Atmospheric circulation definition

Atmospheric circulation is referred to as the large scale movement of air at levels of the atmosphere by which heat is circulated on the surface of the Earth; this is also referred to as the different amount of insolation absorbed by the earth which causes differential heating of the earth and its atmosphere.

A change in temperature produced accounts for the density differences in the air. As the air is heated it expands and when compressed it gets cool, thus leading to changes in the atmospheric pressure. Whenever this happens, the result will be a movement of air from high pressure to low pressure setting the air in three-dimensional motion on a global scale. However, the air that moves in horizontal motion is called wind. Atmospheric pressure also determines when the air will rises or sinks. The wind circulates the heat and moisture across the planet, thereby, maintaining a constant temperature for the planet as a whole. The vertical rising of moist air cools it down to form the clouds and bring precipitation. There is, therefore a close relationship between winds and pressure, and the idea of pressure variations is a prerequisite to understanding the air in motion.

Causes of atmospheric circulation

We understand that the air pressure is not evenly distributed in the atmosphere and air attempts to balance this unevenness. Hence, it flows from high-pressure areas to low-pressure areas. The horizontal flow of air in response to a difference in pressure is referred to as wind while vertical or nearly vertical rising air is called an air current. Both winds and air currents from the system of circulation in the atmosphere. Some of the factors that affect atmospheric circulation are:

Pressure Gradient

The existence of pressure differentials in the atmosphere is the immediate primary force causing air movement. A pressure gradient is the rate of change of pressure with respect to distance. The pressure gradient force always acts down the pressure gradient, attempting to cause the general circulation of air away from high pressure towards low-pressure areas. The force exerted is proportional to the steepness of the gradient. The gentler the pressure gradient, the slower the speed of the wind and vice-versa. If alone this force is exerted to the air, the wind would have direction perpendicular to the isobars. However, there are other forces such as Coriolis force also which usually allow wind to flow more nearly parallel to the isobars.

Coriolis Force

The winds dont blow across the isobars at right angles as the pressure gradient directs them. They deflect from their main paths. One of the most potent influences on wind direction is the deflection caused by the earths rotation on its axis. This deflection is always to the right of the direction of motion in the northern hemisphere and to the left in the southern hemisphere. This influence is known as theCoriolis force. The degree of the deflecting force varies with the speed of the moving air and with latitude. The effect of the earth rotation is dependent on how fast the wind can be. Similarly, the rate of deflection increases with the increasing distance from the Equator, this is because the Coriolis force is zero at the Equator and reaches the maximum at Poles. It must be noted that it is an apparent or relative deflection. When you view from outer space, the moving objects across the face of the earth do not at all appear to be deflected. In relation to star positions, they would travel in a straight line, while the earth rotates beneath them. The changes affect all freely moving objects such as air, rockets, projectiles, ocean currents, etc. Thus, it is not actually any force. But it is simplest to accept that deflection is as a result of a force. Wind always strives to readjust itself until it obtains the new geostrophic speed.

Centripetal Force

This force happens when the isobars are curved, just like in cyclones. The fact that air is following a curved path means that in addition to the pressure gradient and the Coriolis force, a third force is acting centripetally, pulling air inwards. The wind which is in balance with these three forces is known as the gradient wind.

Frictional Force

This force lessens the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1-3 km. Over the sea surface, the friction is small. By reducing the speed of the wind, it makes the Coriolis force to become weak. This allows the pressure gradient to assert its greater strength by causing the air to flow more towards low pressure. Thus, the usual situation is that surface wind moves at a slight angle to the isobars.

Geostrophic Wind

The velocity and direction of the wind are the net results of the wind generating forces. The winds in the upper atmosphere, 2-3 km above the surface are free from the frictional effect of the surface and are controlled by the pressure gradient and the Coriolis force. At such height in the free atmosphere, winds generally blow at right angles to the pressure gradient; this indicates that the pressure gradient force is exactly balanced by the Coriolis force acting in a diametrically opposite direction. This sort of air motion is known as the geostrophic wind Not all winds are exactly geostrophic. As pressure pattern changes, the balance is upset, but the wind always strives to readjust itself until it obtains the new geostrophic speed.

General Circulation of the Atmosphere

General circulation of the atmosphere is the pattern of the movement of the planetary winds. The general circulation of the atmosphere also sets in motion the ocean water circulation which influences the earths climate. The pattern of planetary winds largely depends on :

1. Latitudinal variation of atmospheric heating
2. The emergence of pressure belts
3. The migration of belts following the apparent path of the sun
4. The distribution of continents and oceans
5. The rotation of the earth

Types of atmospheric circulation

The wind is the result of the pressure gradient which is largely caused by differential heating of the earth. Winds in the atmosphere are neither unidirectional nor have the same pattern as we move above the atmosphere. However, winds may change their direction and increase multiple times within the same day. Largely, wind movement in the atmosphere may be classified into three categories:

1. Primary circulation: this circulation includes planetary wind systems that are related to the general arrangement of pressure belts on the earths surface. In fact, it is the primary circulation pattern which prepares the broad framework for the other circulation patterns.
2. Secondary circulation: this is made upand anti-cyclones, and monsoon.
3. Tertiary circulation: it includes all the local winds which are produced by local causes such as topographical features, sea influences, etc. Their impact is visible only in a particular area.

Three cell model of atmospheric circulation

The air at the Inter-Tropical Convergence Zone (ITCZ) rises because of convection caused by high insolation and when low pressure is created. The winds from the tropics converge at this low-pressure zone. The converged air moves along with the convective cell. It reaches the top of the troposphere up to an altitude of 14 km and then moves towards the poles. This causes accumulation of air at about 30⁰ North and South. Part of the accumulated air sinks to the ground and forms a subtropical high. One of the reasons why the air sinks are due to the cooling of air when it reaches 30⁰ North and South latitudes. Just below and near the land surface, the air flows towards the equator as the easterlies. The easterlies from either side of the equator converge in the Inter-Tropical Convergence Zone (ITCZ). Such circulations from the surface upwards and vice-versa are called cells.

Hadley cell

A cell in the tropics is called theHadley Cell. Hence it is a global-scale tropicalatmospheric circulationthat shows a rising air near the Equator, flowing poleward at a height of 10 to 15 kilometers above the earth’s surface, descending in the subtropics, and then returning equatorward near the surface.

Ferrel cell

In the middle latitudes, the circulation is that of sinking cold air which blows from the poles and the rising warm air that blows from the subtropical high. At the surface, these winds are called westerlies and the cell is known as the Ferrel cell. In the Ferrel cell, air flows poleward and eastward near the surface and equatorward and westward at higher altitudes; this movement is the reverse of the airflow in theHadley cell. This belt is the westerly winds or westerlies.

Polar cell

At polar latitudes, the cold dense air subsides near the poles and blows towards middle latitudes as the polar easterlies. This cell is called the Polar cell. The wind belts are named for the directions from which the winds blow.

These three cells set the pattern for the general circulation of the atmosphere. The transfer of heat energy from lower latitudes to higher latitudes maintains the general circulation.