The Earth's Heat Budget
Solar Insolation - The primary source of energy to drive our global climate system (including atmospheric and, to a lesser extent, oceanic circulation) is the heat we receive from the Sun, termed solar insolation. The spectrum of light which comes from the Sun is shown above. The spectrum of light in the atmosphere and at the Earth's surface is shown below. Insolation arrives at the edge of our atmosphere primarily as short wavelength radiation (179 kcal/cm2/yr). The amount of insolation which reaches the Earth's surface depends on site latitude and season. Imagine a disk in front of the Earth that is just big enough to shield all solar radiation from the Earth. Each square meter of the disk surface will receive the same amount of insolation. Regions of the Earth's surface near the equator are almost parallel to the disk and will receive about the same amount of insolation/m2 if we remove the disk, but regions near the poles will receive much less insolation/m2 because the surface is at a large angle to the disk. Also, the sunlight has to go through more atmosphere to reach the poles. Both effects lead to much lower insolation at the poles versus the equator. Transfer of Heat in Atmosphere
Part of the incoming insolation is absorbed by the atmosphere and the rest is absorbed by the Earth's surface or re-radiated back into the atmosphere. Part of re-radiated heat and other heat in atmosphere is re-absorbed by Greenhouse gases. Ultimately, the amount of heat that leaves the atmosphere must match that which enters, otherwise the planet's surface would rise in temperature and cook us up. (Geologic history tells us that has not happened.) The temperature of the atmosphere critically depends on how much energy is absorbed by atmospheric gases and how long that heat is held before it is re-radiated out into outer space.