Adwait Frequency

Atmospheric Pressure

Air is a physical substance. It is a mixture of several gases present in the atmosphere and it has its own weight. Air exerts pressure on the earth's surface. The weight of air on a unit area of the earth is called Air Pressure.

Atmospheric Pressure refers to the force

per unit area exerted against a surface by the weight of the air above that surface. Pressure is expressed in millibars (mb) and measured with a mercury barometer. The average atmospheric pressure at sea level is 1013.25 mb or 760 mm (the height of the column of mercury in a barometer at sea level). Atmospheric Pressure decreases with height.

Atmospheric pressure is an important factor in producing changes in weather. Contrasts in temperature cause changes in air density,

which are responsible for variations in pressure.

These variations cause horizontal movements

of air called winds. Winds transport heat and

moisture from one region to another and thus

help in the occurrence of precipitation and

affect both temperature and humidity.

Measurement of Pressure

Pressure is normally measured in millibars. The variations in pressure are shown on maps by means of Isobars. These are lines joining the places having the same barometric pressure. The barometer consists of a long narrow tube filled with mercury. There are two types of barometers -Fortin's Barometer and Aneroid Barometer.

Pressure Gradient

Pressure gradient is defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly. In other words, the rate of change of atmospheric pressure between two points on the earth's surface is called pressure gradient. On the weather chart this is by the spacing of isobars. The gradient is steep if they are close together and gentle if they are far apart. Close spacing of isobars indicates a strong pressure gradient, while wide spacing suggests a weak gradient.