Question

explain about the heat zones pressure belts and wind in detail

Answer 1

Air Pressure

Since air has mass, it also has weight. The pressure of air at a given place is defined as a force exerted in all directions by virtue of the weight of all the air above it.

The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in various units.

Measurement of Air Pressure

Atmospheric pressure is the weight of the column of air at any given place and time. It is measured by means of an instrument called barometer.

The units used by meteorologists for this purpose are called millibars (mb).

One millibar is equal to the force of one gram on a square centimeter. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimeter.

In other words, it will be equal to the weight of a column of mercury 75 cm high.

The normal pressure at sea level is taken to be about 76 centimeters (1013.25 millibars).

Vertical Variation of Pressure

In the lower atmosphere the pressure decreases rapidly with height.

At the height of Mt. Everest, the air pressure is about two-thirds less than what it is at the sea level.

The decrease in pressure with altitude, however, is not constant. Since the factors controlling air density – temperature, amount of water vapour and gravity are variable, there is no simple relationship between altitude and pressure.

In general, the atmospheric pressure decreases on an average at the rate of about 34 millibars every 300 metres of height.

The vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.

Due to gravity the air at the surface is denser and hence has higher pressure. Since air pressure is proportional to density as well as temperature, it follows that a change in either temperature or density will cause a corresponding change in the pressure.

The pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas.

A rising pressure indicates fine, settled weather, while a falling pressure indicates unstable and cloudy weather.


There are distinctly identifiable zones of homogeneous horizontal pressure regimes or ‘pressure belts’. On the earth’s surface, there are in all seven pressure belts.

The seven pressure belts are :

equatorial low,

the sub-tropical highs,

the sub-polar lows, and

the polar highs.

Except the equatorial low, all others form matching pairs in the northern and southern hemispheres.



Closed Isobars or Closed Pressure centers

Low pressure system is enclosed by one or more isobars with the lowest pressure in the centre. High-pressure system is also enclosed by one or more isobars with the highest pressure in the center