Insolation: the sunlight the Earth receives
Insolation (incoming solar radiation) is the energy from the Sun that reaches the Earth. It comes as short waves. The Earth receives only a tiny part of the Sun's energy, but it runs all weather and life.
What changes the amount of insolation?
- Angle of the Sun's rays: straight rays heat a small area strongly; slanting rays spread out and pass through more air.
- Length of the day: longer days give more heat (summer).
- Rotation on a tilted axis: the axis is tilted 66½° to the orbit plane, so different places get different amounts in different seasons.
- Transparency of the air: clouds, dust and water vapour block and scatter some sunlight.
- Land and sea layout: land heats fast; water heats slowly.
The Earth is nearest the Sun (perihelion) on about 3 January and farthest (aphelion) on about 4 July, but this makes only a small difference. Insolation is highest over subtropical deserts (few clouds) and lowest at the poles.
Passing through the atmosphere
Air is almost see-through for short waves. Tiny particles scatter the blue part of light, so the sky looks blue. At sunrise and sunset light passes through more air and the sky turns red.
Heating and cooling of the atmosphere
- Conduction: the warm ground heats the air touching it.
- Convection: warm air rises, cool air comes down; heat goes upward. Works only in the troposphere.
- Advection: heat carried sideways by wind. In north India the hot summer wind called loo is advection.
At night the ground loses heat and cools the air near it. That is why nights are cooler, and clear nights are cooler than cloudy ones.
Terrestrial radiation
The warm Earth gives off heat as long waves. This is terrestrial radiation. Air lets short waves pass but absorbs long waves, mainly by carbon dioxide and water vapour (greenhouse gases). So the air is heated indirectly, from below, by the Earth, not directly by the Sun. This is why it is colder higher up.
Heat budget of the Earth
Suppose 100 units of sunlight reach the top of the atmosphere:
- 35 units are reflected back to space before heating anything (27 by clouds, 6 by scattering, 2 by snow and ice). This reflected share is the Earth's albedo.
- 14 units are absorbed by the atmosphere.
- 51 units are absorbed by the Earth's surface.
The surface sends its 51 units back as terrestrial radiation: 17 go straight to space and 34 are absorbed by the air (6 directly, 9 by convection, 19 as latent heat of condensation). The air now has 14 + 34 = 48 units and radiates them to space. Out: 17 + 48 = 65 = in (65). So the average temperature stays the same.
Why some places have extra heat
Between about 40° N and 40° S the Earth gains more heat than it loses (surplus). Near the poles it loses more than it gains (deficit). Winds and ocean currents carry the extra heat towards the poles, so the tropics do not keep heating and the poles do not keep freezing.
Distribution of temperature
Things that control temperature at a place:
- Latitude: temperature falls from the equator to the poles.
- Altitude: it falls about 6.5 °C per 1,000 m (normal lapse rate).
- Distance from the sea: the sea keeps coasts mild; places far inland have very hot summers and very cold winters (continentality).
- Air masses and ocean currents: warm currents make coasts warmer, cold currents make them cooler.
- Local features like slope and shade.
Isotherms are lines joining places of equal temperature. On world maps they run roughly east–west, following latitude, but bend over land and sea. In January they bend towards the equator over northern continents (land is colder) and towards the pole over oceans; in July it is the opposite. The highest temperatures are over subtropical lands in the northern summer; the coldest are in Siberia and Antarctica. The yearly range is small near the equator and very large in the interior of big continents.
Inversion of temperature
Normally air gets cooler with height. In an inversion the opposite happens: a layer of cold air lies near the ground with warmer air above it.
Best conditions: long winter nights, clear sky, calm air, dry air, snow-covered ground. The ground cools fast and chills the air touching it.
Air drainage: in hills, cold heavy air slides down slopes at night and collects in valleys. Valley floors can get frost while the slopes stay warmer. Effects: fog, smog trapped near the ground, frost damage to crops, poor visibility for flights.
Try it at home
Place two same cups of water in the sun: one on a white sheet, one on a black sheet. After 30 minutes, touch or measure both. The black sheet absorbs more (low albedo), the white one reflects more (high albedo). Now shine a torch straight down on paper, then at a slant: the slanting patch is larger and dimmer, just like insolation near the poles.
Key formulas and definitions
- Insolation = incoming solar radiation (short waves); terrestrial radiation = outgoing long waves
- Heat budget: 100 in = 35 reflected (albedo) + 14 absorbed by air + 51 absorbed by ground
- Ground gives back 51 = 17 to space + 34 to air; air sends 48 (14 + 34) to space; 35 + 17 + 48 = 100
- Normal lapse rate: −6.5 °C per 1,000 m
- Heat transfer: conduction, convection (vertical), advection (horizontal), radiation
- Inversion: temperature increases with height (cold air below, warm air above)
Worked examples
1. Why is the midday sun hotter than the evening sun?
At midday rays fall almost straight: the same energy falls on a small area and passes through less air. In the evening rays are slanting, spread over a larger area and lose more energy in the thick air they cross.
2. Fill the heat budget: of 100 units, 35 are reflected and 14 absorbed by air. How many reach and heat the ground?
100 − 35 − 14 = 51 units.
3. The ground radiates 51 units; 17 escape to space. How much does the atmosphere absorb? What is the total the atmosphere sends to space?
Absorbed = 51 − 17 = 34 units. Atmosphere total = 14 (direct sunlight) + 34 = 48 units, all radiated to space.
4. A town at sea level is 28 °C. A hill station 2,200 m up is 12 °C on the same day. Does this match the normal lapse rate?
Expected fall = 6.5 × 2.2 = 14.3 °C, so expected temperature ≈ 13.7 °C. Actual 12 °C is close; small differences come from slope, wind and shade.
5. Why are subtropical deserts hotter than the equator, even though the equator gets the most direct sun?
The equator has thick clouds and heavy rain that block sunlight. Subtropical deserts have clear skies, so more insolation reaches the ground.
Common mistakes
- Thinking the Sun heats the air directly. The air is heated mostly by the ground (terrestrial radiation).
- Thinking the Earth is hotter in July because it is nearer the Sun. It is actually farthest in July; seasons come from the tilt.
- Adding the heat budget wrongly: the ground's 51 units are not lost — they go back out as 17 + 34.
- Calling any cold morning an inversion. An inversion means temperature increases with height.