Atmospheric Pressure and Pressure Belts

Air has weight and a column of air extending vertically over a given area on earth’s surface exerts pressure. The atmospheric pressure is measured as a force per unit of area and most common unit of measuring the air pressure is millibar. The instrument used for measuring pressure is Barometer. Some barometers are calibrated to show pressure in mercury inches. At constant temperature of 0°C and latitude of 45°, 1049 millibar is equal to 31 inches of mercury. Barograph is used to take continuous readings of air pressure.

Measuring Atmospheric Pressure

Bar is a unit of pressure equal to 100 kilopascals and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar (symbol: Mbar), kilobar (symbol: kbar), decibar (symbol: dbar), centibar (symbol: cbar), and millibar (symbol: mbar or mb). Bar is neither an SI unit nor a CGS unit. 1 bar is 1% smaller than the atmosphere (symbol: atm), which now is defined to be 1.01325 bar exactly. One millibar is also equal to 1000 dynes per cm².

Pressure Belts of Earth

The distribution of pressure on earth is uneven. Usually pressure is inversely related to the temperature and pressure reduced with altitude. The major factors are earth’s rotation and ascent and descent of air to affect distribution of pressure.

Creation of the Pressure Belts

Due to high amount of insolation over the equator, the air ascends and this air rising in the equatorial region descends at around 30° north and south latitudes. This means that the air at the equatorial region is thrown away from the earth and air at the Polar Regions is pulled towards earth. This implies that there is a low pressure is on equator and there is a high pressure area on poles. This gives rise to two belts of high pressure on Polar Regions each and one belt of low pressure on equator. The air that descends at 30°N and 30°S also created two belts of high pressures in the subtropical regions of both the hemispheres.  Further, the rotation of the earth pulls the air at Polar Regions causes a rarification of air pressure at sub-polar regions. This also produces two belts of low pressure around 60°N and S latitude. This means that there are 7 belts of pressure as shown in the below graphics.

The planetary distribution of pressure, in the 7 belts is determined by two major factors viz. thermal factor and dynamic factor. Please note that equatorial belt of low pressure and polar belts of high pressures are due to the thermal factor while, the subtropical belts of high pressure and subpolar belts of low pressure are primarily due to earth’s rotation or dynamic factors.

Intertropical Convergence Zone / Doldrums

The pressure belt between the 0° to 5°N and S is called Equatorial Low Pressure Belt. This belt is characterized by intense heating, with expanding air and ascending convectional currents. Because the air is largely moving upward, surface winds are light and variable. This region is known as the doldrums.

The term doldrums has been used by the sailors as it has been marked by erratic weather patterns with stagnant calms and violent thunderstorms.

Doldrums are belt of calms and variable winds occurring at times along the equatorial trough. Doldrums are characterised by:

• Low atmospheric Pressure
• High Humidity
• Thunderstorms

The same area is also called the Intertropical Convergence Zone (ITCZ) or Doldrums. This is the area encircling the earth near the equator where winds originating in the northern and southern hemispheres come together. Please note that the location is not precisely defined as location of the Intertropical convergence zone varies over time. Over land, it moves back and forth across the equator following the sun’s zenith point. Over the oceans, where the convergence zone is better defined, the seasonal cycle is more subtle, as the convection is constrained by the distribution of ocean temperatures. Sometimes, a double ITCZ forms, with one located north and another south of the equator. When this occurs, a narrow ridge of high pressure forms between the two convergence zones, one of which is usually stronger than the other. Between 10° and 15° North and South, there are high pressure belts, where air is comparatively dry, light and calm. This region is beneficial to the maritime trade.

Subtropical High / Horse Latitudes

Horse Latitudes or Subtropical High are subtropical latitudes between 30 and 35 degrees both north and south. This region, under a ridge of high pressure receives little precipitation and has variable winds mixed with calm. The air is comparatively dry and calm. This is also the region of descending air current and is marked by some cyclonic activities. The consistently warm, dry conditions of the horse latitudes also contribute to the existence of temperate deserts, such as the Sahara Desert in Africa, the southwestern United States and northern Mexico, and parts of the Middle East in the Northern Hemisphere; and the Atacama Desert, the Kalahari Desert, and the Australian Desert in the Southern Hemisphere.

Other Belts

30°-60°North and South Belt region is of temperate low pressure belt or anti-trade wind area. It is marked by cyclones and anticyclones. 60°North and South are the two Temperate Low Pressure belts which are also called zones of convergence with Cyclonic activity. The 90° North and South are called Polar High belts.