In this article we will discuss about the air pressure and its distribution.
Air being a physical substance is an admixture of several gases present in the atmosphere and thus it has its own weight. Thus, the air exerts pressure through its weight. Air pressure is, thus, defined as total weight of a mass of column of air above per unit area at sea level (unit area being one square inch, one square foot, one square centimetre, one square metre etc.).
The atmospheric pressure is maximum at sea level. It exerts the weight of 14.7 pounds on the area of one square inch at sea level or 1034 grams (about one kilogram) per square centimetre. One can imagine as to how much weight of overlying air is being carried by every man daily but he does not feel such enormous weight on his head and shoulders because the air present inside human body exerts equal amount of outward pressure which balances the inward atmospheric pressure.
Since the atmospheric pressure decreases with increasing altitudes and therefore the balance between the outward pressure exerted by the air of human body and inward pressure exerted by the atmosphere is disturbed, with the result man suffers from nose and ear bleed at higher altitudes on the mountains.
Air pressure is measured in terms of height of mercury in the glass tube. The standard air pressure at sea level is 1013.25 mb (millibars, millibar is a force equal to 1000 dynes per cm2 whereas a dyne is a unit of force approximately equal to the weight of one milligram) or 29.92 inches or 76 cm at a temperature of 15°C at the latitude of 45°.
The height of mercury up to 0.1 inch in the glass tube is equivalent to 3.4 mb. Air pressure is measured with the help of mercurial barometer (Fortin’s barometer), aneroid barometer, altimeter (altitude barometer), barograph, microbarograph etc.
The lines joining the places of equal pressure at sea level are called isobars. Air pressure decreases with increasing altitudes at the rate of 0.1 inch or 3.4 mb per 600 feet but this rate of decrease is confined to the altitude of a few thousand feet only. Normally, half of the total atmospheric pressure is confined to the altitude of 1800 feet.
Air pressure also varies seasonally, diurnally and spatially. On an average, it varies from 982 mb to 1033 mb. The highest sea level pressure of 1075.2 mb was recorded at Irkutsk in Siberia on 14 January, 1893 while the lowest sea level pressure of 877 mb was recorded in Marina Islands.
There is inverse relationship between temperature and pressure i.e., if temperature increases pressure decreases and vice versa. It is very often said that if thermometer is high, barometer is low (pressure is low) and if thermometer is low, barometer is high (pressure is high). The distribution of air pressure is controlled by altitude, temperature, air circulation, rotation of the earth, water vapour etc. The rate of change of pressure per unit horizontal distance is called pressure gradient.
Distribution of Air Pressure:
The horizontal distribution of air pressure on the globe is studied on the basis of isobars.
Air pressure is generally divided in two types viz.:
(1) High pressure, also called as ‘high’ or anticyclone, and
(2) Low pressure, also called as ‘low’ or cyclone or depression.
If we look at the globe then it appears that there is certain definite system of high and low pressure.
If, for generalization, the globe is considered to be homogeneous (either of land or water), then there should be regular and systematic zonal distribution of high and low pressure but the regularity of pressure belts is disturbed due to unequal distribution of land and water on the globe. The pressure belts are discontinued in the northern hemisphere and several centres of pressure bells are developed but the pressure belts are found more or less in regular pattern in the southern hemisphere.
There is no definite trend of distribution of pressure from equator towards the poles. If the air pressure would have been the function of air temperature alone there should have been regular increase of pressure pole-ward because temperature regularly decreases from the equator towards the poles but this is not the case.
There is low pressure near the equator due to high mean annual temperature but the existence of high pressure belts near the tropics of Cancer and Capricorn cannot be explained on the basis of temperature because the tropics record very high temperature and hence there should have been low pressure if the temperature would have been the only control of air pressure.
The air pressure should increase poleward from the tropics of Cancer and Capricorn because there is rapid rate of decrease of temperature poleward but we find low pressure belt near 60° latitude. Again we find high pressure belts near the poles due to exceedingly low temperature throughout the year. It is obvious that pressure belts are not only induced by thermal factor but they are also induced by dynamic factors. In all, there are seven pressure belts on the globe.
Pressure Gradient and Air Circulation:
The difference of pressure between any two places is called pressure gradient. Steep pressure gradient is represented by closely spaced isobars while widely spaced isobars reveal low pressure gradient. The direction of pressure gradient is considered from high pressure to decreasing pressure and the pressure direction is always perpendicular to isobars.
Pressure gradient is also called barometric slope. There is close relationship between pressure gradient and air circulation. The air moves from high pressure to low pressure. In other words, air movement follows barometric slope. The direction of air movement should be perpendicular to the isobars (fig. 35.5) because the direction of pressure gradient is perpendicular to the isobars but the direction is deviated from the expected theoretical direction due to coriolis force caused by the rotation of the earth and thus the winds cross the isobars at acute angle instead of right angle.
Horizontal pressure (pressure gradient), rotation of the earth and coriolis force, frictional force, centrifugal action of wind etc. affect and control air motion. The wind blowing parallel to the isobars generally at the height of 600m is called geostrophic wind.