Pressure is defined as the force exerted on the earth’s surface and other objects by air, because air has weight.

On maps, places having equal atmospheric pressure are joined together by lines called Isobars.



  1. Altitude: When air descends, its pressure and temperature rise but when it ascends, it becomes less dense and its pressure and temperature fall. In other words, pressure decreases as altitude increases.
  2. Temperature: When the temperature of the air increases, its pressure decreases and when the temperature decreases, its pressure increases.
  3. Wind: It is known that wind blows from a high pressure belt to a low pressure belt.
  4. Rotation of the earth: Owing to rotation of the earth , differences in pressure result in the deflection of the wind to the right in the northern hemisphere and to the left in the southern hemisphere.



There are four world pressure belts. These are:

  1. Doldrums or Equatorial Low pressure belt.
  2. Horse latitudes or Sub-Tropical high pressure belt.
  3. Temperate low pressure belt.
  4. Polar high pressure belt.



  1. Doldrums of Equatorial Low Pressure Belt: This belt is found around the equator between 5 degrees north and south of the Equator. Owing to intense heat, the pressure is low and all winds blow to coverage in this belt.
  2. Horse Latitudes or Sub-Tropical High Pressure Belt: This is found in sub-tropical latitudes 20 degrees and 30 degrees north and south of the equator. The pressure in this belt is high and winds tend to diverge and anticyclone is formed.
  3. Temperate Low or Mid-Latitude Low Pressure Belt: This pressure belt is found between 30 degrees and 60 degrees north and south of the equator. They are zones of convergence with cyclone activities.
  4. Polar High (High Pressure Belts): These are found around the polar regions and planetary winds shift with seasons.



Humidity is defined as a measure of the dampness of the atmosphere due to the presence of water vapour in the air. The actual amount of water vapour present in the air is called absolute humidity and it is expressed in g/m3.

Relative humidity therefore, is the ratio between the actual amount of water vapour and the total amount the air can hold at a given temperature. Relative humidity is expressed as percentage.

If air contains only half the amount it can hold, the relative humidity is 50%. When the relative humidity reaches 100%, the air is said to be saturated. At this stage, the air temperature is said to be at dew point.

Further cooling will condense the water vapour into clouds or rain.

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