Atmospheric pressure is important to us because it is related to winds and it helps to determined, weather conditions of a place. In this article you will study air pressure, its distribution. This is an important topic for UPSC Mains GS paper 1 syllabus.
- Air Pressure
- Pressure Belts
- Seasonal Distribution of Pressure
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. The weight of the column of air at a given place and time is called air pressure or atmospheric pressure.
Measurement of Air Pressure
Atmospheric pressure is measured as force per unit area. The unit used for measuring pressure is called millibar. Its abbreviation is ‘mb’. One millibar is equal to the force of one gram per square centimetre approximately. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimetre at sea level. It is equal to the weight of a column of mercury which is 76 centimetre high. The international standard pressure unit is the “pascal”, a force of one Newton per square meter. In practice atmospheric pressure is expressed in kilopascals, (one kpa equals 1000 Pa).
Atmospheric pressure is measured by an instrument called barometer. Now a days Fortin’s barometer and Aneroid barometer I are commonly used for measuring air pressure.
- The weight of a column of air at a given place and time is called air
- Barometer is the instrument which measures air or atmospheric
- The unit of measurement of atmospheric pressure is millibar (kilopascals).
- One millibar is equal to the force of nearly one gram per square centimetre.
The mean atmospheric pressure at sea level is 1013.25 millibars. However the actual pressure at a given place and at a given time fluctuates and it generally ranges between 950 and 1050 millibars
Distribution of Air Pressure
Distribution of atmospheric pressure on the surface of the earth is not uniform. It varies both vertically and horizontally.
Air is a mixture of various gases. It is highly compressible. As it compresses, its density increases. The higher the density of air, the greater is the air pressure and vice versa. The mass of air above in the column of air compresses the air under it hence its lower layers are more dense than the upper layers; As a result, the lower layers of the atmosphere have higher density, hence, exert more pressure. Conversely, the higher layers are less compressed and, hence, they have low density and low pressure. The columnar distribution of atmospheric pressure is known as vertical distribution of pressure. Air pressure decreases with increase in altitude but it does not always decrease at the same rate. Dense components of atmosphere are found in its lowest parts near the mean sea level. Temperature of the air, amount of water vapour present in the air and gravitational pull of the earth determine the air pressure of a given place and at a given time. Since these factors are variable with change in height, there is a variation in the rate of decrease in air pressure with increase in altitude. The normal rate of decrease in air pressure is 34 millibars per every 300metres increase in altitude. The effects of low pressure are more clearly experienced by the people living in the hilly areas as compared to those who live in plains. In high mountainous areas rice takes more time to cook because low pressure reduces the boiling point of water. Breathing problem such as faintness and nose bleedings are also faced by many trekkers from outside in such areas because of low pressure conditions in which the air is thin and it has low amount of oxygen content.
The distribution of atmospheric pressure over the globe is known as horizontal distribution of pressure. It is shown on maps with the help of isobars. An isobar is a line connecting points that have equal values of pressure. Isobars are analogous to the contour lines on a relief map. The spacing of isobars expresses the rate and direction of change in air pressure. This charge in air pressure is referred to pressure gradient. Pressure gradient is the ratio between pressure difference and the actual horizontal distance between two points. Close spacing of isobars expresses steep pressure gradient while wide spacing indicates gentle pressure gradient (see fig. 11.5)
The horizontal distribution of atmospheric pressure is not uniform in the world. It varies from time to time at a given place; it varies from place to place over short distances. The factors responsible for variation in the horizontal distribution of pressure are as follows:
- Air temperature
- The earth’s rotation
- Presence of water vapour
Air Temperature: In the previous lesson, we have studied that the earth is not heated uniformly because of unequal distribution of insolation, differential heating and cooling of land and water surfaces. Generally there is an inverse relationship between air temperature and air The higher the air temperature, the lower is the air pressure. The fundamental rule about gases is that when they are heated, they become less dense and expand in volume and rise. Hence, air pressure is low in equatorial regions and it is higher in polar regions. Along the equator lies a belt of low pressure known as the “equatorial low or doldrums”. Low air pressure in equatorial regions is due to the fact that hot air ascends there with gradual decrease in temperatur causing thinness of air on the surface. In polar region, cold air is very dense hence it descends and pressure increases. From this we might expect, a gradual increase in average temperature thords equator. However, actual readings taken on the earth’s surface at different places indicate that pressure does not increase latitudinally in a regular fashion from equator to the poles. Instead, there are regions of high pressure in subtropics and regions of low pressure in the subpolar areas.
The Earth’s Rotation: The earth’s rotation generates centrifugal This results in the deflection of air from its original place, causing decrease of pressure. It is believed that the low pressure belts of the sub polar regions and the high pressure belts of the sub-tropical regions are created as a result of the earth’s rotation. The earth’s rotation also causes convergence and divergence of moving air. Areas of convergence experience low pressure while those of divergence have high pressure
Pressure of Water Vapour : Air with higher quantity of water vapour has lower pressure and that with lower quantity of water vapour has higher In winter the continents are relatively cool and tend to develop high pressure centres; in summer they stay warmer than the oceans and tend to be dominated by low pressure, conversely, the oceans are associated with low pressure in winter and high pressure in summer.
- An isobar is a line connecting points that have equal values of Pressure
- Pressure gradient is the ratio between pressure difference and horizontal distance between two points
- On an average air pressure decreases by 34 millibars per 300 metres increase in heights
The horizontal distribution of air pressure across the latitudes is characteriesd by high or low pressure belts. This is however, a theoretical model because pressure belts .are not always found as such on the earth. We will see it later how the real condition departs from the idealized model. and examine why these differences occur.
The Equatorial Low Pressure Belt
The sun shines almost vertically on the equator throughout the year. As a result the air gets warm and rises over the equatorial region and produce equatorial low pressure. This belt extends from equator to 100N and 100S latitudes. Due to excessive heating horizontal movement of air is absent here and only conventional currents are there. Therefore this belt is called doldrums (the zone of calm) due to virtual absence of surface winds. These are the regions of convergence because the winds flowing from sub tropical high pressure belts converge here. This belt is also known as-Inter Tropical Convergence Zone (ITCZ).
The Sub-tropical High Pressure Belts
The sub-tropical high pressure belts extend from the tropics to about 350 latitudes in both the Hemispheres. In the northern hemisphere it is called as the North sub-tropical high pressure belt and in the southern hemisphere it is known as the South sub-tropical high pressure belt. The existence of these pressure belts is due to the fact that the up rising air of the equatorial region is deflected towards poles due to the earth’s rotation. After becoming cold and heavy, it descends in these regions and get piled up. This results in high pressure. Calm conditions with feeble and variable winds are found here. In olden days vessels with cargo of horses passing through these belts found difficulty in sailing under these calm conditions. They used to throw the horses in the sea in order to make the vessels lighter. Henceforth these belts or latitudes are also called ‘horse latitudes’. These are the regions of divergence because winds from these areas blow towards equatorial and sub- polar low pressure belts.
The Sub-polar low Pressure Belts
The sub-polar low pressure belts extend between 450N and the Arctic Circle in the northern hemisphere and between 45°S and the Antarctic Circle in the southern hemisphere. They are known as the North sub-polar low and the South sub-polar low pressure belts respectively. Winds coming from the sub-tropical and the polar high belts converge here to produce cyclonic storms or low pressure conditions. This zone of convergence is also known as polar front.
The Polar High Pressure Belts
In polar regions, sun never shines vertically. Sun rays are always slanting here resulting in low temperatures. Because of low temperature, air compresses and its density increases. Hence, high pressure is found here. In northern hemisphere the belt is called the North polar high pressure belt while it is known as the South polar high pressure belt in the southern hemisphere. Winds from these belts blow towards sub-polar low pressure belts.
This system of pressure belts that we have just studied is a generalised picture. In reality, the location of these pressure belts is not permanent. They shift northward in July and southward in January, following the changing position of the sun’s direct rays as they migrate between the Tropics of Cancer and Capricorn. The thermal equator (commonly known as the belt of highest temperature) also shifts northwards and southwards of the equator. With the shifting of thermal equator northwards in summer and southwards in winter, there is also a slight shift in pressure belts towards north and south of their annual average location.
- Sub-tropical high pressure belts are also called horse
- Subsidence and piling of air in sub-tropical belts cause high
- Convergence of subtropical and polar winds result in the formation of cyclones in the sub-polar
- High pressure belts are dry while low pressure belts are
- With the movement of sun northwards and southwards thermal equator also shifts northwards and
- Pressure belts also shift northwards and southwards with the shift of thermal
SEASONAL DISTRIBUTION OF PRESSURE
The variation of pressure from place to place and from season to season over the earth plays an important role in affecting the weather and climate. Therefore we study pressure distribution through isobar maps. While drawing isobar maps, the pressures of all places are reduced to sea level to avoid the effect of altitude on air pressure.
In January, with the south-ward apparent movement of the Sun, the equatorial low pressure belt shifts a little south of the mean equatorial position. Areas of lowest pressure occurs in South America, Southern Africa and Australia. This is because the land tends to get hotter rapidly than water. Sub-tropical high pressure cells are centered over the ocean in the southern hemisphere. The belt of high pressure is interrupted by the continental land masses where the temperature is much higher. They are well developed in eastern part of the ocean where cold ocean currents dominate.
In the northern hemisphere, ridges of high pressure occur in the sub-tropical latitudes over the continent. A well developed high pressure cell occurs in the interior parts of Eurasia. This is due to the fact that land cools more rapidly than oceans. Its temperatures are lower in winter than the surrounding seas. In the southern hemisphere, the sub-polar low pressure belt circles the earth as a real belt of low pressure and is not divided into cells, because there is virtually no landmass. In northern hemisphere two cells of low pressure namely Iceland low and Aleutian low develop over the North Atlantic and the North Pacific oceans respectively.
In July, the equatorial low pressure belt shifts a little north of the mean equatorial position because of the northward apparent movement of the Sun. All the pressure belts shift northwards in July.
The Aleutian and Icelandic lows disappear from the oceans while the landmasses, which developed high pressure during winter months, have extensive low pressure cells now. In Asia, a low pressure develops. The sub- tropical hights of the northern hemisphere are more developed over the oceans- Pacific and Atlantic. In the southern hemisphere, the sub-tropical high pressure belt is continuous. Sub-polar low forms a continuous belt in the southern hemisphere while in northern hemisphere, there is only a faint oceanic low.
Air pressure distribution decides the wind flow on the surface of earth. This article helps you understand the mechanism of the air pressure. To read more articles on geography click here