Pressure: The pressure exerted by the atmosphere as a result of its weight, above a unit area of the earth’s surface is called atmospheric pressure. It is expressed in millibars (mb) and measured. The following are the major pressure belts operating in the earth’s atmosphere.
1. Equatorial Low Pressure Belt: It extends from the equator to about 10°N and S. It is characterised by extremely low pressure with calm conditions, and due to calm air movement it is also known as doldrums.
2. The Sub Tropical High Pressure Belt : It extends from near the tropics to about 35°N and S. A calm condition with variable and feeble winds is created in these high pressure belts, called Horse Latitudes.
3. Sub Polar Low Pressure Belt: This belt is located between 45°N and S to the Arctic and Antarctic circles. Cyclonic storms or ‘Lows’ are produced due to great contrast between the temperature of the winds from sub-tropical and polar source regions.
4. Polar High : These belts surround the poles. Due to high intensity of cold at the poles, the air becomes very cold and develops high pressure belts around the poles.
Wind: Due to horizontal differences in air pressure, airflows from areas of high pressure to areas of low pressure. Horizontal movement of the air is called wind. The three types of winds are given below :
Planetary Winds : The winds blowing throughout the year from one latitude to another in response to latitudinal differences in air pressure are called planetary or prevailing winds. Trade winds, Westerlies and Polar winds are planetary winds.
Periodic Winds : Periodic winds change their direction periodically with the change in season, e.g. Monsoons, Land and Sea Breeze etc.
Local Wind : Local winds develop as a result of localdifferences in temperature and pressure. Loo is an example of local winds.
Hurricane : This is also known as tropical cyclone or tropical storm. This is a disturbance of about 650 km across, spinning around a central area of very low pressure, with winds over 140 km per hour.
Clouds: Cloud is a mass of minute droplets of water or tiny crystals of ice formed by the condensation of water vapour in free air at considerable elevations. Clouds are formed mainly by the adiabatic cooling of air below its dew point. Clouds are found up to the height of 12,000 meters.
Zones : The globe is divided into three temperature zones.
1. Tropical Zone : It extends between Tropic of Cancer and Capricorn. High temperature prevails throughout the year.
2. Temperate Zone : It extends between 23 l/2° and 66l/2° latitudes in both the hemispheres.
3. Frigid Zone : It extends between 66 l/2° latitude and poles in both hemispheres. Low temperature prevails throughout the year.
