Since air has mass, it also has weight. The pressure of air at a given place is defined as a force exerted in all directions by virtue of the weight of all the air above it. Since air pressure is proportional to density as well as temperature, it follows that a change in either temperature or density will cause a corresponding change in the pressure. The following equation called ‘the gas law’, describes the relationship between pressure, temperature and density—
Pressure = Density x Temperature x Constant
According to the gas law, an increase in either density or temperature will cause an increase in pressure, provided the other variable (density or temperature) remains constant.
The atmosphere exerts a pressure of 1034 gm per square cm at sea level. This amount of pressure is exerted by the atmosphere at sea level on all animals, plants, rocks, etc.
Measurement of Air Pressure:
Atmospheric pressure is the weight of the column of air at any given place and time. It is measured by means of an instrument called barometer. The units used by meteorologists for this purpose are called millibars (mb). One millibar is equal to the force of one gram on a square centimetre.
A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimetre. In other words, it will be equal to the weight of a column of mercury 75 cm high. The normal pressure at sea level is taken to be about 76 centimetres (1013.25 millibars). It may, however, fluctuate on either side of this value.
The distribution of atmospheric pressure is shown on a map by isobars. An isobar is an imaginary line drawn through places having equal atmospheric pressure reduced to sea level. The spacing of isobars expresses the rate and direction of pressure changes and is referred to as pressure gradient.
Close spacing of isobars indicates a steep or strong pressure gradient, while wide spacing suggests weak gradient. The pressure gradient may thus be defined as the decrease in pressure per unit distance in the direction in which the pressure decreases most rapidly.
The actual direction of pressure change is always perpendicular to the isobar lines. (Fig. 2.11). A rising pressure indicates fine, settled weather, while a falling pressure indicates unstable and cloudy weather.
Distribution of Atmospheric Pressure:
The distribution of atmospheric pressure is not uniform over the earth’s surface. It varies vertically as well as horizontally.
Air being a mixture of gases is highly compressible. Its density is, therefore, greatest at the lower layers, where it is compressed under the mass of air above. As a result, the lower layers of the atmosphere have high density and high pressure. In contrast, the higher layers are less compressed and hence have low density and low pressure. The higher we go, the thinner the atmosphere becomes, and thus the molecules are more diffused, and there is less pressure because inter-molecular space is greater.
At the height of Mt. Everest, the air pressure is about two-thirds less than what it is at the sea level. The decrease in pressure with altitude, however, is not constant. Since the factors controlling air density—temperature, amount of water vapour and gravity are variable, there is no simple relationship between altitude and pressure. In general, the atmospheric pressure decreases on an average at the rate of about 34 millibars every 300 metres of height.
The distribution of atmospheric pressure across the latitudes is termed as global horizontal distribution. This distribution is characterised by presence of distinctly identifiable zones of homogeneous pressure regimes or ‘pressure belts’. On the earth’s surface, there are in all seven pressure belts.
The seven pressure belts are: equatorial low, the sub-tropical highs, the sub-polar lows, and the polar highs. Except the equatorial low, all others form matching pairs in the northern and southern hemispheres.
1. Equatorial Low Pressure Belt or ‘Doldrums’:
This belt lies between 10°N and 10°S latitudes, although the width may vary between 5°N and 5°S and 20°N and 20°S. Due to intense heating, air gets warmed up and rises over the equatorial region and produces the equatorial low pressure belt. This belt is characterised by extremely low pressure with calm conditions.
Surface winds are generally absent since winds approaching this belt begin to rise near its margin. Thus, only vertical currents are found. This belt happens to be the zone of convergence of trade winds from two hemispheres from sub-tropical high pressure belts. This belt is also called the Doldrums, because of the extremely calm air movements.
2. Sub-Tropical High Pressure Belt or ‘Horse Latitudes’:
The sub-tropical highs extend from near the tropics to about 35°N and S. The high pressure along this belt is due to subsidence of air coming from the equatorial region which descends after becoming heavy. The high pressure is also due to the blocking effect of air at upper levels because of the Coriolis force. The subsiding air is warm and dry, therefore, most of the deserts are present along this belt, in both hemispheres.
The descending air currents feed the winds blowing towards adjoining low pressure belts. A calm condition with variable and feeble winds is created in these high pressure belts, called horse latitudes. In early days, the sailing vessels with a cargo of horses found it difficult to sail under such calm conditions. They used to throw horses into the sea when fodder ran out. This belt is frequently invaded by tropical and extra-tropical disturbances.
3. Sub-Polar Low Pressure Belt:
This belt is located between 45°N and S latitudes and the Arctic and the Antarctic circles. The low pressure exists along this belt due to ascent of air as a result of convergence of westerlies and polar easterlies. During winter, because of a high contrast between land and sea, this belt is broken into two distinct low centres—one in the vicinity of the Aleutian Islands and the other between Iceland and Greenland. During summer, a lesser contrast results in a more developed and regular belt. Also, due to a great contrast between the temperatures of the winds from sub-tropical and polar source regions, cyclonic storms or ‘lows’ are produced in the region.
4. Polar High:
The lowest temperatures are found over the poles, which cause subsidence of air and hence the polar highs. The polar highs are small in area and extend around the poles.
Factors Controlling Pressure Systems:
There are two main causes, thermal and dynamic, for the pressure differences resulting in high and low pressure systems.
An important factor while studying the pressure systems is temperature and its variations from equator to the poles, since a chain of events takes place due to heating and cooling of the earth’s surface and its atmosphere. When air is heated, it expands and, hence, its density decreases. This naturally leads to low pressure. On the contrary, cooling results in contraction. This increases the density and thus leads to high pressure. Formations of equatorial low and polar highs are examples of thermal lows and thermal highs, respectively.
Apart from variations of temperature, the formation of pressure belts may be explained by dynamic controls arising out of pressure gradient forces and rotation of the earth. The warm equatorial air cools during its ascent and, upon reaching the upper layers, it starts moving towards the pole. It further cools and begins to subside in a zone between 20° and 35° latitude. Two factors are responsible for the general subsidence of air in this belt.
First, cooling of the air results in increased density, which accounts for its subsidence. Second, owing to the rotation of the earth, the pole-ward directed winds are deflected eastwards, which is also called the Coriolis force—after a French scientist who first expressed its magnitude quantitatively.
The rate of deflection increases with the distance from the equator. As a result, by the time the pole-ward directed winds reach 25° latitude, they are deflected into a nearly west-to-east flow. It produces a blocking effect and the air piles up. This causes a general subsidence in the areas between the tropics and 35°N and S, and they develop into high pressure belts.
The location of these pressure belts is further affected by differences in net radiation resulting from apparent movement of the sun and from variations in heating of land and water surfaces. In the northern hemisphere, during summer, with the apparent northward shift of the sun, the thermal equator (belt of highest temperature) is located north of the geographical equator.
The pressure belts shift slightly north of their annual average locations. During winter, these conditions are completely reversed and the pressure belts shift south of their annual mean locations. Opposite conditions prevail in the southern hemisphere. The amount of shift is, however, less in the southern hemisphere due to predominance of water.
Similarly, distribution of continents and oceans has a marked influence over the distribution of pressure. In winter, the continents are cooler than the oceans and tend to develop high pressure centres, whereas in summer, they are relatively warmer and develop low pressure. It is just the reverse with the oceans.
Seasonal Distribution of Pressure:
Figures 2.14 and 2.15 shows the seasonal contrasts in world distribution of pressure.
In January, the equatorial low pressure belt shifts a little south of its mean equatorial position, due to the apparent southward movement of the sun. The lowest pressure pockets occur on the land masses of South America, South Africa and Australia, because land masses become much hotter than the adjoining oceans. Sub-tropical high pressure belt of the southern hemisphere is broken over the continents and remains confined to the oceans only.
Its development is maximum in the eastern parts of the oceans where the cool ocean currents are effective. In the northern hemisphere, a well-developed sub-tropical high pressure area extends over the continents. Finally, sub-polar low of the southern hemisphere extends as a trough whereas in the northern hemisphere, there are two cells of low presrure extending over the North Atlantic and the North Pacific. These are known as the Icelandic low and the Aleutian low respectively. (Fig. 2.14)
In July, the equatorial low pressure belt shifts towards the north following the apparent movement of the sun. This shift is maximum in Asia. The landmasses of the northern hemisphere become excessively hot and low pressure areas develop over them. The sub-tropical high pressure belt of the southern hemisphere extends continuously. In contrast, in the northern hemisphere, it is broken over the continents and remains confined to the North Atlantic and North Pacific Oceans. Sub-polar low is deep and continuous in the southern hemisphere, while in the northern hemisphere, there is only a faint oceanic low. (Fig. 2.15)
Diurnal Variation of Pressure:
The atmospheric pressure shows a definite rhythm when observed diurnally. Insolational heating and terrestrial radiation are mainly responsible for diurnal variations in pressure. During the equinoxes, the maxima occurs at 10 A.M. and 10 P.M. and the minima at 4 A.M. and 4 P.M. In the tropics, higher diurnal range occurs at places located at sea level and a lower range occurs at places located at higher altitudes.
The continents experience a larger range during daytime and a smaller range during the night. The oceans and coasts have a large diurnal range. The irregularities in the diurnal range occur due to cyclones, anti-cyclones and other atmospheric disturbances. These irregularities are larger and more pronounced in mid-latitudes and less pronounced in high latitudes.