Atmospheric circulation is the global movement of the air, distributing the heat received from solar radiation from warmer to cooler regions. If the Earth did not rotate on its axis and had a smooth and uniform surface, hot air would rise at the equator and flow towards the poles. It would there cool and sink, creating a return flow to the equator along the surface of the Earth. There would be two large, uniformly rotating convection cells, one in the Northern Hemisphere and one in the southern. Instead of this simplified pattern, the earth’s atmospheric circulation is vastly more complex.
An overly simplified model of the Earth’s atmospheric patterns has one large convection cell in each hemisphere. In reality, there are three convection cells in each hemisphere. Heated, moist tropical air near the equator rises and flows away from the equator forming the Hadley Cell. Over the poles, cool, dry air descends, driving the Polar Cells. Ferrel Cells are more variable and are found between the Hadley and Polar Cells.
The rotation of the Earth, the tilt of its axis, surface features, ocean currents and local weather patterns all affect global atmospheric patterns. Instead of winds flowing in a straight line, the rotation of the Earth causes them to curve. The Coriolis Force deflects Northern Hemisphere winds to the right and Southern Hemisphere winds to the left. It contributes to the formation of the westerly winds in the mid-latitudes and the easterly winds in the tropical and polar zones. The Coriolis Force also produces the rotational winds around high and low-pressure cells.
Seasonal shifts in atmospheric circulation are caused by the tilt of the Earth’s axis. As the sun’s direct rays seasonally move north and south of the equator, circulation patterns alter. The features on the surface of the Earth also affect global atmospheric circulation. Larger land area in the Northern Hemisphere and the corresponding greater oceanic area in the Southern Hemisphere cause variations in the three convection cells in each hemisphere.
The many complex factors influencing atmospheric circulation made it difficult for humans to adequately model global air circulation patterns. It wasn’t until the 20th century that accurate atmospheric circulation models were produced using computers and satellite data. These models closely resembled the actual workings of the atmosphere, helping scientists better understand climate and weather patterns. Early advances in weather predictions using computer modeling evolved as much more realistic and complex models allowed more accurate predictions. Atmospheric circulation models are used to understand long-term climate changes in the past and predict the effects of changes in the future.