Atmospheric Convection: Hadley Cells

There is a second, larger scale effect that also plays a key role in the global distribution of precipitation and evaporation. Fundamentally, these patterns are also explained by the rise and fall, and cooling and warming of air masses – as is the case with the orographic effect – but in this case, their movement is a result of atmospheric convection rather than transport over topographic features.

As you have seen, there are regular climate and precipitation bands on the Earth – latitudes where most of the Earth's tropical and temperature rainforests, deserts, polar deserts (also known as tundra) tend to occur. This global pattern – along with prevailing global wind patterns and storm tracks, are driven by atmospheric convection.
It all starts with solar radiation. Because of the Earth's curvature, the tropics (between 23.5 ° N and S latitude) receive a larger flux of solar radiation per unit area on average than higher latitudes. Because the Earth's axis is tilted, during Northern hemisphere summer, the peak influx of solar radiation occurs at 23.5 ° N latitude. During the Southern hemisphere summer, the maximum occurs at 23.5 ° S. (Incidentally, these latitudes define the tropics of Cancer and Capricorn.) Annually, the highest flux of solar energy per unit area occurs at the equator, as shown below.

As a result, air around the equator becomes warmest. It holds quite a bit of water, too – based on the fact that, as you've seen above, warm air has a higher capacity to carry moisture.

Video Review

Take a few minutes to review the video below to help your understand Global Circulation a little better.