Pressure Gradient

You know that air pressure is the weight of the atmosphere over a unit area of the earth's surface. The average air pressure at sea level is approximately 1 kilogram per square centimetre. At any point at sea level the air pressure is the same. We know that gravity which holds everything on earth's surface, also holds the atmospheric gases in an envelope
around the earth. Gravity compresses the atmosphere on the earth's surface so that air pressure decreases with increasing altitude.

Weather forecasters on the TV and Radio usually report air pressure in rnm i.e. in unit of length. This in fact refers to the height to which a column of mercury can rise in the barometer at a specific station at a given time. However, it is more appropriate to express pressure in millibars (mb). The average pressure at sea level is 1013.25 mb.

Let see what happens to the air pressure and density as we go higher up in the atmosphere. Air density which is the mass per unit volume also diminishes with altitude. Ninety-nine per cent of the atmosphere's mass lies between the earth's surface and an altitude of approximately 32 km. Approximately half of the atmosphere's mass lies between the surface of the earth and an altitude of 5.5 km. At this altitude the air pressure too remains only one-half of the pressure at sea level.

In most cases the reduction of pressure is not a limiting factor for the distribution of plants and animals at high altitudes. There arelother adverse conditions like low temperature, lack of food, unsuitable soil etc. Many species of beetles have been found in the highest meadows of Himalayas. While, earthworms have been found up to the snowline in the Andes mountains. However, for warm blooded vertebrates reduced air pressure and density
at high altitudes causes impairment in respiration.

The expansion and thinning of air accompanying the lower air pressure at high altitudes triggers physiological changes in human beings. For example, a person at high altitude may experience dizziness, headaches and shortness of breath, but gradually adjusts or acclimatises to the low oxygen levels. However, people cannot adjust to pressure at altitudes higher than approximately 5.5 km.

Relatively a slight change in the air pressure can trigger important changes in the weather. A large volume of air which is relatively uniform in terms of its temperature and water vapour content is called an air mass. As air masses move from one place to another, surface air pressure falls or rises causing changes in weather. As a general rule low pressure causes
stormy weather and when air pressure rises the weather improves.