English · हिन्दी · తెలుగు · தமிழ் · മലയാളം · मराठी · ಕನ್ನಡ

The atmosphere, insolation and temperature

Climatology: the atmosphere, winds, weather systems and world climates · Part 1 / 4 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

What the air is made of, the layers of the atmosphere from the troposphere to the exosphere and the ozone layer, insolation and the factors that control it, how the atmosphere is heated and the greenhouse effect, the heat budget and albedo, the distribution of temperature and isotherms, and temperature inversion.

Weather and climate happen in a thin envelope of air wrapped around the planet, the atmosphere. Weather is the state of the air at a given place and time; climate is the average pattern of weather over a long period, usually thirty years or more. This chapter builds the subject in four steps: first the air itself and how the Sun heats it, then pressure and winds, then moisture and storms, and finally the climate types into which the world is divided. This first part explains what the atmosphere is made of, how it is layered, how sunlight warms the Earth, and why temperatures differ from place to place.

Dry air is a mixture of gases in remarkably constant proportions near the surface. By volume, nitrogen makes up about 78 per cent and oxygen about 21 per cent; argon, an inert gas, accounts for most of the remaining one per cent. Carbon dioxide forms only a tiny share, a little over 0.04 per cent according to NOAA's measurements in the mid-2020s, but it matters far more than its share suggests, because it absorbs heat radiated by the Earth and is used by plants in photosynthesis. Its concentration has been rising steadily since the Industrial Revolution, mainly because of the burning of fossil fuels. About half of the atmosphere's mass lies below roughly 5.5 km, so the air thins quickly with height, which is why climbers on high peaks need extra oxygen.

Water vapour may be almost absent in cold, dry desert air but can make up as much as about four per cent of the volume of warm, humid tropical air. It is the raw material of clouds, rain and snow, it absorbs heat radiated by the Earth, and it carries energy from one place to another when it evaporates and later condenses. The air also holds tiny solid and liquid particles, called aerosols: dust, sea salt, smoke, pollen and soot. These particles act as nuclei around which water vapour condenses into droplets, so without them clouds would form far less easily. They also scatter sunlight, which is one reason the sky turns red and orange at sunrise and sunset.

Scientists divide the atmosphere into layers by the way temperature changes with height. The lowest layer, the troposphere, holds most of the air's mass and nearly all its water vapour, and almost all weather happens in it. Its thickness varies: about 18 km over the equator, where strong heating drives tall convection currents, but only about 8 km over the poles. Within it, temperature generally falls with height at an average of about 6.5 °C per kilometre, known as the normal lapse rate, because the air is heated mainly from the ground below. This is why hill stations such as Shimla or Ooty are cooler than the plains around them. The top of the troposphere is the tropopause, a boundary zone where the fall in temperature stops.

Above the tropopause lies the stratosphere, extending to about 50 km. Here temperature stays steady at first and then rises with height, the opposite of the troposphere. The reason is the ozone layer, a region within the stratosphere where ozone, a form of oxygen with three atoms in each molecule, is most concentrated. Ozone absorbs most of the Sun's harmful ultraviolet radiation, and the energy it absorbs warms the upper stratosphere; without this shield, far more ultraviolet light would reach the ground, damaging skin, eyes and crops. Because the stratosphere is calm and almost free of clouds and water vapour, long-distance jet aircraft often fly in its lowest part. From the 1980s, scientists recorded a severe seasonal thinning of ozone over Antarctica, the 'ozone hole', caused mainly by chlorine from chlorofluorocarbons; the Montreal Protocol of 1987 set out to phase out these chemicals.

Above the stratosphere, the mesosphere stretches to roughly 80–85 km. Temperature falls again with height and reaches the lowest values anywhere in the atmosphere, around minus 90 °C, near its top. Most meteors burn up in this layer, producing 'shooting stars'. Higher still is the thermosphere, where the thin air absorbs high-energy solar radiation and temperatures climb very steeply, although the air is so sparse that it would not feel hot. Within this region lies the ionosphere, where solar radiation strips electrons from atoms to create electrically charged particles; these reflect certain radio waves back to Earth, allowing long-distance radio communication, and they glow as auroras near the poles. The International Space Station orbits in the thermosphere. The outermost zone, the exosphere, has no sharp upper edge and fades gradually into space; the lightest gases, hydrogen and helium, escape from here.

The energy that drives weather comes from the Sun. The solar energy received at the Earth's surface is called insolation, short for incoming solar radiation, and it arrives mostly as short-wave radiation. The amount a place receives depends on several factors. The most important is the angle of the Sun's rays: when the Sun is high, a beam of sunlight falls on a small area and passes through less air, so it heats more strongly than a slanting beam that is spread thinly. The length of the day matters too. Clouds, dust and water vapour reduce what reaches the ground, and on slopes the direction a hillside faces makes a difference. Because cloud cover near the equator is heavy, the greatest insolation at the surface is received not at the equator but over the clear-skied subtropical deserts.

Most sunlight passes through the air and is absorbed by land and sea, which then heat the air above them. The air is warmed in four main ways. Conduction passes heat from the warm ground to the air in direct contact with it. Convection carries that heated air upward in rising currents, mainly within the troposphere. Advection moves heat horizontally with the wind; the hot, dry Loo that blows across north India in May and June is an example. Finally, the warmed Earth radiates energy back as long-wave infrared radiation, and the atmosphere absorbs much of it; this is the main reason the air is heated from below. Water vapour, carbon dioxide, methane and nitrous oxide absorb this outgoing heat and send part of it back down, a process called the greenhouse effect. Without it, the Earth's average surface temperature would be about minus 18 °C instead of about 15 °C.

Over a year the energy the Earth absorbs is balanced by the energy it sends back to space. This balance is called the heat budget. NASA's estimates put it roughly as follows: of the sunlight arriving at the top of the atmosphere, about 30 per cent is reflected straight back to space by clouds, the air and bright surfaces such as ice and deserts; about a quarter is absorbed by the atmosphere; and close to half is absorbed at the surface. The share reflected is called the albedo. Fresh snow and ice reflect most of the light that falls on them, while forests and the open ocean reflect very little, so melting ice exposes darker ground and water and makes warming faster. The balance is not even across latitudes: low latitudes receive more energy than they radiate away, while high latitudes lose more than they receive. Winds and ocean currents carry the surplus heat poleward.

Surface temperatures depend on several controls. Latitude comes first, because it sets the angle of the Sun and the length of day. Altitude follows from the lapse rate, so high plateaus are cooler than lowlands at the same latitude. Distance from the sea matters because water heats and cools far more slowly than land: coastal places have mild, maritime climates with a small difference between summer and winter, while the interiors of large continents have continental climates with hot summers and very cold winters. Ocean currents and prevailing winds carry warmth or cold from elsewhere. Temperature patterns are shown on maps by an isotherm, a line joining places of equal temperature. In January, isotherms in the Northern Hemisphere bend towards the equator over the cold continents and poleward over the warmer oceans; in July the pattern reverses. The largest annual range of temperature is found in the interior of north-eastern Asia, in Siberia.

Normally air grows colder with height, but sometimes a layer of warmer air sits above colder air near the ground. This reversal is called temperature inversion. It forms most easily on long winter nights with clear skies and calm, dry air: the ground loses heat rapidly by radiation, chills the air just above it, and there is no wind to mix the layers. In hilly areas, cold, dense air also slides down the slopes at night and collects in valley floors, a process called air drainage, so valley bottoms can be frosty while the slopes above stay milder; frost-sensitive crops are therefore often grown on slopes rather than in hollows. An inversion acts like a lid over the cold air. Fog forms easily beneath it, and smoke and pollutants are trapped close to the ground, which is one reason winter smog in the cities of north India is so severe.

The pieces of this part fit into one chain. The Sun supplies short-wave insolation, most of which passes through the atmosphere and heats the ground; the ground heats the air from below by conduction, convection and long-wave radiation, which is why temperature falls with height through the troposphere and why the greenhouse effect keeps the planet habitable. The heat budget balances over the globe but not at each latitude, so the tropics run a surplus and the poles a deficit, and the atmosphere and oceans must move heat between them. That movement of air, driven by differences in temperature and therefore in pressure, is the subject of the next part: pressure belts, winds and jet streams.

Exam traps

• The troposphere is thickest over the equator (about 18 km) and thinnest over the poles (about 8 km), not the other way round.
• Temperature falls with height in the troposphere and mesosphere but rises in the stratosphere (because of ozone) and the thermosphere.
• The mesosphere, not the stratosphere or thermosphere, has the lowest temperatures in the atmosphere; meteors burn up there.
• The ozone layer lies in the stratosphere; radio waves are reflected by the ionosphere in the thermosphere.
• Maximum insolation at the surface is over the subtropical deserts, not at the equator, because the equator is cloudy.
• The atmosphere is heated mainly from below by terrestrial (long-wave) radiation, not directly by incoming short-wave sunlight.
• Advection is horizontal transfer of heat by wind (the Loo); convection is vertical.
• Temperature inversion needs long, clear, calm winter nights; cloudy or windy nights prevent it.

Recap

• Dry air: nitrogen about 78%, oxygen about 21%, argon most of the rest; carbon dioxide a little over 0.04% (NOAA, mid-2020s) and rising.
• Water vapour (up to about 4%) and aerosols vary; aerosols act as condensation nuclei.
• Troposphere: weather layer, about 18 km at the equator and 8 km at the poles; temperature falls about 6.5 °C per km.
• Stratosphere: ozone layer absorbs ultraviolet; temperature rises; Montreal Protocol (1987) targets ozone-depleting chemicals.
• Mesosphere coldest (meteors); thermosphere with the ionosphere (radio waves, auroras); exosphere fades into space.
• Insolation depends on the Sun's angle, day length, transparency of the air and slope; highest over subtropical deserts.
• Air is heated by conduction, convection, advection and long-wave radiation; greenhouse gases keep the surface near 15 °C instead of −18 °C.
• Heat budget: about 30% reflected (albedo), about a quarter absorbed by air, about half by the surface; surplus at low latitudes moves poleward.
• Isotherms bend equatorward over continents in January in the Northern Hemisphere; largest annual range in Siberia.
• Inversion on clear, calm winter nights; cold air drains into valleys; fog and smog are trapped beneath.

Practice questions

Sources

• NOAA National Weather Service, JetStream online school: the atmosphere, its composition and layers, temperature inversions (US government) — https://www.noaa.gov/jetstream/atmosphere
• NASA Earth Observatory: Climate and Earth's Energy Budget; incoming sunlight, albedo and the greenhouse effect (US government) — https://earthobservatory.nasa.gov/features/EnergyBalance
• India Meteorological Department, Ministry of Earth Sciences: meteorological glossary and public material on temperature, heat waves, fog and cold waves — https://mausam.imd.gov.in
• Montreal Protocol on Substances that Deplete the Ozone Layer (1987), UNEP Ozone Secretariat — https://ozone.unep.org/treaties/montreal-protocol
• NOAA Global Monitoring Laboratory: trends in atmospheric carbon dioxide (Mauna Loa record) — https://gml.noaa.gov/ccgg/trends/