Heat Budget of the Earth: Insolation, Albedo and Temperature Inversion

Insolation: Solar Radiation Reaching the Earth

Insolation and the Solar Constant

The earth receives almost all of its energy from the sun, mostly in short wavelengths. This incoming solar radiation is called insolation. The sun's surface is at about 5,800 K (about 5,500 °C), and at that temperature most of the energy it radiates is visible and near-infrared light. The earth intercepts only a very small portion of the sun's energy. Because the earth is a geoid resembling a sphere, the sun's rays fall obliquely on most of it at the top of the atmosphere.

The solar constant is the energy arriving on a surface held at right angles to the sun's rays at the top of the atmosphere, at the earth's mean distance from the sun. In older units it is 1.94 calories per square centimetre per minute; satellite measurements now put it at about 1.361 kilowatts per square metre. Averaged over the whole globe, by day and night and over every angle of the rays, the sunlight arriving at the top of the atmosphere is one-fourth of this, approximately 340 watts per square metre.

The solar output received at the top of the atmosphere varies slightly through the year because the earth's distance from the sun changes during its revolution.

• Perihelion: On 3 January the earth is nearest to the sun, at 147 million km, so the insolation received then is slightly more than in July.
• Aphelion: On 4 July the earth is farthest from the sun, at 152 million km.
• Size of the change: Direct solar irradiance at the top of the atmosphere fluctuates by about 6.9 per cent during a year, from 1.408 kilowatts per square metre in early January to 1.317 kilowatts per square metre in early July.

These dates are approximate, because the exact dates shift by up to three days from one year to another. Aphelion fell on 3 July 2025 and on 6 July 2026. The effect of this variation is masked by other factors, chiefly the distribution of land and sea and the atmospheric circulation, so it has no great effect on daily weather.

Factors Controlling Insolation: Angle of Incidence and Length of Day

The amount and intensity of insolation vary during a day, in a season and in a year. Five factors cause these variations, and the last two have less influence than the first three.

• Rotation of the earth: The earth's rotation on its axis brings day and night, so a place receives insolation only for part of each day.
• Angle of incidence of the rays: This depends on latitude. The higher the latitude, the smaller the angle the rays make with the surface. A slanting ray spreads the same energy over a larger area, so the energy received per unit area falls; it also passes through a greater depth of atmosphere, where more is absorbed, scattered and diffused.
• Length of the day: A longer day gives more hours of insolation, and day length changes with latitude and season.
• Transparency of the atmosphere: Clouds, water vapour and dust reduce the energy that reaches the ground.
• Configuration of the land: The aspect of a slope, the direction it faces, affects how much sunlight it receives.

The earth's axis makes an angle of 66½° with the plane of its orbit round the sun. This tilt has a great influence on the amount of insolation received at different latitudes.

Passage of Solar Radiation Through the Atmosphere: Absorption, Scattering and Reflection

The atmosphere is largely transparent to short-wave solar radiation, but three processes act on the incoming beam before it strikes the surface.

• Absorption: Within the troposphere, water vapour, ozone and other gases absorb much of the near-infrared radiation. Ozone in the stratosphere absorbs the ultraviolet rays and prevents them from reaching the surface.
• Scattering: Very small suspended particles and the gas molecules of the air scatter part of the visible spectrum, both back to space and down towards the surface.
• Reflection: The tops of clouds and bright surfaces such as snow and ice send part of the insolation straight back to space.

Scattering gives the sky its colour. The atmosphere scatters short wavelengths more strongly than long ones, so blue light is scattered far more than red, and the sky away from the sun looks blue. At sunrise and sunset the sun's light crosses a much longer path through the air; much of the blue is scattered out of the beam on the way, and the sun appears red.

Spatial Distribution of Insolation: Tropics, Subtropical Deserts and Poles

The insolation received at the surface varies from about 320 watts per square metre in the tropics to about 70 watts per square metre at the poles. The pattern has four features.

• Subtropical deserts: Maximum insolation is received over the subtropical deserts, where cloudiness is the least.
• Equator: The equator receives comparatively less insolation than the tropics, because its skies are cloudier.
• Land and sea: At the same latitude, insolation is generally more over the continents than over the oceans.
• Seasons: In winter, the middle and higher latitudes receive less radiation than in summer.

Heating and Cooling of the Atmosphere

Conduction, Convection and Advection

The atmosphere is heated mainly from below. The earth, after being heated by insolation, passes heat to the air in contact with it, and the air carries that heat upward and sideways in three ways.

• Conduction: When two bodies of unequal temperature are in contact, energy flows from the warmer to the cooler until both reach the same temperature or the contact is broken. Air in contact with the land is heated slowly in this way, and each layer then heats the layer above it. Conduction is important in heating the lower layers of the atmosphere.
• Convection: Air heated at the surface rises vertically in currents and carries heat upward. This vertical transfer of energy is confined to the troposphere.
• Advection: Heat is also carried by the horizontal movement of air, which is relatively more important than the vertical movement. In middle latitudes most of the day and night variation in daily weather is caused by advection alone.

The loo of northern India is a product of advection. It is a strong, dusty, hot and dry summer wind from the west that blows over the Indo-Gangetic Plain, strongest in May and June. Its temperatures of 45 °C to 50 °C make exposure to it dangerous, and it dries vegetation over wide areas.

Terrestrial Radiation: Long-Wave Heat From the Earth

The earth, once heated, becomes a radiating body itself and gives out energy in long-wave form. This is terrestrial radiation. The difference in wavelength follows from the difference in temperature, since a hotter body radiates more energy, at shorter wavelengths, than a cooler one. The very hot sun therefore sends short waves, and the far cooler earth sends long waves.

The atmosphere treats the two kinds of radiation differently. It lets most of the short-wave insolation pass, but the gases of the atmosphere, particularly carbon dioxide and the other greenhouse gases, absorb the long-wave radiation. The atmosphere is thus heated indirectly, by the earth's radiation, and not directly by the sun. The atmosphere in turn radiates heat to space, and in the end the heat received from the sun is returned to space.

Greenhouse Effect and the Temperature of the Earth

The greenhouse effect is the warming that results when gases in the atmosphere absorb the long-wave radiation emitted by the earth's surface and slow its loss to space. The name comes from the glass greenhouse used in cold areas. Glass is transparent to incoming short-wave radiation but opaque to outgoing long-wave radiation, so the inside stays warmer than the outside. A car or bus with its windows closed on a summer day grows hot inside for the same reason.

The main greenhouse gases are carbon dioxide (CO₂), chlorofluorocarbons (CFCs), methane (CH₄), nitrous oxide (N₂O) and ozone (O₃). Carbon dioxide has the largest concentration of these, and its emission comes mainly from the burning of oil, gas and coal. Water vapour also absorbs part of the insolation and preserves the earth's radiated heat, acting like a blanket.

The natural greenhouse effect makes the earth habitable. Judged by the long-wave energy it emits to space, the earth's effective temperature is −18 °C. Its average near-surface air temperature is about 15 °C, which is 33 °C warmer. The rise in greenhouse gases through human activity is strengthening this effect, which is the cause of global warming.

Heat Balance of the Earth: The 100-Unit Account

Heat Budget of the Earth in 100 Units: Where Insolation Goes

The heat budget is followed most easily by taking the insolation at the top of the atmosphere as 100 units. While passing through the atmosphere, some of this energy is reflected, scattered and absorbed, and only the remainder reaches the surface. The account runs in five steps.

1. Reflected and scattered to space, 35 units: About 35 units are sent back to space before reaching the surface: 27 units from the tops of clouds, 2 units from snow and ice-covered areas, and 6 units scattered to space by the atmosphere. This reflected amount is the albedo of the earth.
2. Absorbed, 65 units: The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth's surface. Of the 51 units, 34 arrive as direct radiation and 17 as scattered radiation.
3. Radiated by the surface, 51 units: The earth radiates its 51 units back as terrestrial radiation. Of these, 17 units go directly to space and 34 units are absorbed by the atmosphere: 6 units directly, 9 units through convection and turbulence, and 19 units through the latent heat of condensation.
4. Radiated by the atmosphere, 48 units: The atmosphere now holds 48 units, the 14 units it took from insolation and the 34 units it took from the earth, and it radiates all of them back to space.
5. Balance, 65 units: The total returned by the earth and the atmosphere is 17 + 48 = 65 units, which balances the 65 units absorbed from the sun.

The account explains why the earth neither warms up nor cools down despite the huge transfer of heat that takes place. It also shows that the atmosphere receives far more of its heat from the earth (34 units) than directly from the sun (14 units).

Albedo of the Earth: Reflection by Clouds, Ice and Land

Albedo is the share of the incoming radiation that a surface reflects, usually stated for visible light. The average albedo of the earth is about 0.3. NCERT's heat budget counts 35 of every 100 units as reflected, while other accounts give about 29 per cent; both show the same balance, with about two-thirds or a little more of the insolation absorbed and the same amount radiated back to space.

Surfaces differ widely in how much they reflect, which is why ice cover and land use affect climate.

• High albedo: Snow-covered sea ice reflects far more than open sea water, and desert areas have some of the highest albedos among landforms.
• Low albedo: The ocean surface has a low albedo, as do most forests, which absorb most of the ultraviolet and visible light in photosynthesis.
• Human change: Deforestation, farming and urbanisation change the albedo of large areas.

Clouds in the Earth's Energy Budget

Clouds act on both kinds of radiation. Their high albedo reflects sunlight back to space, a cooling effect; the water and ice inside them also absorb long-wave radiation from the surface and send part of it back down, a warming effect. Which effect is stronger depends on the cloud's altitude, its size and the particles that form it.

• Low, thick clouds: They primarily reflect solar radiation and so cool the surface of the earth.
• High, thin clouds: They primarily transmit incoming solar radiation, but they trap some of the outgoing infrared radiation and radiate it back downward, so they warm the surface.

Averaged over all the clouds around the globe, the cooling effect predominates. The warming effect is also why a cloudy night stays warmer than a clear one, since the ground loses less heat under cloud.

Latitudinal Heat Balance and Transfer of Heat

Surplus and Deficit of Net Radiation by Latitude

The heat budget balances for the earth as a whole but not for each latitude. Because the earth is a sphere, the sun heats the equatorial regions more than the polar regions. Net radiation, the insolation absorbed minus the energy radiated away, is therefore positive in low latitudes and negative near the poles. There is a surplus of net radiation between about 40° north and south, and the regions near the poles have a deficit.

In September 2008, the month of an equinox, areas around the equator absorbed about 200 watts per square metre more on average than they reflected or radiated, areas near the poles lost about 200 watts per square metre more than they absorbed, and the mid-latitudes were roughly in balance.

Poleward Transfer of Heat by Winds and Ocean Currents

If the surplus stayed where it is gained, the tropics would grow progressively hotter and the high latitudes would become permanently frozen. Neither happens, because the surplus heat of the tropics is redistributed poleward. The atmosphere and the ocean work without pause to even out the imbalance through the evaporation of surface water, convection, rainfall, winds and ocean circulation. This coupled circulation of the atmosphere and the ocean is called the earth's heat engine.

The transfer works in two linked ways.

• Through the atmosphere: Unequal heating causes pressure differences, and the winds that blow from high to low pressure carry heat from one region to another. Evaporation stores heat as latent heat in water vapour, and the heat is released into the atmosphere when the vapour condenses into cloud and rain.
• Through the oceans: Warm currents carry tropical heat towards higher latitudes. The warm Gulf Stream and North Atlantic Drift make the northern Atlantic Ocean warmer.

The heat engine also returns heat from the surface and the lower atmosphere to space. Without that outflow the earth would warm without end; with it, the global account stays balanced.

Earth's Energy Imbalance and Global Warming

Energy Imbalance and the Storage of Excess Heat

The earth's energy budget depends on many factors, including aerosols, greenhouse gases, surface albedo, clouds and land use. When the incoming and outgoing flows are equal, the earth is in radiative equilibrium and the global temperature stays relatively stable. When the earth receives more energy than it gives back to space, it warms.

The added greenhouse gases now hold back part of the outgoing long-wave radiation, so less energy leaves than arrives. This gap is the earth's energy imbalance. From 2005 to 2019 it averaged about 0.90 watts per square metre over the whole globe.

The extra energy is shared unevenly within the climate system.

• Ocean: More than 91 per cent of the excess heat is stored in the ocean, which buffers the land against higher temperatures.
• Land: About 5 per cent is stored in the continental land masses.
• Ice: About 3 per cent warms and melts ice.
• Atmosphere: Only about 1 per cent warms the atmosphere, including the air near the surface that people feel.

Because the ocean takes by far the largest share, ocean heat content is a critical indicator of climate change. In the 100-unit account, the imbalance would appear as a small, steady surplus of energy absorbed over energy returned.

Distribution of Temperature: Factors, Isotherms and Annual Range

Factors Controlling the Distribution of Temperature

The temperature of the air at any place is the result of the heat it receives and loses, and five factors control it.

• Latitude: The temperature of a place depends on the insolation it receives, and insolation varies with latitude.
• Altitude: The atmosphere is heated from below by terrestrial radiation, so places near sea level are warmer than places at higher elevations.
• Distance from the sea: Compared with land, the sea heats up slowly and loses heat slowly, while land heats up and cools down quickly. The variation of temperature over the sea is therefore less than over land, and places near the sea come under the moderating influence of land and sea breezes.
• Air masses and ocean currents: Places under warm air masses record higher temperatures, and places under cold air masses lower ones. Coasts washed by warm ocean currents are warmer than coasts washed by cold currents.
• Local aspects: Features of the place itself, such as the direction a slope faces, also modify its temperature.

The land and sea contrast rests on specific heat, the energy needed to raise the temperature of one gram of a substance by 1 °C. Liquid water has one of the highest specific heats among common substances, so the sea warms and cools far more slowly than the land.

Altitude and the Normal Lapse Rate

Temperature generally decreases with increasing height in the troposphere. The rate of this decrease is the normal lapse rate, 6.5 °C per 1,000 m. Three reasons explain it. The atmosphere is heated from below, by the earth's surface, so the air farthest from the surface receives the least heat. Water vapour, which absorbs part of that heat, decreases with altitude. The air also becomes less dense with height, and air at lower pressure has a lower temperature.

Rising air cools at a different rate from the still air around it. The actual fall of temperature with height at a given time and place is the environmental lapse rate, of which the normal lapse rate is an average. A parcel of dry air that rises cools by expansion at the dry adiabatic lapse rate of about 9.8 °C per 1,000 m. When the surrounding air cools more slowly with height than this, a rising parcel soon becomes colder and heavier than its surroundings, and the air is stable.

Isotherms in January and July: Horizontal Distribution of Temperature

The global distribution of temperature is studied through maps of January and July drawn with isotherms, lines joining places of equal temperature. In general the isotherms run parallel to the lines of latitude, which shows how strongly latitude controls temperature. The deviation from this pattern is more pronounced in January than in July, and more in the northern hemisphere, whose land area is much larger than that of the southern hemisphere.

• January, northern hemisphere: The isotherms deviate to the north over the oceans and to the south over the continents. Over the North Atlantic Ocean the warm Gulf Stream and North Atlantic Drift make the water warmer and bend the isotherms north. Over the land the temperature falls sharply and the isotherms bend south over Europe, most of all over the Siberian plain. Along 60° E longitude the mean January temperature is −20 °C at both 80° N and 50° N.
• January, by region: The mean January temperature is over 27 °C over the equatorial oceans, over 24 °C in the tropics, 2 °C to 0 °C in the middle latitudes and −18 °C to −48 °C in the interior of Eurasia.
• January, southern hemisphere: The effect of the ocean dominates, so the isotherms run more or less parallel to the latitudes and temperature changes more gradually. The 20 °C, 10 °C and 0 °C isotherms run parallel to 35° S, 45° S and 60° S respectively.
• July: The isotherms generally run parallel to the latitudes. The equatorial oceans record more than 27 °C, and over the land more than 30 °C is found in the subtropical continental region of Asia along 30° N.

Continentality: Why Continental Interiors Have the Widest Range

The annual range of temperature is the difference between the mean temperatures of the warmest and the coldest months. It is the clearest measure of the land and sea contrast. The highest range, more than 60 °C, is found over the north-eastern part of the Eurasian continent, and the cause is continentality. The interior lies far from the moderating influence of the sea, so it heats strongly in summer and cools severely in winter. The least range, 3 °C, is found between 20° S and 15° N.

The isotherms show when land and sea differ most. In January the cold northern continents and the warmer oceans bend the isotherms sharply away from the parallels, while in July the isotherms run much closer to them, so the temperature contrast between continents and oceans is greater in winter than in summer.

Temperature Inversion: Meaning, Causes, Types and Effects

Conditions Favouring a Temperature Inversion

Normally, temperature decreases with increase in elevation at the normal lapse rate. At times the situation is reversed, and a layer of warmer air lies over cooler air. This reversal of the normal lapse rate is called temperature inversion. An inversion is usually of short duration, but it is quite common.

The conditions that favour an inversion near the ground are well defined.

• Long nights: The heat of the day has time to be radiated away, so by the early morning hours the earth is cooler than the air above it.
• Clear skies: Without clouds to send long-wave radiation back down, the ground loses heat quickly.
• Still air: Without wind, the cold air near the ground is not mixed with the warmer air above.

Over polar areas these conditions hold so often that an inversion is normal throughout the year. Elsewhere, such a surface inversion commonly lasts a few hours, until the sun comes up and begins to warm the earth. This kind of inversion is virtually confined to land, because the ocean retains heat far longer.

Types of Temperature Inversion: Radiation, Valley, Frontal and Subsidence

Inversions are classified by the way the cold layer comes to lie beneath the warm one.

The radiation and valley types form at the ground and usually clear once the sun warms the earth; the frontal and subsidence types form aloft and can cover a wide area.

Effects of Temperature Inversion on Weather and People

An inversion makes the lower atmosphere stable. The cold, dense air at the bottom does not rise, so the vertical mixing that normally carries smoke and moisture upward stops. Most effects of an inversion follow from that one change.

• Smoke, dust and smog: Smoke and dust particles collect beneath the inversion layer and spread horizontally to fill the lower layers of the atmosphere. During a severe inversion, trapped pollutants form a brownish haze that can cause respiratory problems.
• Fog: Dense morning fogs are common, especially in winter. With enough moisture in the cool layer, fog forms below the inversion cap.
• Cities: Cities suffer most, because they produce more pollutants and store more heat than the countryside, and the effect is stronger where hills surround a city and block the circulation of air.
• Capped convection and thunderstorms: A warm layer aloft can shut off convection, acting as a cap. If the cap is broken, the stored energy can be released suddenly as violent thunderstorms.
• Freezing rain and ice pellets: In winter, snow that melts in a warm layer aloft and falls into a cold layer near the ground can freeze again, as ice pellets in a thick cold layer or as freezing rain in a shallow one.
• Mirages and long-range signals: An inversion reverses the usual bending of light, so distant objects can appear stretched or above the horizon; it also bends very high frequency radio waves, such as FM radio, back towards the earth, so their signals travel unusually far.

North India shows these effects every winter. In Delhi and the National Capital Region, the post-monsoon and winter months bring lower temperatures, inversion conditions and calm winds. Pollutants then mix through a shallower layer of air and stay near the ground, so the Air Quality Index of the region generally deteriorates in winter.

Urban Heat Island: Causes, Effects and Remedies

Dark Surfaces, Lost Vegetation and Waste Heat

An urban heat island is a city that is markedly warmer than the rural area around it. The difference is usually larger at night than during the day and is most apparent when winds are weak. Urban areas occupy about 0.5 per cent of the earth's land surface but hold more than half of the world's population, so the effect reaches a very large number of people. Its causes follow directly from the heat budget of a city surface.

• Changed land surfaces: The main cause is the modification of land surfaces. Dark roads and roofs absorb significantly more solar radiation than the suburban and rural surfaces around them, and their materials act as a reservoir of heat that warms the city into the night.
• Loss of evapotranspiration: With little vegetation and open water, a city loses the shade and evaporative cooling of trees, so more of the absorbed energy goes into warming the air.
• Urban canyons: Tall buildings offer many surfaces that reflect and absorb sunlight, and they block the wind that would otherwise carry heat away.
• Waste heat: Energy used in homes, industry and transport adds heat directly, a secondary cause.
• Air pollution: Many pollutants change the radiative properties of the air above a city and add to the heating.

Effects on Rainfall, Air Quality and Health, and Cooling Measures

Urban heat islands affect weather, health and the environment as well as comfort. Monthly rainfall is greater downwind of cities, partly because the extra heat drives stronger upward motion of air. Heat in city centres lengthens the growing season, worsens air quality by increasing the production of pollutants such as ozone, and lowers water quality, since warmer water running off into streams stresses their ecosystems. A warmer world makes the effect worse, and heatwaves are likely to affect half of the future global urban population, with harm to health and productivity.

The remedies work on the city's heat budget.

• Trees and green space: Planting trees and creating green infrastructure, including green roofs, restores shade and evaporative cooling.
• Cool roofs and light surfaces: Roofs painted white or given a reflective coating, and light-coloured concrete in place of black asphalt, raise the albedo of the city.

Sources

• NCERT: Fundamentals of Physical Geography, Class XI, Chapter 8, Solar Radiation, Heat Balance and Temperature (Reprint 2026‑27)
• NCERT: Fundamentals of Physical Geography, Class XI, Chapter 7, Composition and Structure of Atmosphere (Reprint 2026‑27)
• NCERT: Fundamentals of Physical Geography, Class XI, Chapter 9, Atmospheric Circulation and Weather Systems (Reprint 2026‑27)
• NCERT: Fundamentals of Physical Geography, Class XI, Chapter 11, World Climate and Climate Change (Reprint 2026‑27)
• NASA Earth Observatory: Climate and Earth's Energy Budget
• NASA Earth Observatory: Clouds and Radiation
• World Meteorological Organization: Earth's climate swings increasingly out of balance (State of the Global Climate 2025)
• World Meteorological Organization: WMO confirms 2025 was one of warmest years on record
• PIB (Ministry of Environment, Forest and Climate Change): Air Quality Index, 4 December 2023
• Wikipedia: Solar constant
• Wikipedia: Apsis
• Wikipedia: Diffuse sky radiation
• Wikipedia: Loo (wind)
• Wikipedia: Greenhouse effect
• Wikipedia: Albedo
• Wikipedia: Earth's energy budget
• Wikipedia: Cloud feedback
• Wikipedia: Specific heat capacity
• Wikipedia: Lapse rate
• Wikipedia: Inversion (meteorology)
• Wikipedia: Urban heat island
• UPSC: Civil Services Examination question papers