Absorbing and emitting radiation

1. Every object absorbs some of the radiation that falls on it and emits radiation of its own at the same time.
2. Absorbing radiation transfers energy to the object's thermal store, and emitting radiation transfers energy away from it.
3. The temperature of the object depends on the balance between the rate at which it absorbs radiation and the rate at which it emits radiation.
4. If an object absorbs radiation faster than it emits it, its thermal store increases and its temperature rises.
5. If an object emits radiation faster than it absorbs it, its thermal store decreases and its temperature falls.
6. If the two rates are equal, there is no overall change in the thermal store, so the temperature stays constant.
7. A constant temperature does not mean that radiation has stopped, because the object is still absorbing and emitting, only at equal rates.

Reaching equilibrium

1. A hotter object emits radiation at a greater rate, so the rate of emission depends on the object's temperature.
2. This makes the balance self-correcting.
3. When an object absorbs more than it emits, it warms up, so it emits faster and faster until emission catches up with absorption.
4. When an object emits more than it absorbs, it cools down, so it emits more and more slowly until emission falls to match absorption.
5. Either way, the object settles at an equilibrium temperature where the two rates are equal.
6. Anything that changes the rate of absorption or the rate of emission upsets the balance, so the object warms or cools until it reaches a new equilibrium temperature.

Everyday examples

1. A mug of hot tea in a room emits radiation faster than it absorbs radiation from the cooler walls around it, so it cools.
2. As the tea cools, it emits radiation more slowly, so it cools more and more slowly until it reaches room temperature and the two rates balance.
3. A car parked in sunshine absorbs radiation from the Sun faster than it emits radiation, so its temperature rises until its own emission balances the sunlight it absorbs.
4. A dark-coloured car reaches a higher equilibrium temperature than a white car in the same sunshine, because a dark, matt surface absorbs a greater share of the radiation falling on it.
5. On a clear night, the ground emits infrared into space but receives little radiation back, so it cools quickly and frost can form.
6. On a cloudy night, the clouds absorb infrared from the ground and emit some of it back down, so the ground cools more slowly.

Radiation reaching the Earth

1. The Earth is heated by radiation from the Sun, which arrives mainly as visible light, together with some infrared and ultraviolet.
2. The amount of radiation reaching each square metre of the Earth depends on how much radiation the Sun emits and on the distance between the Earth and the Sun.
3. About $30\%$ of the incoming sunlight is reflected straight back into space by clouds, ice, snow and the atmosphere, so it never heats the Earth.
4. Light-coloured surfaces such as ice and snow have a high albedo, so they reflect most of the radiation falling on them.
5. Dark surfaces such as oceans and forests have a low albedo, so they absorb most of the radiation falling on them.
6. When ice melts, the darker land or sea underneath is uncovered, so more radiation is absorbed and the Earth warms further.
7. The atmosphere also changes the Sun's radiation as it passes through, because ozone high in the atmosphere absorbs much of the harmful ultraviolet, while most visible light passes through to the surface.

Radiation leaving the Earth

1. The Earth's surface is much cooler than the Sun, so it emits radiation at much longer wavelengths, mainly in the infrared.
2. Visible light from the Sun passes through the atmosphere easily, but some gases in the atmosphere absorb the longer-wavelength infrared emitted by the surface.
3. These greenhouse gases, such as carbon dioxide, methane and water vapour, then emit infrared in all directions, including back down towards the surface.
4. Less energy escapes into space each second than would escape without these gases, so the surface warms until it emits enough radiation to restore the balance.
5. This greenhouse effect keeps the Earth's average surface temperature at about $15{}^{\circ }C$, rather than the $-18{}^{\circ }C$ it would be without any greenhouse gases.
6. Burning fossil fuels has increased the amount of carbon dioxide in the atmosphere, so less infrared escapes and the Earth now absorbs slightly more energy each second than it emits.
7. As a result, the Earth's average temperature is rising towards a new, higher equilibrium temperature.