What happens when solar radiation enters the atmosphere?
First, let us look what happens to the sunlight.
1. The Sun is the source of all radiation and energy coming to the Earth from space.
2. Part of the sunlight reaches the Earth's surface - the forests, oceans, deserts, savannah, cities, ice and snow.
3. The Earth's surface doesn't take up (absorb) all the sunlight, but sends (reflects) a certain part of it directly back into space. Very light coloured surfaces (e.g. ice and snow) are excellent reflectors.
4. Reflection doesn't just occur at the Earth's surface. Some light is reflected back into space by the top of the clouds and by particles in the air.
5. Absorption of sunlight doesn't only take place at the surface. Gas molecules and particles in the air also absorb sunlight.
The sunlight which reaches the Earth, warms its surface. The Earth sends this warmth back into space as infra-red heat radiation.
What happens to this heat radiation?
6. The warm surface of the Earth is a source of infra-red heat radiation to space.
7. A portion of this energy is used to evaporate water (think about a kettle - energy in the form of electricity is used to heat the liquid water up and in the process some is converted into water vapour or steam).
8. A small fraction of the infrared radiation goes directly back into space.
9. Clouds not only reflect sunlight, they also absorb and re-emit heat radiation from the Earth. A cloudy sky keeps the Earth warm, like a blanket.
10. Particles and gases in the air absorb infrared heat radiation. The gases are called greenhouse gases. They trap the heat near the ground.
We must take all these processes into account if we want to understand our climate. But why do we call it a greenhouse effect?