Atmospheric Pressure is the weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere. Atmospheric Pressure is measured by an instrument called ‘barometer’. Atmospheric Pressure is expressed in the units of Millibars and Pascal. Pressure belts are unstable. They oscillate with the movement of the Sun.

The forces, which affect the velocity and direction of the wind, are: Pressure Gradient Force Frictional Force and, Coriolis Force Pattern of the movement of planetary winds depends on: Latitudinal variation of atmospheric heating Emergence of pressure belts Migration of belts following apparent path of the Sun Distribution of continents and oceans Rotation of the earth Movements of air repetitively and predictably, driven by changes in the weather patterns are called seasonal winds. Differential heating and cooling of the earth’s surface can create several local or regional winds. The air with distinctive characteristics in terms of temperature and humidity is called an air mass. The five major types of air masses are: Maritime Tropical (mT) Continental Tropical (cT) Continental Polar (cP) Continental Arctic (cA) and, Maritime Polar (mP) A ‘front’ is the meeting boundary zone of two different air masses. There are four types of fronts: Cold Fronts Warm Fronts Stationary Fronts and, Occluded Fronts Extra Tropical Cyclones develop in the mid and high latitudes, beyond the tropics. Tropical cyclones are violent storms that originate over oceans in tropical areas and move over to the coastal areas resulting in violent winds, heavy rainfall and storm surges.

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