Atmospheric Pressure - Notes | Geography for UPSC 2024 (Pre & Mains) PDF Download

 Page 1


 
ATMOSPHERIC PRESSURE  
? 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. 
? Air pressure is measured with the help of a mercury barometer or the 
aneroid barometer 
? Due to gravity the air at the surface is denser and hence has higher 
pressure.  
 
VERTICAL VARIATION OF PRESSURE 
? In the lower atmosphere the pressure decreases rapidly with 
height. 
? The decrease amounts to about 1 mb for each 10 m increase in 
elevation. 
? 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. 
? At any elevation, pressure varies from place to place and 
variation is the primary cause of air motion i.e. winds move from 
high pressure to low pressure areas.  
 
ADDITIONAL INFO: 
? A rising pressure indicates fine, settled weather, while a falling 
pressure indicates unstable and cloudy weather. 
 
? 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.  
 
Page 2


 
ATMOSPHERIC PRESSURE  
? 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. 
? Air pressure is measured with the help of a mercury barometer or the 
aneroid barometer 
? Due to gravity the air at the surface is denser and hence has higher 
pressure.  
 
VERTICAL VARIATION OF PRESSURE 
? In the lower atmosphere the pressure decreases rapidly with 
height. 
? The decrease amounts to about 1 mb for each 10 m increase in 
elevation. 
? 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. 
? At any elevation, pressure varies from place to place and 
variation is the primary cause of air motion i.e. winds move from 
high pressure to low pressure areas.  
 
ADDITIONAL INFO: 
? A rising pressure indicates fine, settled weather, while a falling 
pressure indicates unstable and cloudy weather. 
 
? 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.  
 
 
HORIZONTAL DISTRIBUTION OF PRESSURE  
? Horizontal distribution of pressure is studied by drawing isobar  
? Isobars are lines connecting places having equal pressure reduced to 
sea level  
? 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.  
 
 
? 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. 
Page 3


 
ATMOSPHERIC PRESSURE  
? 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. 
? Air pressure is measured with the help of a mercury barometer or the 
aneroid barometer 
? Due to gravity the air at the surface is denser and hence has higher 
pressure.  
 
VERTICAL VARIATION OF PRESSURE 
? In the lower atmosphere the pressure decreases rapidly with 
height. 
? The decrease amounts to about 1 mb for each 10 m increase in 
elevation. 
? 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. 
? At any elevation, pressure varies from place to place and 
variation is the primary cause of air motion i.e. winds move from 
high pressure to low pressure areas.  
 
ADDITIONAL INFO: 
? A rising pressure indicates fine, settled weather, while a falling 
pressure indicates unstable and cloudy weather. 
 
? 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.  
 
 
HORIZONTAL DISTRIBUTION OF PRESSURE  
? Horizontal distribution of pressure is studied by drawing isobar  
? Isobars are lines connecting places having equal pressure reduced to 
sea level  
? 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.  
 
 
? 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. 
 
 
 
FACTORS CONTROLLING PRESSURE SYSTEMS 
THERMAL FACTORS 
When air is heated, it expands and, hence, its density decreases. This 
naturally leads to low pressure.  
Page 4


 
ATMOSPHERIC PRESSURE  
? 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. 
? Air pressure is measured with the help of a mercury barometer or the 
aneroid barometer 
? Due to gravity the air at the surface is denser and hence has higher 
pressure.  
 
VERTICAL VARIATION OF PRESSURE 
? In the lower atmosphere the pressure decreases rapidly with 
height. 
? The decrease amounts to about 1 mb for each 10 m increase in 
elevation. 
? 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. 
? At any elevation, pressure varies from place to place and 
variation is the primary cause of air motion i.e. winds move from 
high pressure to low pressure areas.  
 
ADDITIONAL INFO: 
? A rising pressure indicates fine, settled weather, while a falling 
pressure indicates unstable and cloudy weather. 
 
? 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.  
 
 
HORIZONTAL DISTRIBUTION OF PRESSURE  
? Horizontal distribution of pressure is studied by drawing isobar  
? Isobars are lines connecting places having equal pressure reduced to 
sea level  
? 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.  
 
 
? 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. 
 
 
 
FACTORS CONTROLLING PRESSURE SYSTEMS 
THERMAL FACTORS 
When air is heated, it expands and, hence, its density decreases. This 
naturally leads to low pressure.  
 
On the contrary, cooling results in contraction. This increases the density 
and thus leads to high pressure. 
Formation of equatorial low and polar highs . 
 
 
 
DYNAMIC FACTORS 
Apart from variations of temperature, the formation of pressure belts may be 
explained by dynamic controls arising out of pressure gradient forces and 
rotation of the earth (Coriolis force). 
The rest of the belts are examples of dynamic factors. ( to be discussed in 
some other lecture). 
Page 5


 
ATMOSPHERIC PRESSURE  
? 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. 
? Air pressure is measured with the help of a mercury barometer or the 
aneroid barometer 
? Due to gravity the air at the surface is denser and hence has higher 
pressure.  
 
VERTICAL VARIATION OF PRESSURE 
? In the lower atmosphere the pressure decreases rapidly with 
height. 
? The decrease amounts to about 1 mb for each 10 m increase in 
elevation. 
? 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. 
? At any elevation, pressure varies from place to place and 
variation is the primary cause of air motion i.e. winds move from 
high pressure to low pressure areas.  
 
ADDITIONAL INFO: 
? A rising pressure indicates fine, settled weather, while a falling 
pressure indicates unstable and cloudy weather. 
 
? 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.  
 
 
HORIZONTAL DISTRIBUTION OF PRESSURE  
? Horizontal distribution of pressure is studied by drawing isobar  
? Isobars are lines connecting places having equal pressure reduced to 
sea level  
? 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.  
 
 
? 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. 
 
 
 
FACTORS CONTROLLING PRESSURE SYSTEMS 
THERMAL FACTORS 
When air is heated, it expands and, hence, its density decreases. This 
naturally leads to low pressure.  
 
On the contrary, cooling results in contraction. This increases the density 
and thus leads to high pressure. 
Formation of equatorial low and polar highs . 
 
 
 
DYNAMIC FACTORS 
Apart from variations of temperature, the formation of pressure belts may be 
explained by dynamic controls arising out of pressure gradient forces and 
rotation of the earth (Coriolis force). 
The rest of the belts are examples of dynamic factors. ( to be discussed in 
some other lecture). 
 
 
WORLD DISTRIBUTION OF SEA LEVEL PRESSURE  
PRESSURE BELTS IN JULY 
In the northern hemisphere, during summer, with the apparent northward 
shift of the sun, the thermal equator (belt of highest temperature) is 
located north of the geographical equator. 
The pressure belts shift slightly north of their annual average locations. 
 
PRESSURE BELTS IN JANUARY 
During winter, these conditions are completely reversed and the pressure 
belts shift south of their annual mean locations. Opposite conditions prevail 
in the southern hemisphere. The amount of shift is, however, less in the 
southern hemisphere due to predominance of water. 
Similarly, distribution of continents and oceans have a marked influence 
over the distribution of pressure.