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, the 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: Global Atmospheric Circulation (2:24)
Take a few minutes to review the video below to help you understand Global Circulation a little better.
Global Atmospheric Circulation
Click here for a transcript of the Global Atmospheric Circulation Video
In this animation, we're going to look at global wind patterns and talk about the reasons why the air circulates the way it does and also patterns of rising and sinking air and how that relates to precipitation. The engine that drives it all, I guess you could say, is the intense heating by the Sun that occurs only in the equator areas where the sun is shining is at a very high angle of incidence and this hot air near the equator being less dense Rises upward. It rises up, going to move toward the poles and then it gradually sinks at about 30 degrees north and south latitude. So we create these big spinning circles of air that we call the Hadley cells near the equator where the air is rising it loses its ability to hold moisture and you get a band of high rainfall and low pressure because there's air leaving the equator where the air sinks. In these, it belts at around 30 degrees north and south you get high pressure sinking air which creates areas of clear skies and desert climates now as this air circulates and tries to flow back toward the equator along the surface of the earth or as some of it heads toward the North Pole or toward the South Pole. The Coriolis effect, the spin of the earth, causes it to bend and turn and it's going to create the too big wind belts that prevail on our earth two out of three the trade winds north-northeast trade winds and southeast trade winds and then the prevailing westerlies. Now these winds curve the way they do because of the Coriolis effect the winds curve to the right of their path north of the equator, they curve to the left of their past south of the equator, and they end up flowing to the from east to west or from west to east. Now the other big factor is what's happening at the poles. At the poles the air is cold and the cold air wants to sink and as that cold polar air sinks it heads toward the equator and it bumps into this air heading toward the pole here and toward the South Pole here and it creates an area of rising air and again rising air produces high precipitation belts at about 60 degrees north and about 60 degrees south latitude. At the polls themselves, the precipitation is quite modest because the air is sinking and that creates low precipitation.
Credit: Keith Meldahl