Air pressure is measured with an instrument called a mercury barometer or the aneroid barometer. On maps, places of equal atmospheric pressure are joined together by lines called ISOBARS.
Meaning of Atmospheric Pressure
Atmospheric pressure is the weight of the volume of air which extends from the ground surface to the outermost layers of the atmosphere. Air is made up of a number of mixed gases and it has weight. The atmosphere is held on the earth by the gravitational pull of the earth. A column of air exerts weight in terms of pressure on the surface of the earth. However, as we move higher above the earth surface, there is a decrease in atmospheric pressure. Apart from the effects of the altitude, temperature and the rotation of the earth also play a vital role. This could be the reasons for the difference in pressure for all parts of the earths surface.
However, it is very important to know that atmospheric pressure over a place and at any given time does not remain the same for a very long time due to both daily and seasonal variations. The pressure is normally measured in millibars (Mb)or pascals hence the pressure of a given place ranges from 950- 1050 millibars. At the sea level, this pressure of the air is about 6.7 kg. per 6.45 square centimetres. The pressure of the wind decreases with the increase in altitude. Sea level height is internationally accepted as zero (0) height to measure the air pressure. The gradual change of pressure between different areas is known as the barometric slope or pressure gradient. The closer the isobars are together, the greater the pressure gradient; for example, widely spaced isobars indicate a weak pressure gradient.
Air pressure is not evenly distributed in the atmosphere, as a result, the air tries to balance the unevenness. The air moves from the high-pressure area towards the low-pressure area. Due to difference in temperature and pressure the air moves from one part of the earth to another, when the air movement takes a horizontal pattern, it is known as the wind but when it is vertical or nearly vertical moving air it is termed air current. Both winds and air currents from the system of circulation in the atmosphere.
The wind has certain specific characteristics as follows: The cold and heavy air moves from the areas of high pressure to low pressure area. According to Ferrel s Law, the air moves towards the right in the northern hemisphere and towards left in the southern hemisphere. The wind system of the earth can be divided into four types. These are Planetary winds, Seasonal wind, Local wind, and Irregular wind.
Planetary wind: The winds are controlled by the pressure belts, it blows towards the same direction throughout the whole year. The planetary wind is of three types such as the trade wind, the westerlies and the polar wind.
The trade wind: The suns rays fall vertically over the equatorial region, so the air becomes hot and goes upwards. Since the pressure is less upward, the rising air gets a room for expansion and consequently, the air becomes cool and dense. The cool air could not come down directly due to warm air at the bottom. As a result, the air moves towards north and south directions through the upper atmosphere. Moving up to 30 latitudes some part of this air finds its way to come down and blows towards the equatorial low-pressure belt. This part of the air is known as the Trade Wind. According to Ferrels Law, the trade wind blows from north-east in the northern hemisphere and south-east in the southern hemisphere. The trade wind of northern hemisphere is known as North-East Trade Wind and that of the southern hemisphere as South-East Trade Wind. The sky remains clear and the weather is hot and dry as this trade wind originates in the high-pressure zones. The big deserts of the world are situated nearer to this area. For example, the Sahara desert, the Lybian desert, the Arabian desert in the northern hemisphere and the Kalahari desert in the southern hemisphere are some of the examples.
The pressure reduces rapidly in the lower atmosphere with height and with a decrease in the density of air. This doesnt always lower at the same rate. But for simpler calculations, a decrease of about 1Mb for each 10m increase in elevation is what is considered. Even with a high vertical pressure gradient, we do not experience strong vertical air currents. This is possible because of equal and opposite gravitation force acting upon the air.
The low-pressure effects are mostly and regularly experienced by the people living in the hilly areas as compared to people living in the plains. In high mountainous areas, rice takes more time to cook because low pressure reduces the boiling point of water. Similarly, many trekkers from outside in such areas also experience breathing problem such as faintness and nose bleedings because of low-pressure conditions in which makes the air is thin and with a low amount of oxygen content. Unlike vertical high-pressure gradient, small horizontal pressure gradients are highly significant in terms of the wind direction and velocity. In order to eliminate the effect of altitude on pressure, it is measured at any station after being reduced to sea level for purposes of comparison. The pressure system is enclosed by one or more isobars with the lowest pressure in the centre. A high-pressure system is also enclosed by one or more isobars with the highest pressure in the centre. The terms high pressure and low pressure do not usually indicate any particular absolute values but are used relatively.
The northern hemisphere tends to have the greater seasonal contrasts in its pressure distributions and the southern hemisphere exhibits much simpler average pressure patterns overall. These differences are largely related to the unequal distribution of land and sea between the two hemispheres. Ocean areas, which dominate the southern hemisphere, tend to be much more equable than continents in both temperature and pressure variations. Due to the variation of temperature in different latitudes and the rotation of the earth, several pressure zones have been formed at the lower layer of the atmosphere which are known as pressure belts.
Due to the variation of temperature in different latitudes and the rotation of the earth, several pressure zones have been formed at the lower layer of the atmosphere which are referred to as pressure belts
Distribution of Pressure Belts
- Equatorial low-pressure belt
- Tropical high-pressure belt
- Subpolar low-pressure belt
- Polar high-pressure belt
Equatorial Low-Pressure Belt
The Equatorial Low-Pressure Belt lies on both sides of the equator extending from 00N 10S0. This belt receives vertical suns rays throughout the whole year. The air pressure is low over this region as the air in comparison to its surrounding areas is hot and light. Moreover, this region has more water bodies. As a result, low pressure which is known as Equatorial Low-Pressure Belt has been formed. this belt, therefore, is called doldrums (the zone of calm) and it is also known as-Inter Tropical Convergence Zone (ITCZ) because the trade winds flowing from sub-tropical high-pressure belts converge here.
Tropical High-Pressure Belt
This belt extends averagely between 250 and 350 north and south latitudes. The existence of these pressure belts is simply because of the fact that the uprising air of the equatorial region is deflected towards the poles as a result of earths rotation. The more the equatorial hot, humid and light air goes up, the more it becomes cold. In this case, the warm air rises up continuously but it cannot come down in the equatorial zone. As a result, the upper air starts moving towards the north and south. In this way, the upper cold and heavy air start coming down in the tropical region between 25 to 30. Thus two heavy pressure belts are formed in the mid. These two pressure belts are known as the Tropical High-Pressure Belts (One over the Tropical of Cancer and the other one over the Tropic of Capricorn). These belts are also called Horse latitudes.
Sub Polar Low-Pressure Belt
It extends between 550 – 650 latitude in both north and south hemisphere. These belts are not thermally induced instead the winds coming from the sub-tropics and the polar regions converge in this belt and rise upward. The high-temperature contrast between the subtropical and the polar regions and result in cyclonic storms in this belt. In the southern hemisphere, this low-pressure belt is more common due to the vast presence of ocean But in the northern hemisphere, there are large land masses along 600 latitude which are very cold. Therefore, the pressures over these landmasses are increased. Thus, the continuity of the belt is broken. This belt is also referred to as the sub-antarctic low.
Polar High-Pressure Belt
The wind nearer to the two poles is very cold and heavy due to the location in the cold regions. As a result of low temperature, the air compresses and its density increases. Hence, high pressure is found here throughout the year. This is more marked over the land area of the Antarctic continent than over the ocean of the North Pole. However,two high-pressure belts have been formed in two polar regions. These two are known as Polar High-Pressure Belts. The wind from these two high-pressure belts moves towards the sub-polar low-pressure belts.