Atmospheric circulation and motion

Atmospheric circulation and motion

The general model relating to the movement of circulation of the atmosphereis still the tri-cellular model. This model helps explain differences in pressure belts, temperature and precipitation that exist across the globe.

This shows how energy is redistributed across the globe and ensures there is not a surplus at the equator and deficit at the Poles. There are three major cells present: Hadley, Ferrel and Polar. As shown on the two diagrams below, the boundaries coincide with particular latitudes. However, they do shift with the movement of the Sun along with the ITCZ.

Atmospheric circulation and motion

At the equator, trade winds meet and form the Intertropical Convergence Zone (ITCZ). Winds are light and known as the doldrums. Air is warm and unstable, having crossed warm oceans, and rises due to convection currents. As the air rises it cools and large cumulonimbus clouds develop. The pressure at the equator is low.

Eventually, the rising air has cooled to the same temperature as its surroundings and begins to spread out. It falls at approximately 30 degrees north and south of the equator - the descending limb of the Hadley cell. Pressure is high here.

Air is now either returned to the equator at ground level, or travels to the Poles as warm south-westerly winds. At approximately 60 degrees north the air rises creating an area of low pressure and the rising limbs of the Ferrel and Polar cells. It is here that the Polar front jet stream is located and depressions form.

Rossby waves are high altitude, fast moving westerly winds, which often follow an irregular path. The path that they take changes throughout seasons, as shown in the diagram below:

Atmospheric circulation and motion


NP = North Pole.

R = Ridge.

T= Trough.

Of greater importance, are jet streams that are found to exist within Rossby waves. They help in the rapid transfer of energy around the globe, as they are very fast, narrow bands of air that can reach speeds of over 200km per hour. Five jet streams exist, with three having significant importance:

Jet stream: Location: Characteristics:
Polar front 40 degrees North and South of the equator. Divides the Ferrel and polar cells. Gives wet or fine weather on Earth's surface, and is strongly associated with anticyclones and depressions.
Subtropical 25-30 degrees North and South of the equator. Divides the Hadley and Ferrel cells.
Easterly equatorial Equatorial regions. A seasonal jet stream linked with the Indian summer monsoon.

There are two ways air can move in the atmosphere - vertically and horizontally (winds). Winds occur due to differences in pressure, shown by isobars on a synoptic chart.

The Coriolis force

This relates to the apparent deflection of winds to the right in the northern hemisphere and the left in the southern hemisphere due to the spinning of the Earth.

Atmospheric circulation and motion

Pressure gradient

Explains the movement of air between areas of high and low pressure. Isobars show this phenomenon and the closer the isobars, the stronger the winds. Winds act to balance out differences in pressure, humidity and temperature.

Atmospheric circulation and motion