The areas located between the 30⁰ and area's parallels of latitude in both hemispheres are mostly affected by the shi of air pressure belt. Explain with suitable examples
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Answer:
Air Pressure
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.
The weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere is called the atmospheric pressure. The atmospheric pressure is expressed in various units.
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 centimeter. A pressure of 1000 millibars is equal to the weight of 1.053 kilograms per square centimeter.
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 centimeters (1013.25 millibars).
Vertical Variation of Pressure
In the lower atmosphere the pressure decreases rapidly with height.
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 vertical pressure gradient force is much larger than that of the horizontal pressure gradient. But, it is generally balanced by a nearly equal but opposite gravitational force. Hence, we do not experience strong upward winds.
Due to gravity the air at the surface is denser and hence has higher pressure. 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 pressure decreases with height. At any elevation it varies from place to place and its variation is the primary cause of air motion, i.e. wind which moves from high pressure areas to low pressure areas.
A rising pressure indicates fine, settled weather, while a falling pressure indicates unstable and cloudy weather.
Horizontal Distribution of Pressure
Small differences in pressure are highly significant in terms of the wind direction and velocity. Horizontal distribution of pressure is studied by drawing isobars at constant levels.
Isobars are lines connecting places having equal pressure. 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 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.
There are distinctly identifiable zones of homogeneous horizontal 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.
Answer:
(i) Equatorial Low-Pressure Belts
This low-pressure belt extends from 0 to 5° North and South of Equator. Due to the vertical rays of the sun here, there is intense heating. The air, therefore, expands and rises as convection current causing low pressure to develop here. This low-pressure belt is also called as doldrums because it is a zone of total calm without any breeze.
(ii) Subtropical High-Pressure Belts
At about 30°North and South of Equator lies the area where the ascending equatorial air currents descend. This area is thus an area of high pressure. It is also called as the Horse latitude. Winds always blow from high pressure to low pressure. So the winds from subtropical region blow towards the Equator as Trade winds and another wind blow towards Sub-Polar Low-Pressure as Westerlies.
(iii) Circum-Polar Low-Pressure Belts
These belts located between 60° and 70° in each hemisphere are known as Circum-Polar Low-Pressure Belts. In the Subtropical region, the descending air gets divided into two parts. One part blows towards the Equatorial Low-Pressure Belt. The other part blows towards the Circum- Polar Low-Pressure Belt. This zone is marked by the ascent of warm Subtropical air over cold polar air blowing from poles. Due to the earth’s rotation, the winds surrounding the Polar region blow towards the Equator. Centrifugal forces operating in this region create the low-pressure belt appropriately called the Circumpolar Low-Pressure Belt. This region is marked by violent storms in winter.
(iv) Polar High-Pressure Areas
At the North and South Poles, between 70° to 90° North and South, the temperatures are always extremely low. The cold descending air gives rise to high pressures over the Poles. These areas of Polar high pressure are known as the Polar Highs. These regions are characterized by permanent Ice Caps
Explanation:
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