lacking in humus and often in clay.
• Spodosols are most widespread in areas of coniferous forest where there is a subarctic climate. Alfisols, Histosols, and Inceptisols also occupy these regions, however, and Spodosols are sometimes found in other environments, such as poorly drained portions of Florida.

Oxisols (Highly Weathered and Leached)

The most thoroughly weathered and leached of all soils are the Oxisols, which invariably display a high degree of mineral alteration and profile development.

• They occur mostly on ancient landscapes in the humid tropics, particularly in Brazil and equatorial Africa, and to a lesser extent in Southeast Asia. The distribution pattern is often spotty, with Oxisols mixed with less developed Entisols, Vertisols, and Ultisols. Oxisols’ are totally absent from the United States, except for Hawai‘i, where they are common.
• Oxisols are essentially the products of laterization (and in fact were called Latosols in older classification systems). They have evolved in warm, moist climates, although some are now found in drier regions, an indication of climatic change since the soils developed.
• The diagnostic horizon for Oxisols is a subsurface dominated by oxides of iron and aluminum and with a minimal supply of nutrient bases (this is called an oxic horizon).
• These are deep soils but not inherently fertile. The natural vegetation is efficient in cycling the limited nutrient supply.

Soil Profile | Diagram, Horizons and Layers  | UPSC – IAS

Image – Idealized soil profile. The true soil, or solum, consists of the O, A, E, and B horizons.

The vertical variation of soil properties is not random but rather an ordered layering with depth. Soil tends to have more or less distinctly recognizable layers, called horizons, each with different characteristics.  The horizons are positioned approximately parallel with the land surface, one above the other, normally, but not always, separated by a transition zone rather than a sharp line.

• A vertical cross section (as might be seen in a road cut or the side of a trench dug in a field) from the Earth’s surface down through the soil layers and into the parent material is referred to as a soil profile. The almost infinite variety of soils in the world are usually grouped and classified on the basis of differences exhibited in their profiles.

Soil Horizons Explanation | UPSC – IAS

• The O horizon is sometimes the surface layer, and in it organic matter, both fresh and decaying, makes up most of the volume. This horizon results essentially from litter derived from dead plants and animals. It is common in forests and generally absent in grasslands. It is actually more typical for soils not to possess an O horizon; the surface horizon of most soils is the A horizon.
• The A horizon, colloquially referred to as topsoil, is a mineral horizon that also contains considerable organic matter. It is formed either at the surface or immediately below an O horizon. A horizons generally contain enough partially decomposed organic matter to give the soil a darker color than underlying horizons. They are also normally coarser in texture, having lost some of the finer materials by erosion and eluviation. Seeds germinate mostly in the A horizon.
• The E horizon is normally lighter in color than either the overlying A or the underlying B horizon. It is essentially an eluvial layer from which clay, iron, and aluminum have been removed, leaving a concentration of abrasion-resistant sand or silt particles.
• The B horizon, usually called subsoil, is a mineral horizon of illuviation where most of the materials removed from above have been deposited. A collecting zone for clay, iron, and aluminum, this horizon is usually of heavier texture, greater density, and relatively greater clay content than the A horizon.
• The C horizon is unconsolidated parent material (regolith) beyond the reach of plant roots and most soil-forming processes except weathering. It is lacking in organic matter.
• The R horizon is bedrock, with little evidence of weathering. True soil, which is called solum, only extends down through the B horizon.

Soil-forming Processes | UPSC – IAS

Image Explanation -Image Explanation – four soil-forming processes: addition, loss, translocation, transformation. Geologic, climatic, topographic, biological, and chronological (time) soil-forming factors influence the rate at which these four processes occur and therefore the rate at which soil is formed.

The development of any soil is expressed in two dimensions: Depth and time. There is no straight-line relationship between depth and age, however; some soils deepen and develop much more rapidly than others. Four processes deepen and age soils:

• Addition (ingredients added to the soil),
• Loss (ingredients lost from the soil),
• Translocation (ingredients moved within the soil), and
• Transformation (ingredients altered within the soil).

The five soil-forming factors discussed earlier – Read it from Here –  Soil forming factors Which are responsible for soil development.

• Geologic,
• Climatic,
• Topographic,
• Biological, and
• Time – 

These factors influence the rate of these four processes, the result being the development of various soil horizons and the soil profile.

Leaching of Soil | UPSC – IAS | Geography Optional

For plants, capillary water is the most important and gravitational water is largely superfluous. After gravitational water has drained away, the remaining volume of water represents the field capacity of the soil.

• If drought conditions prevail and the capillary water is all used up by plants or evaporated, the plants are no longer able to extract moisture from the soil; then, the wilting point is reached.

What is Leaching of Soil ? | UPSC – IAS

• leaching is the loss of water from water-soluble plant nutrients from the soil, due to rain and irrigation.
• Water performs a number of important functions in the soil. It is an effective solvent, dissolving essential soil nutrients and making them available to plant roots.
• These dissolved nutrients are carried downward in solution, to be partly redeposited at lower levels. This process, called leaching, tends to deplete the topsoil of soluble nutrients. Water is also required for many of the chemical reactions of clay and for the actions of the microorganisms that produce humus.
• In addition, it can have considerable influence on the physical characteristics of soil by moving particles.

What is Eluviation and Illuviation ? | UPSC – IAS

Water can have considerable influence on the physical characteristics of soil by moving particles around and depositing them elsewhere in the soil.

• For example, as water percolates into the soil, it picks up fine particles of mineral matter from the upper layers and carries them downward in a process called eluviation. These particles are eventually deposited at a lower level, and this deposition process is known as illuviation.

Image Explanation – In the process of eluviation, fine particles in upper soil layers are picked up by percolating water and carried deeper into the soil. In the process of illuviation, these particles are deposited in a lower soil layer.

Genesis of Soil Structure | Geography Optional | UPSC IAS

Although the lithosphere encompasses the entire planet, from surface to core, the part that holds our attention here is soil, the topmost layer. Soil is the essential medium in which most terrestrial life is nurtured. Almost all land plants sprout from this precious medium, spread so thinly across the continental surfaces that it has an average worldwide depth of only about 15 centimeters (6 inches).

Despite the implication of the well-known simile “as common as dirt,” soil is remarkably diverse. It is a nearly infinitely varying mixture of weathered mineral particles, decaying organic matter, living organisms, gases, and liquid solutions.

• Preeminently, however, soil is a zone of plant growth. Soil is a relatively thin surface layer of mineral matter that normally contains a considerable amount of organic material and is capable of supporting living plants.
• It occupies that part of the outer skin of Earth that extends from the surface down to the maximum depth to which living organisms penetrate, which means basically the area occupied by plant roots. Soil is characterized by its ability to produce and store plant nutrients, an ability made possible by the interactions of such diverse factors as water, air, sunlight, rocks, plants, and animals.
• Although thinly distributed over the land surface, soil functions as a fundamental interface where atmosphere, lithosphere, hydrosphere, and biosphere meet. The bulk of most soil is inorganic material, so soil is usually classified as part of the lithosphere, but it is intimately related to the other three Earth spheres.

Image – Vertical cross section from surface to bedrock, showing the relationship between soil and regolith

Genesis of Soil Structure | UPSC IAS

Soil development (Genesis of Soil Structure) begins with the physical and chemical disintegration of rock exposed to the atmosphere and to the action of water percolating down from the surface. This disintegration is called weathering. The basic result of weathering is the weakening and breakdown of solid rock, the fragmentation of coherent rock masses, and the making of little rocks from big ones.

• The principal product is a layer of loose inorganic material called regolith (“blanket rock”) because it lies like a blanket over the unfragmented rock below. Typically then, the regolith consists of material that has weathered from the underlying rock and that has a crude gradation of particle sizes, with the largest and least fragmented pieces at the bottom, immediately adjacent to the bedrock.
• Sometimes, however, the regolith consists of material that was transported from elsewhere by the action of wind, water, or ice. Thus, the regolith may vary significantly in composition from place to place.
• The upper half meter or so of the regolith normally differs from the material below in several ways, most notably in the intensity of biological and chemical processes taking place.
• This upper portion is soil. It is composed largely of finely fragmented mineral particles, and is the ultimate product of weathering.  It normally also contains an abundance of living plant roots, dead and rotting plant parts, microscopic plants and animals both living and dead, and a variable amount of air and water. Soil is not the end product of a process, but rather a stage in a neverending continuum of physical – chemical–biological processes.

Image explanation – Soil develops through a complex interaction of physical, chemical, and biological processes. Parent-material bedrock weathers to regolith, and then plant litter combines with the regolith to form soil. Some of that soil washes to the ocean floor, where, over the expanse of geologic time, it is transformed to sedimentary rock. Someday that ocean floor may be uplifted above sea level and the exposed sedimentary rock will again be weathered into soil.

Genesis of Soil Structure – Principal Soil Forming Factors | UPSC IAS

Soil is an ever-evolving material. Metaphorically, soil acts like a sponge – taking in inputs and being acted upon by the local environment – changing over time and when the inputs or local environment change. Five principal soil forming factors are responsible for soil development:

• Geology,
• Climate,
• Topography,
• Biology, and
• Time

Genesis of Soil Structure – The Geologic Factor | UPSC IAS

The source of the rock fragments that make up soil is parent material, which may be either bedrock or loose sediments transported from elsewhere by water, wind, or ice. Whatever the parent material, it is sooner or later disintegrated and decomposed at and near Earth’s surface, providing the raw material for soil formation. The nature of the parent material often influences the characteristics of the soil that develop from it; this factor sometimes dominates all others, particularly in the early stages of soil formation.

• The chemical composition of parent material is obviously reflected in the resulting soil, and parent–material physical characteristics may also be influential in soil development, particularly in terms of texture and structure.
• Bedrock that weathers into large particles (as does sandstone, for example) normally produces a coarse-textured soil, one easily penetrated by air and water to some depth. Bedrock that weathers into minute particles (shale, for example) yields fine-textured soils with a great number of pores but of very small size, which inhibits air and water from easily penetrating the surface.
• Young soils are likely to be very reflective of the rocks or sediments from which they were derived. With the passage of time, however, other soil-forming factors become increasingly important, and the significance of the parent material diminishes.
• Eventually the influence of the parent material may be completely obliterated, so that it is sometimes impossible to ascertain the nature of the rock from which the soil evolved.

Genesis of Soil Structure – The Climatic Factor | UPSC IAS

Temperature and moisture are the climatic variables of greatest significance to soil formation. As a basic generalization, both the chemical and biological processes in soil are usually accelerated by high temperatures and abundant moisture and are slowed by low temperatures and lack of moisture.

• One predictable result is that soils tend to be deepest in warm, humid regions and shallowest in cold, dry regions. It is difficult to overemphasize the role of moisture moving through the soil.
• The flow is mostly downward because of the pull of gravity, but it is sometimes sideways in response to drainage opportunities and sometimes, in special circumstances, even upward. Wherever and however water moves, it always carries dissolved chemicals in solution and usually also carries tiny particles of matter in suspension. Thus, moving water is ever engaged in rearranging the chemical and physical components of the soil, as well as contributing to the variety and availability of plant nutrients.
• In terms of general soil characteristics, climate is likely to be the most influential factor in the long run. This generalization has many exceptions, however, and when soils are considered on a local scale, climate is likely to be less prominent as a determinant.

Genesis of Soil Structure – The Topographic Factor | UPSC IAS

Slope and drainage are the two main features of topography that influence soil characteristics. Wherever soil develops, its vertical extent undergoes continuous, if usually very slow, change. This change comes about through a lowering of both the bottom and top of the soil layer.  The bottom slowly gets deeper as weathering penetrates into the regolith and parent material and as plant roots extend to greater depths. At the same time, the soil surface is being lowered by sporadic removal of its uppermost layer through normal erosion, which is the removal of individual soil particles by running water, wind, and gravity.

• Where the land is flat, soil tends to develop at the bottom more rapidly than it is eroded away at the top. This does not mean that the downward development is speedy; rather it means that surface erosion is extraordinarily slow.  Thus, the deepest soils are usually on flat land. Where slopes are relatively steep, surface erosion is more rapid than soil deepening, with the result that such soils are nearly always thin and immaturely developed. If soils are well drained, moisture relationships may be relatively unimportant factors in soil development. If soils have poor natural drainage,
• However, significantly different characteristics may develop. For example, a waterlogged soil tends to contain a high proportion of organic matter, and the biological and chemical processes that require free oxygen are impeded (because air is the source of the needed oxygen and a waterlogged soil contains essentially no air).
• Most poorly drained soils are in valley bottoms or in some other flat locale because soil drainage is usually related to slope. In some cases, such subsurface factors as permeability and the presence or absence of impermeable layers are more influential than slope.

Genesis of Soil Structure – The Biological Factor | UPSC IAS

From a volume standpoint, soil is about half mineral matter and about half air and water, with only a small fraction of organic matter. However, the organic fraction, consisting of both living and dead plants and animals, is of utmost importance.  The biological factor in particular gives life to the soil and makes it more than just “dirt.” Every soil contains a quantity (sometimes an enormous quantity) of living organisms, and every soil incorporates some (sometimes a vast amount of) dead and decaying organic matter.

• Vegetation of various kinds growing in soil performs certain vital functions. Plant roots, for instance, work their way down and around, providing passageways for drainage and aeration, as well as being the vital link between soil nutrients and the growing plants.
• Many kinds of animals contribute to soil development as well. Even such large surface-dwelling creatures as elephants and bison affect soil formation by compaction with their hooves, rolling in the dirt, grazing the vegetation, and dropping excreta. Ants, worms, and all other land animals fertilize the soil with their waste products and contribute their carcasses for eventual decomposition and incorporation into the soil. Many small animals spend most or all of their lives in the soil layer, tunneling here and there, moving soil particles upward and downward, and providing passageways for water and air.
• Mixing and plowing by soil fauna is sometimes remarkably extensive. Ants and termites, as they build mounds, also transport soil materials from one layer to another. The mixing activities of animals in the soil, generalized under the term bioturbation, tend to counteract the tendency of other soil-forming processes to accentuate the vertical differences among soil layers.
• The abundance and variety of animal life connected with the soil are quite surprising. Such organisms vary in size from the gigantic to the microscopic, and in numbers from a few per hectare to billions per gram. The organic life of the soil ranges from microscopic protozoans to larger animals that may accidentally alter certain soil characteristics. Of all creatures, however, it is probable that the earthworm is the most important to soil formation and development.

The Biological Factor – (i) – Earthworms | UPSC IAS

The cultivating and mixing activities of earthworms are of great value in improving the structure, increasing the fertility, lessening the danger of accelerated erosion, and deepening the profile of the soil.  The distinctive evidence of this value is that the presence of many well- nourished earthworms is almost always a sign of productive, or potentially productive, soil.

• The mere presence of earthworms, however, does not guarantee that a soil will be highly productive, as there may be other kinds of inhibiting factors such as a high water table.
• Nevertheless, an earthworm-rich soil has a higher potential productivity than similar soils lacking earthworms. In various controlled experiments, the addition of earthworms to wormless soil has enhanced plant productivity by several hundred percent.

At least seven beneficial functions have been attributed to earthworms:

• Their innumerable tunnels facilitate drainage and aeration and the deepening of the soil  profile.
• The continual movement of the creatures beneath the surface tends to bring about the formation  of a crumbly structure, which is generally favorable for plant growth.
• The soil is further mixed by material being carried and washed downward into their holes from  the surface. This is notably in the form of leaf litter dragged downward by the worms, which  fertilizes the subsoil.
• The digestive actions and tunneling of earthworms form aggregate soil particles that increase  porosity and resist the impact of raindrops, helping to deter erosion.
• Nutrients in the soil are increased by the addition of casts excreted by earthworms (casts are  expelled by earthworms and consist of mineral material bound together with decomposed organic  material), which have been shown to be 5 times richer in available nitrogen, 7 times richer in  available phosphates, and 11 times richer in available potash than the surrounding soil.
• They rearrange material in the soil, especially by bringing deeper matter to the surface, where  it can be weathered more rapidly. Where earthworms are numerous, they may deposit as much as 9000  kg/ hectare (25 tons/acre) of casts on the surface in a year.
• Nitrification is also promoted by the presence of earthworms, due to increased aeration,  alkaline fluids in their digestive tracts, and the decomposition of earthworm carcasses. In many parts of the world, of course, earthworms are lacking. They are, for example almost totally  absent from arid and semiarid regions. In these dry lands, some of the earthworm’s soil-enhancing  functions are carried out by ants and earth-dwelling termites, but much less effectively.

The Biological Factor – (ii) – Microorganisms in the Soil | UPSC IAS

• Another important component of the biological factor is microorganisms, both plant and animal, that occur in uncountable billions. An estimated three-quarters of a soil’s metabolic activity is generated by microorganisms.
• These microbes help release nutrients from dead organisms for use by live ones by decomposing organic matter and by converting nutrients to forms usable by plants. Algae, fungi, protozoans, actinomycetes, and other minuscule organisms all play a role in soil development, but bacteria probably make the greatest contribution overall. This is because certain types of bacteria are responsible for the decomposition and decay of dead plant and animal material and the consequent release of nutrients into the soil.

Genesis of Soil Structure – The Time Factor | UPSC IAS

• For soil to develop on a newly exposed land surface requires time, with the length of time needed varying according to the nature of the exposed parent material and the characteristics of the environment.
• Soil-forming processes are generally very slow, and many centuries may be required for a thin layer of soil to form on a newly exposed surface. A warm, moist environment is conducive to soil development. Normally of much greater importance, however, are the attributes of the parent material. For example, soil develops from sediments relatively quickly and from bedrock relatively slowly.

The Time Factor – Soil Erosion | UPSC IAS

Most soil develops with geologic slowness – so slowly that changes are almost imperceptible within a human life span. It is possible, however, for a soil to be degraded, either through the physical removal associated with accelerated erosion or through depletion of nutrients, in only a few years.

• Soils that have fine textures – especially those with low rates of rainwater infiltration – tend to be those that are most easily eroded by rainwater runoff and wind;
• Steep slopes and lack of a vegetation cover also increase the likelihood of erosion. In regions where single-crop agriculture (“monoculture”) is practiced, fields are often left bare and unplanted for several months each year, increasing the likelihood of erosion.
• Estimates of the amount of agricultural land lost to soil erosion vary greatly. In the United States, some researchers estimate that nearly 40 percent of the productive soil in the wheat growing Palouse region of Washington and Idaho, and as much as 50 percent of the topsoil of Iowa, has been lost to erosion over the last 150 years.
• Globally, perhaps 10 million hectares of cropland are lost each year to soil erosion – a rate that is 10 to 40 times faster than productive soil can develop. It is important to realize that in the grand scale of geologic time, soil can be formed and reformed, but in the dimension of human time, it is a mostly nonrenewable resource.

The Köppen Climate Classification System | UPSC – IAS 

To cope with the great diversity of information encompassed by the study of global climate distribution, we need a classification scheme to simplify, organize, and generalize a vast array of data.

The Koppen climate classification system is by far the most widely used modern climate classification system. Wladimir Köppen (1846–1940; pronounced like “kur-pin” with a silent r) was a Russian-born German climatologist who was also an amateur botanist.

• The first version of his climate classification scheme appeared in 1918, and he continued to modify and refine it for the rest of his life, the last version being published in 1936.
• The system uses as a database only the average annual and average monthly values of temperature and precipitation, combined and compared in a variety of ways. Consequently, the necessary statistics are commonly tabulated and easily acquired.
• Data for any location (called a station) on Earth can be used to determine the precise classification of that place, and the geographical extent of any recognized climatic type can be determined and mapped. This means that the classification system is functional at both the local and the global scale.
• Koppen defined four of his five major climatic groups primarily by temperature characteristics, the fifth (the B group) on the basis of moisture. He then subdivided each group into climate types according to various temperature and precipitation relationships.

Image Explanation – The modified Köppen climatic classification. There are 5 major climate groups (and the special category of Highland) and 15 individual climate types.

Modified Köppen system | UPSC IAS | Geography Optional

Köppen was unsatisfied with his last version and did not consider it a finished product. Thus, many geographers and climatologists have used the Köppen system as a springboard to devise systems of their own or to modify Koppen’s classifications.

Modified Köppen system encompasses the basic design of the Koppen system but with a variety of minor modifications. Some of these modifications follow the lead of Glen Trewartha, who was a geographer and climatologist at the University special category of highland (H) climate.

Image: Climatic regions over land areas (modified Koppen system).

Köppen Letter Code System | UPSC IAS | Geography Optional 

In the modified Koppen system, each climate type is designated by a descriptive name and by a series of letters defined by specific temperature and/or precipitation values (image).

• The first letter designates the major climate group,
• The second letter usually describes precipitation patterns, and
• The third letter (if any) describes temperature patterns.

Although the letter code system seems complicated at first, it provides a shorthand method for summarizing key characteristics of each climate.

For example, – If we look for the definitions of the letters in Csa, one of the letter code combinations for a mediterranean climate, we see that:

C = Mild mid latitude climate
s = Summer dry season
a = Hot summers

Time zones and International date line | UPSC – IAS

Comprehending time around the world depends on an understanding of both the geographic grid of latitude and longitude, and of Earth–Sun relations. As Malcolm Thomson, a Canadian authority on the physics of time has noted, there are really only three natural units of time:

• The tropical year, marked by the return of the seasons;
• The lunar month, marked by the return of the new moon; and
• The day, marked by passage of the Sun.

All other units of time measurement – such as a second, an hour, or a century—are human-made to meet the needs of society.

Image Explanation – The 24 time zones of the world, each based on central meridians spaced 15° apart. Especially over land areas, these boundaries have been significantly adjusted.

International Date Line | UPSC – IAS

In 1519, Ferdinand Magellan set out westward from Spain, sailing for East Asia with 241 men in five ships. Three years later, the remnants of his crew (18 men in one ship) successfully completed the first circumnavigation of the globe. Although a careful log had been kept, the crew found that their calendar was one day short of the correct date. This was the first human experience with time change on a global scale, the realization of which eventually led to the establishment of the International Date Line.

• One advantage of establishing the Greenwich meridian as the prime meridian is that its opposite arc is in the Pacific Ocean. The 180th meridian, transiting the sparsely populated mid-Pacific, was chosen as the meridian at which new days begin and old days exit from the surface of Earth.
• The International Date Line deviates from the 180th meridian in the Bering Sea to include all of the Aleutian Islands of Alaska within the same day and again in the South Pacific to keep islands of the same group (Fiji, Tonga) within the same day.
• The extensive eastern displacement of the date line in the central Pacific is due to the widely scattered locations of the many islands of the country of Kiribati. The International Date Line is in the middle of the time zone defined by the 180° meridian. Consequently, there is no time (i.e., hourly) change when crossing the International Date Line – only the calendar changes, not the clock. When you cross the International Date Line going from west to east, it becomes one day earlier (e.g., from January 2 to January 1); when you move across the line from east to west, it becomes one day later (e.g., from January 1 to January 2).

Standard Time | UPSC – IAS

At the 1884 International Prime Meridian Conference in Washington, D.C., countries agreed to divide the world into 24 standard time zones, each extending over 15° of longitude. The mean local solar time of the Greenwich (prime) meridian was chosen as the standard for the entire system. The prime meridian became the center of a time zone that extends 7.5° of longitude to the west and 7.5° to the east of the prime meridian. Similarly, the meridians that are multiples of 15° both east and west of the prime meridian, were set as the central meridians for the 23 other time zones

Although Greenwich Mean Time (GMT) is now referred to as Universal Time Coordinated (UTC), the prime meridian is still the reference for standard time. Because it is always the same number of minutes after the hour in all standard time zones (keeping in mind that a few countries, such as India, do not adhere to standard one-hour-interval time zones).

• To know the exact local time, we usually need to know only how many hours later or earlier our local time zone is compared to the time in Greenwich. Figure 1-31 shows the number of hours later or earlier than UTC it is in each time zone of the world.
• Most of the countries of the world are sufficiently small in their east–west direction so as to lie totally within a single time zone. However, large countries may encompass several zones: Russia occupies nine time zones; including Alaska and Hawai‘i, the United States spreads over six, Canada, six; and Australia, three.
• In international waters, time zones are defined to be exactly 7°30´ to the east and 7°30´ to the west of the central meridians. Over land areas, however, zone boundaries vary to coincide with appropriate political and economic boundaries. For example, continental Europe from Portugal to Poland shares one time zone, although longitudinally covering about 30°.
• At the extreme, China extends across four 15° zones, but the entire nation, at least officially, observes the time of the 120° east meridian, which is the one closest to Beijing.

Image Explanation – Times zones for Canada, the United States, and northern Mexico. The number in each time zone refers to the number of hours earlier than UTC (GMT).

In each time zone, the central meridian marks the location where clock time is the same as mean Sun time  (i.e., the Sun reaches its highest point in the sky at 12:00 noon). On either side of that meridian, of course, clock time does not coincide with Sun time. The deviation between the two is shown for one U.S. zone in Figure 1-33. From the map of time zones of the United States (Figure 1-32), we can recognize a great deal of manipulation of the time zone boundaries for economic and political convenience. For example, the Central Standard Time Zone, centered on 90° W extends all the way to 105° W (which is the central meridian of the Mountain Standard Time Zone) in Texas to keep most of that state within the same zone. By contrast, El Paso, Texas, is officially within the Mountain Standard Time Zone in accord with its role as a major market center for southern New Mexico, which observes Mountain Standard Time. In the same vein, northwestern Indiana is in the Central Standard Time Zone with Chicago.

Daylight-Saving Time | UPSC – IAS

To conserve energy during World War I, Germany ordered all clocks set forward by an hour. This practice allowed the citizenry to “save” an hour of daylight by shifting the daylight period into the usual evening hours, thus reducing the consumption of electricity for lighting.

The United States began a similar policy in 1918, but many localities declined to observe “summer time” until the Uniform Time Act made the practice mandatory in all states that had not deliberately exempted themselves. Hawai‘i, and parts of Indiana and Arizona, have exempted themselves from observance of daylight-saving time under this act.

Image Explanation – Standard clock time versus Sun time. The Sun reaches its highest point in the sky at 12:00 noon in St. Louis and New Orleans because these two cities lie on the central meridian. For places east of the central meridian, the Sun is highest in the sky a few minutes before standard time noon; for locations west, local solar noon is a few minutes after. In Chicago, for instance, the Sun is highest in the sky at 11:50 A.M. and in Dallas it is highest in the sky at 12:28 P.M.

• Russia has adopted permanent daylight-saving time (and double daylight-saving time – two hours ahead of Sun time – in the summer).
• In recent years, Canada, Australia, New Zealand, and most of the nations of western Europe have also adopted daylight-saving time.
• In the Northern Hemisphere, many nations, like the United States, begin daylight-saving time on the second Sunday in March (in the spring we “spring forward” one hour) and resume standard time on the first Sunday in November (in the fall we “fall back” one hour).
• In the tropics, the lengths of day and night change little seasonally, and there is not much twilight. Consequently, daylight-saving time would offer little or no savings for tropical areas.