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Climate and Weather: Information and Education

General Circulation of the Atmosphere

Figure 1

Figure 1, The Balance of Earth's Energy

The energy from the sun that comes in the form of electromagnetic rays, heats up the earth's surface. The earth radiates that energy in the form of heat and cools down. the process of heating and cooling is not even throughout the globe a equatorial and troical zones of the earthget more energy from the sun than the polar zones. This creates a zone of deficit due to uneven distribution of heat energy (Figure 1).

This uneven distribution of heat on the globe definitely starts a process of heat transformation to turn into a state for balancing the energy; otherwise tropic would have been hotter and hotter and poles cooler and cooler with time. The process of balancing the energy drives an oscillation in atmosphere and ocean which redistributes 60% and 40% of the heat through atmospheric winds and oceanic currents respectively.

Figure 2
Figure 2, Hadley Cell or Single Cell of
Atmospheric Circulation

The process of heat transformation was first proposed by George Hadley in 1700's as a single cell model (Figure 2). This atmospheric circulation can be described as warming up of air near equatorial region followed by rising of hot tropical air and replacement of denser and cooler air from poles. This would have been happening truly if Coriolis force did not exit. Coriolis force deflects the path of an object to the right in north hemisphere and left in south hemisphere due to earth's rotation (Figure 4). Due to the presence of Coriolis force, the tilted axis of Earth and also the more land surface area in northern hemisphere than the south made this process even more complicated. As a result, a theory of three cell circulation has been well accepted to describe the process of heat transformation in atmosphere and ocean. These three cells which are mainly used to describe the atmospheric energy transformation are: 1) Tropical Cell (Hadley Cell), 2) Mid-latitude Cell (Ferrel Cell) and 3) Polar Cell (Figure 3).

Figure 3
Figure 3, Earth's Rotation

Figure 4

Figure 4, Three Cell Atmospheric Circulation

Figure 5

Figure 5, The Three Cell General Circulation Model

  1. Tropical Cell (Hadley Cell): The intense solar radiation heats up the air in the equatorial region and the hot and light air goes up vertically and starts flowing towards poles in the upper atmosphere. This phenomenon forms a convection cell within tropical and subtropical regions. This also creates some other atmospheric phenomena such as Tropical Doldrum or Inter-Tropical Converzence Zone (ITCZ), Subtropical Jets, Subtropical Highs, presence of Trade winds etc.

  2. Ferrel Cell: Some of the diverging air near 30° N starts moving towards poles but due to the Coriolis force it gets deflected leftward in North hemisphere and start blowing as Westerlies (Figure 5). Near 60° N it gets cooler, gets condensed and precipitates. This creates the Midlatitude or Ferrel Cell.

  3. Polar Cell: The air that moves towards poles sinks at poles and creates Polar Highs and that result to wind blows as easterlies (as Coriolis force diverts the winds to the right in Northern Hemisphere). Near about 60° N, the cold easterlies confronts with subtropical warm air and the zone which is known as Polar Front acts as the main source of changing weather in the US as well as Jet Stream.