Air Masses and Frontogenesis: Types of Air Masses, Fronts, Frontogenesis and Frontolysis

Air Masses: Definition, Characteristics and Source Regions

What Is an Air Mass? Air Masses Definition and Characteristics

When air stays over a uniform area, a vast ocean or a wide plain, for long enough, it takes on the temperature and humidity of that surface. The result is an air mass: a large body of air with little horizontal variation in temperature and moisture, covering many hundreds or thousands of square miles.

• Source region: The uniform surface over which an air mass forms is its source region, and air masses are classified by it.
• Time matters: The longer an air mass stays over its source region, the more it takes on the properties of the surface below.
• Movement: Air masses usually move with the flow of the upper air; as the jet stream changes strength and position, it changes their motion and strength.
• Density: Cold air masses are denser than warm ones, which is why cold air undercuts warm air where the two meet.

Source Regions of Air Masses: Five Kinds of Surface

Air masses form only where the surface is uniform over a large area. There are five major source regions, and each gives a type of air mass its name.

1. Warm tropical and subtropical oceans: Maritime tropical (mT) air.
2. Subtropical hot deserts: Continental tropical (cT) air; the Sahara is the major source, with the Arabian Peninsula, central Australia and the deserts of the south-western United States.
3. Cold high-latitude oceans: Maritime polar (mP) air.
4. Cold, snow-covered continents in high latitudes: Continental polar (cP) air, as over interior Canada.
5. Permanently ice-covered Arctic and Antarctica: Continental arctic (cA) air; air over northern Siberia in winter can also be cA.

The source decides both halves of the name: tropical air masses are warm and polar air masses are cold, while continental ones are dry and maritime ones moist.

Types of Air Masses: Classification, Modification and Air Masses over India

Air Masses Classification: Types, Bergeron Letters and Modification

The Bergeron classification is the most widely used way of naming air masses. It joins a letter for moisture to a letter for the latitude of the source region.

• First letter, moisture: The letter c marks continental air, which is dry, and m marks maritime air, which is moist.
• Second letter, source: T for tropical, P for polar, A for arctic or antarctic, and in some schemes M for monsoon, E for equatorial and S for superior air.
• Third letter, stability: The letter k means the air mass is colder than the ground below it, and w that it is warmer.

An air mass does not keep its character for ever. Once it leaves its source region, the vegetation and water below can quickly modify it.

• Over the sea: In winter, very cold and dry arctic air moving out over a warmer ocean picks up warmth and moisture and becomes maritime polar air, still fairly cold but moist.
• Over land: The same polar air moving south over land warms up but stays very dry, as continental polar air.
• Stability changes: Polar air over the sea loses its stability as it gains moisture over warmer water.
• Notation: Meteorologists write the change as a transformation, for example cA-mPk for arctic air turning into cold maritime polar air.

Air Masses over India: Dry Winter Air, Local Storms and Western Disturbances

Air masses help explain India's seasons. In winter a high-pressure centre north of the Himalayas sends surface winds towards India that arrive as a dry continental air mass; over north-western India they meet the trade winds, in a contact zone that sometimes shifts as far east as the middle Ganga valley.

• Pre-monsoon storms: In the hot season a sudden contact between dry and moist air masses gives local storms of great intensity, with violent winds, torrential rain and even hail.
• Western disturbances: Extra-tropical storms from the Mediterranean region, driven by the westerlies, bring sudden winter rain to north-western India; four to five form in an average winter.
• Their weight: About 30 per cent of the annual precipitation of Jammu and Kashmir, Ladakh, Himachal Pradesh and Uttarakhand falls in winter, mostly from western disturbances, and it is vital for the rabi crop.
• After they pass: Cold waves and cold days follow their passage, and they bring dense fog to the Indo-Gangetic plains.

Air masses drive India's winter, summer and monsoon seasons, and the winter rain of north-west India comes with western disturbances.

Fronts in Geography: Where Air Masses Meet

What Are Fronts in Weather? Definition and Features

When two different air masses meet, the boundary zone between them is called a front. It is a zone of transition between air masses of different density, and so usually of different temperature; a moving front is named after the advancing air mass, so a cold front is one where colder air is advancing.

• Where they occur: Fronts occur in the middle latitudes and have a steep gradient of temperature and pressure.
• What they bring: Abrupt changes in temperature; they make the air rise, forming clouds and precipitation.
• A sure sign: Some fronts bring no rain and little cloud, but there is always a shift in the wind.
• Dry line: A narrow zone across which moisture, rather than temperature, changes sharply.

Types of Fronts: Cold, Warm, Stationary and Occluded

There are four types of fronts. Which one forms depends on which air mass is advancing, and each brings its own kind of weather because cold air is denser than warm air.

• Chart symbols: Blue triangles for a cold front, red semicircles for a warm front, alternating blue triangles and red semicircles for a stationary front, and a purple line of both for an occluded front.
• Two occlusions: In a cold occlusion the overtaking air is colder than the air ahead; in a warm occlusion it is warmer and rides over it.

Frontogenesis and Frontolysis: How Fronts Form, Decay and Make Cyclones

Frontogenesis and Frontolysis: How Fronts Form and Decay

A front forms by frontogenesis: the horizontal temperature gradient between two air masses tightens until a sharp boundary appears. It happens as a developing wave in the atmosphere draws cold air from the poles and warm air from the equator together.

• What drives it: Eight mechanisms influence the temperature gradient: horizontal deformation, horizontal shearing, vertical deformation, differential vertical motion, latent heat release, surface friction, turbulence and mixing, and radiation.
• Where it happens: Where air masses converge, fronts form and sharpen; where they diverge, fronts weaken.
• Frontolysis: The weakening or dissipation of a front, as the temperature gradient between the air masses decreases or the wind pattern shifts; on maps it is drawn as a dashed line.
• Shear line: When the density contrast fades, for instance after the air flows over a uniformly warm ocean, a front can degenerate into a mere line separating different wind speeds.

Fronts and Extra-Tropical Cyclones: The Norwegian Cyclone Model

Fronts are the birthplace of the extra-tropical cyclones that bring much of the weather of the middle latitudes, between 30 and 60 degrees. Norwegian meteorologists described their life cycle in the 1910s and 1920s, and it is still known as the Norwegian cyclone model.

1. Stationary front: Along the polar front, warm air lies to the south and cold air to the north.
2. A wave forms: When pressure drops along the front, the warm air moves north and the cold air south, setting up an anticlockwise circulation; the front splits into a warm front and a cold front.
3. Mature stage: A warm sector lies wedged between the fronts; the warm air glides over the cold air ahead of the warm front, and cumulus clouds build along the cold front.
4. Occlusion and death: The faster cold front overtakes the warm front, the warm air is lifted off the ground, and without it the cyclone dissipates.

Fronts are where temperate cyclones form, as the polar front theory explains; tropical cyclones are a different kind of storm.

Air Masses and Climate: Heat, Rain, Seasons and the Jet Stream

Each air mass carries the weather of its source region to distant places, and the fronts between them make much of the weather of the middle latitudes. Their role in climate can be seen at several scales.

• Carriers of heat and moisture: Arctic air turns into moist maritime polar air over the sea, while polar air moving over land stays dry, so the same outbreak brings different weather to different places.
• Rain and storms: Fronts lift air to form cloud and precipitation, and the extra-tropical cyclones that grow on them, with anticyclones, drive the weather over much of the middle latitudes.
• Seasons in India: Dry continental air in winter, storms where dry and moist air meet before the monsoon, and western disturbances that supply north-west India’s winter rain and snow.
• Link to the jet stream: Shifts in the jet stream’s strength and position change how air masses move and how strong they are.

Air masses over India move with the jet streams and pressure belts of each season.

Sources

• NCERT: Fundamentals of Physical Geography (Class XI), Atmospheric Circulation and Weather Systems
• NOAA JetStream: Air Masses
• NOAA JetStream: Norwegian Cyclone Model
• WMO International Cloud Atlas: Glossary
• IMD Srinagar: Western Disturbances and Weather Fronts
• PIB (MoES): Impact of western disturbances on Indian climate, 5 February 2021
• Wikipedia: Air mass
• Wikipedia: Weather front
• Wikipedia: Frontogenesis
• Wikipedia: Frontolysis
• Wikipedia: Cold front
• Wikipedia: Warm front
• Wikipedia: Stationary front
• Wikipedia: Occluded front
• Wikipedia: Extratropical cyclone
• Wikipedia: Western disturbance
• UPSC: Civil Services Examination question papers