• Insolation, incoming solar radiation, is greatest at the equator and least at the poles
• Global atmospheric circulation transfers heat from the equator toward the poles
• Air flow on a non-rotating earth would generate one convection cell per hemisphere
• The Coriolis effect causes atmospheric circulation to be divided into three cells per hemisphere (Hadley, Ferrel, Polar Cells)
• Global cloud patterns are linked to the distribution of low and high pressure systems that separate the convection cells

olar radiation strikes the earth more directly at the equator and tropics than in polar regions. Radiation strikes the earth at a lower angle near the poles and the sun’s rays must therefore penetrate a greater thickness of atmosphere. Some of the solar radiation is scattered in the atmosphere and heat energy is lost near the poles as a result of scattering. Furthermore, the same amount of heat energy is distributed over a larger area near the poles than at the equator. Consequently, the atmosphere above the equator receives 2.5 times more insolation, incoming solar radiation, than the atmosphere above the poles. More heat is therefore transferred to the earth in the tropics than at the poles. Contrast in insolation at the poles and equator creates a heat gradient that causes warm air to be transferred toward the poles. Global air circulation patterns represent the planet’s attempt to move warm air toward the poles and cold air toward the equatorial region.

A Non-Rotating Earth
Global atmospheric circulation patterns would be simple if the earth did not rotate. Warm air would rise at the equator, forming one limb of a Hemisphere-scale convection cell that carried cold dense air to the tropics and warm, less dense air to the poles. Air flow in this idealized world would be driven by pressure differences between the equator and poles and would be meridional (parallel to longitude, north-south).

Rotating earth
Of course the earth does rotate once on its axis each day and the resulting Coriolis effect causes the meridional flow to be disrupted as winds are deflected to the right of their course in the northern hemisphere and to the left of their course in the southern hemisphere. Atmospheric circulation can be divided into three convection cells in each hemisphere. From equator to the poles these cells are the Hadley Cell, Ferrel Cell, and Polar Cell.

A model of atmospheric circulation showing the convection cells in each hemisphere and the low and high pressure systems and wind patterns that result from their interaction. Sun is assumed to be overhead at the equator. Earth has been rotated to better illustrate distribution of circulation cells.

Hadley Cell: Warm air converges on the equator and rises, forming a belt of low pressure (equatorial low). The humidity of the air increases as it cools during its ascent causing condensation and cloud formation. Precipitation follows as temperatures continue to decline with elevation, consequently, equatorial regions are characterized by ecosystems dependent upon heavy rainfall (e.g tropical jungles). This air then moves north or south toward the tropics.

A high-pressure zone, a subtropical high, of descending air is present between 20-35^o latitude in the northern and southern hemispheres. The descending air becomes warmer and its relative humidity decreases as elevation decreases, preventing condensation and resulting in clear skies over the tropics. Most of the descending air flows toward the equator, forming the last leg in a convection cell. These winds are deflected to the west in the Northern Hemisphere and to the east in the Southern Hemisphere creating the trade winds (northeast trades, southwest trades).

The image shows global cloud cover, sea surface temperatures and land surface temperatures for the March 24, 1998. Note the cloud cover over the equator (equatorial low) and clear skies over the tropics (subtropical high). A current version of the image can be viewed here. Image courtesy of the Space Science and Engineering Center at the University of Wisconsin, Madison.

Ferrel Cells: Mid-latitude cells in both hemispheres are termed the Ferrel Cells. Circulation in these cells results from the air flowing toward the poles from the subtropical highs which collides with cold air flowing from the Poles. The zone of convergence is the polar front, a zone of high pressure characterized by ascending air and cloud formation.

Polar Cells: Cold, dense air descends in a polar high-pressure system and moves toward the equator. The polar front is a zone of convergence where the surface winds from the Ferrel and Polar cells meet.

Bands of clouds form where condensation takes place above rising, cooling air at the equatorial low and the polar fronts. In contrast the skies are relatively clear over areas of descending, warming air such as the subtropical highs and the poles. The global distribution of temperature and precipitation is directly related to variations in incoming solar radiation and the atmospheric circulation patterns described above.