Our atmosphere is a blanket of large amount of air. The pressure at any point on the earth’s surface due to the weight of the column of air is also quite large. It is almost equivalent to the weight of three elephants on our body or 15 pounds per square inch.
Measurement and Units of Pressure
A barometer is the instrument used to measure atmospheric pressure. Atmospheric pressure is usually expressed in centimetres of mercury. The standard pressure at sea level is 76 cm or 29.92 inches.
Milibar is also a unit of atmospheric pressure used in weather charts. One bar is divided in to 1000 milibars. Milibar is known as hectopascals.
One bar = one atmospheric pressure
= 76 cm of mercury
= 1013 mb
= 760 mm of mercury
Factors affecting Atmospheric Pressure
(a) Altitude : Atmospheric pressure decreases with increases in altitude.
(b) Temperature : Atmospheric pressure decreases with increase in temperature.
(c) Water vapour : More water vapour means less pressure than dry air.
Effects of Air Pressure
1. Winds are caused by the differences in air pressure.
2. Atmospheric pressure affects weather.
3. Weather Forecasting.
Higher temperatures are assocaited with :
1. Low pressure
2. Upward motion of the current of air
3. Inflowing winds
Low temperatures are associated with :
1. High pressure
2. Downward motion of the current of air
3. Outflowing winds
Isobars : The pressure of air is shown on weather maps by means of lines known as isobars, meaning ‘equal weight.’
Major Pressure Belts of the Earth
Pressure belts are due to—
1. The unequal heating of the Earth and its atmosphere
2. The rotation of the Earth.
Major pressure belts on the Earth are:
1. Equatorial Low Pressure Belt : This belt extends upto 5O North and South of the Equator.
2. Sub-tropical High Pressure Belt : This belt coincides with latitudes 30O to 35O North and South of the Equator.
3. Sub-polar Low Pressure Belt : This belt lies between latitudes 60O and 65O North and South of the Equator.
4. Polar High Pressure Belt.
Winds
Air in motion is called wind. It is caused by the unequal heating of the Earth by the Sun.
Wind direction : The name of the wind is given according to the direction from which it blows.
Deflection of Winds : If the Earth was not rotating on its axis, all winds would blow straight from high pressure to low pressure and die down when the pressure equalized.
But due to the earth rotation winds curve as they low. This curving motion is called the Coriolis Effect.
Wind Speed : The speed of the wind has a great effect on our lives. When it blows gently, it refreshes us. When it blows strongly creating storms that cause widespread damage.
Trade Winds
1. Trade winds blow from the Sub-tropical High Pressure Belt to the Equatorial Low Pressure Belt in the tropics betweeen 30O North and 30O South latitudes.
2. Trade winds blow as the N.E. Trades in the Northern Hemisphere and as the S.E. Trades in the Southern Hemisphere.
3. Their strength and direction is regular.
4. Trade winds are permanent winds but in the Indian Ocean and some parts of the south-west Pacific Ocean, they get reversed in summer by the Monsoons.
5. As per the movement of the sun the belts of the Trade Winds move north and south.
6. The term ‘Trade’ means a nautical expression to blow along a regular path of ‘tread’.
Westerlies
1. They blow from west to east or the Sub-tropical High Pressure Belt to the Sub-polar Low Pressure Belt in the temperature latitudes between 30O and 60O, on either side of the Equator.
2. They blow from the South-West direction in the Northern Hemisphere and the North-West direction in the Southern Hemisphere.
3. Westerlies are not as constant in strength and direction as the Trade Winds. The areas of the Westerlies is characterised by frequent cyclones and anticyclones.
4. They are more constant and strong in the Southern Hemisphere. In some areas they become so strong that these winds are known as the Roaring Forties or the Brave West Winds, and the Furious Fifties.
5. As per the Sun’s movement, the belts of the Westerlies move north and south.
6. They blow out of the west so these are known as the Westerlies.
Polar Winds
1. Polar winds blow from the High Pressure Belt to the Sub-polar Low Pressure Belt between latitudes 60O and the poles on both sides of the equator.
2. These winds form the Polar Easterlies and blow from the east. In the Northern Hemisphere their direction is N.E. and S.E. in the Southern Hemisphere.
3. The direction of the polar winds changes as much as 90O from their normal course until they blow directly from the east.
4. These winds move in different directions due to local weather disturbances in the Northern Hemisphere.
5. In the Southern Hemisphere they are more regular.
6. Polar winds are very cold and dry as they blow from the ice-cap region.
Periodic Winds
Periodic winds are the land and sea breezes and the Monsoon winds.
Day : Sea Breeze
1. Land is heated quickly than the sea. The warm air above the land rises, creating a low pressure.
2. The adjoining region of the sea is more cooler. The cold air above the water is heavy and it creates a high pressure.
3. Air moves from high pressure region to low pressure region. Winds blow from the sea to the land. These are called sea breezes.
These winds blow during the day.
Night : Land Breeze
1. The sea during night remains warm while the land cools more quickly. There is low pressure over the sea.
2. Since the land cools more quickly so there is high pressure over the land.
3. During the night the wind blows from the land to the sea.
Monsoon Winds
These are seasonal winds and have a reversal in their direction from one season to another.
1. In summer they blow from the sea to the land.
2. In winter they blow from the land to the sea.
Variable Winds : Examples of variable winds are cyclones and anti-cyclones.
Cyclones : A cyclone is a small low pressure area with winds blowing anti-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Depressions : Depressions are the cyclones of the temperate latitudes. Winds with rain are moderate and do not cause any damage.
Hurricanes : Hurricanes are torpical cyclones in the Gulf of Mexico, the Caribbean sea and Western Pacific Ocean. The wind velocity in hurricanes goes over 119 km/hr. They cause big damage to the buildings and other property.
Typhoons : Typhones are tropical cyclones which start from China Sea. The winds of typhoons are very violent and cause severe damage to property.
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