How the air is heated

Why does the sun send short waves and the Earth long ones?

Short and long waves

Tap Next. In the chart, wavelength runs across and energy up; the two curves are not drawn to the same height.

1. Everything radiates Every object gives out energy as electromagnetic waves (radiation): the sun, a hot stove, the ground, even you. The hotter it is, the more it gives out. An object that is a perfect radiator is called a black body.
2. The sun: short waves The sun's surface is about 6000 °C, so its radiation is strongest at short wavelengths, in visible light.
3. What sunlight is made of Visible light (violet to red, VIBGYOR) is 45% of it. Invisible infra-red, beyond the red end, is 46%. Ultraviolet, beyond the violet end, is 9%. All of it counts as short-wave (solar) radiation.
4. The Earth: long waves The Earth, at about 15 °C, gives out much longer waves: invisible infra-red. This is terrestrial radiation. At night, with no sunshine, it is called nocturnal radiation. The warmer the Earth, the stronger it is.
5. Hotter means much stronger The Stefan-Boltzmann law: the total energy given out goes with the fourth power of the absolute temperature (in kelvin). So doubling the temperature gives 2 × 2 × 2 × 2 = 16 times the energy. Hot bodies radiate very strongly.
6. Hotter means shorter waves Wien's law: the wavelength of the strongest radiation gets shorter as the absolute temperature rises (λmax ∝ 1/T). Hot bodies send short waves, cool ones long waves. Planck's law gives the shape of each curve. E ∝ T⁴. λmax ∝ 1/T. Sun: short wave. Earth: long wave.

Memory hook: Hot = short and strong. Cool = long and weak.

Takeaway: Everything gives out radiation. A hotter body gives out far more (energy goes with T⁴: the Stefan-Boltzmann law) and at shorter wavelengths (Wien's law). So the sun sends short waves (46% infra-red, 45% visible, 9% UV), and the Earth sends long, invisible infra-red waves.

Why is midday sun hotter than evening sun?

Insolation

The same four rays of sunlight, two sun heights.

1. Sun overhead When the sun is high, a beam of sunlight lands on a small patch: lots of energy per square metre.
2. Sun low in the sky The same beam arriving at a low angle is spread over a much larger area, so each square metre gets less.
3. So where and when? Insolation is greatest when the sun is overhead and least at sunrise and sunset. Over the year the tropics (between the Tropic of Cancer and the Tropic of Capricorn) get the most, the poles the least.

Takeaway: Insolation is the total solar radiation received over an area. The more slanting the rays, the more ground the same energy is spread over: most when the sun is overhead (and in the tropics), least at sunrise, sunset and the poles.

What happens to 100 units of sunlight?

The heat budget

Follow 100 units of sunshine. The bar at the bottom keeps count. Tap Next.

1. 100 units arrive Picture all the sunlight reaching the top of the atmosphere as 100 units. Counting in units makes the sharing easy to follow. Every unit ends up either bounced back to space or soaked up by the air or the ground.
2. 30 go straight back Air scatters 6 units back to space, clouds reflect 20 and the ground reflects 4. These 30 units warm nothing. This share is the Earth's albedo, its reflecting power. Albedo = reflected radiation ÷ incoming radiation.
3. 19 warm the atmosphere On the way down, water vapour, dust and ozone absorb 16 units, and clouds absorb 3 more. So the air takes in only about a fifth of the sunshine directly. Most of its heat comes later, from the warm ground below.
4. 51 reach and warm the ground The rest, 51 units, is soaked up by the Earth's surface. On a clear day about 5/6 of the sunshine reaches the ground, which reflects about 10% of it. Fresh snow reflects about 80%.
5. In balance The Earth's average temperature hardly changes from year to year. So all the energy it takes in must go back out to space, as long-wave radiation. What comes in equals what goes out: the budget balances. Albedo = reflecting power ≈ 30%. Snow reflects about 80%.

Memory hook: 30 back, 19 in the air, 51 in the ground.

Takeaway: Of every 100 units of sunlight, 30 bounce straight back to space (the albedo): 6 off the air, 20 off clouds and 4 off the ground. 19 are absorbed in the atmosphere (16 by water vapour, dust and ozone, 3 by clouds), and 51 by the surface.

How does heat get from the ground into the air?

How heat moves

Six ways, all at work in the troposphere.

1. Conduction Molecules pass heat on by touch. Only important in the air right against the ground.
2. Convection: free and forced Hot air rises and carries its heat up. More than 70% of the Earth is water, so convection matters everywhere. Free convection is started by strong sunshine heating the ground; forced convection happens when the wind is pushed up over hills.
3. Radiation Every body radiates at its temperature. Radiation needs no medium and does not heat the air it passes through: sunlight heats the ground directly. It is the biggest player in heat transfer.
4. Advection and turbulence Advection moves heat sideways with the wind (a cold wave reaching Delhi on north-westerlies). Turbulence, the irregular eddies of the air, mixes heat around.
5. Latent heat Heat taken in by evaporation (and melting) at the surface is released higher up when the vapour condenses or freezes. In the stratosphere none of these work: there, only ozone absorbing short-wave sunlight heats the air.

Takeaway: Conduction (very near the ground), convection (free, from heating; forced, by hills), radiation, advection (sideways by wind), turbulence, and latent heat. In the stratosphere only the short-wave radiation absorbed by ozone heats the air.

Which way is the heat travelling?

How heat moves

Tap a case, then the way heat moves in it.

• Sunshine warming the ground through empty space
• The lowest few centimetres of air warmed by hot tarmac
• Hot afternoon bubbles rising over Rajasthan
• Air forced up a mountain slope
• A cold wave reaching Delhi on north-westerly winds
• Water evaporating from the Bay of Bengal, then condensing in a cloud
• The Earth cooling on a clear night
• The stratosphere warming up

Takeaway: Radiation needs no medium. Conduction works by contact, so only right at the ground. Convection lifts warm air; advection carries it sideways; latent heat moves with water vapour.