Lecture – 3 Atmospheric Pressure (English)

 

Definition: - The air of the atmosphere is a mixture of different gases, which has its own weight, due to which the air exerts pressure on the lithosphere and the hydrosphere, this is called atmospheric pressure or air pressure.

þ  It is the total weight of all the layers of the atmosphere per unit area on the ground or sea level. The mean sea level is considered to be the zero level on which the atmospheric pressure is highest.

þ  One square cm of area is approximately equivalent to 1036 grams.

þ  About 50% of the total mass of air in the atmosphere is found within the height of the atmosphere up to 5 km.

þ  The average density of air, which is about 1.2 kg m -3 at the ground level, decreases with increasing altitude and remains only 0.7 kg m -3 at an altitude of 5 km.

v  Types of atmospheric pressure: -

1.   High air pressure

2.   Low air pressure

F There is an inverse or inverse relationship between air pressure and temperature.

F Winds flow from high pressure center to low pressure center.

F The winds move in the anticlockwise direction in the low-pressure centers, due to which the air around the center starts rising, which cools due to lower temperature in the height and increases the chances of rain in the low-pressure center.

F On the contrary, in high pressure the winds move in clockwise direction and move outward from the center causing the air towards the center to move downwards due to which the air gets heated.

F This is the reason why the sky around the high-pressure centers remains clear.

F Imaginary lines passing through places of equal air pressure are called isobaric lines.

F The gradient from higher air pressure to lower air pressure is called air pressure gradient.

v  Decrease in air pressure with altitude: -

Height above sea level (Km)	Air pressure (MB)	Decrease in air pressure (MB)
0.0	1013.25	------
0.50	954.61	58.64
1.00	898.76	55.85
2.00	845.59	53.17
2.50	795.01	50.58
3.00	746.91	48.10
25.00	25.49	29.80
30.00	11.97	13.52

v  Variations in atmospheric pressure: -

1.   Variation with altitude/vertical variation

F Air pressure depends on the density of the air.

F The density of air depends on the temperature, composition of the air and gravity.

F The air pressure at sea level is 1013.25 MB, 0.93 MB at an altitude of 50 km, 0.03 MB at an altitude of 80 km. It decreases by 34 MB at an average of 300 meters.

F Horizontal variation of pressure: - Horizontal variation of pressure depends on temperature, water vapor, and latitude. It has many belts like -

A.   Equatorial low-pressure belt: -

On either side of the equator, the air pressure is low up to 5 degree latitude, it is called Doldrum belt or calm belt.

B.   Sub-tropical high-pressure belt: -

It is found between 24-30 degree latitudes in both the hemispheres.

C.   Low pressure belt in both the hemisphere near 60 degree latitude: -The air from this region is thrown outwards due to the cyclic motion of the earth and belts of low mass are formed here.

D.  Polar high-pressure belts: -

Due to extreme temperatures in the polar regions, the air remains cold and heavy throughout the year. Therefore, the air pressure remains high in both the polar regions.

2.   Diurnal Variation: - At the meteorological station

Two maxima - one at 10 a.m. and one at 10 p.m.

Two minima - one at 4 in the evening and one at 4 in the morning are observed. This is done due to the alternate heating and cooling of the atmosphere.