The climate we experience and the location of the world’s deserts and rainforests are controlled by how air moves around the globe, a process known as Global Circulation. It lets us understand how air moves across different parts of the world.
So what causes Global Circulation?
This is caused by different parts of the world heating up differently.
So our next question comes why do different parts of the world heat up differently? The Sun is our main source of heat. This heat travels to us through space in the form of radiation. This radiation is absorbed by clouds, atmospheric gases, and the Earth's surface. Some parts of the earth receive more of this radiation than others.
The main reason for this is the curvature of the earth. This curvature means that, at higher latitudes, the radiation from the Sun is spread over a much larger surface area while the same amount of energy is much more concentrated near the equator. The radiation also travels through a greater depth of atmosphere nearer the poles, in comparison to the equator. So more radiation is lost to scattering and absorption by gases and particles in the atmosphere.
The tilt of the earth also means that polar regions do not see daylight during the winter.
Here the earth is continuously sending out heat to space without any compensating incoming heat from the Sun.
These cold polar regions are where we see lots of snow and ice. Snow and ice and also thick cloud reflect a lot of the sun's radiation back into space.
The reflectivity or albedo of the underlying surface is an important factor in determining how much of the sun's radiation is used for heating the earth. The combined effect of all of these processes sets up a thermal gradient between the equator and the poles.
In fact, the global radiation balance is such that polewards of 40 degrees latitude the outgoing heat radiation from the earth exceeds the incoming radiation from the sun. Global circulation acts as a clever air conditioning system redistributing the heat, stopping the equator from becoming hotter and hotter and the poles from becoming colder and colder.
This circulation takes the form of three large atmospheric cells namely Hardley cell, Ferrel cell, and Polar cell which exist in both northern and southern hemispheres. Our next blog will talk about these cells in more detail.
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