Atmospheric pressure
There is an uneven distribution of temperature, on the surface of the earth. This is because of the heating and as a result, the air expands and gets compressed on cooling.
Due to the compression and expansion of Air, variations occur in the atmospheric pressure. This variation in atmospheric pressure is the cause of the movement of air. The air moves from a high-pressure area to low pressure area.
The atmospheric pressure also determines, when will the air rise or sink. The heat and moisture are redistributed across the planet by Winds. As a result, Constant temperature is maintained on the whole planet.
The moist is raised vertically and gets cooled to form clouds, which brings down Precipitation in form of rain or snow.
Atmospheric Pressure
Every living being on the earth is subjected to air pressure. As one moves vertically, the air pressure gets varied and one gets difficulties in breathing.
Atmospheric Pressure is defined as “the weight of a column of air contained in a unit area from the mean sea level to the top of the atmosphere.” It is expressed in the units of Milibar.
Atmospheric Pressure a sea level is 1013.2 Milibar.
The air is denser at the surface of the earth and has higher pressure due to the effect of gravity.
Mercury Barometer or the aneroid barometer is used to measure Air pressure.
The air pressure decreases with height and at any height or elevation, the air pressure varies from place to place and its variation is the main reason for the air motion.
As a result, the wind moves from high pressure to low-pressure regions.
Vertical Variation of Pressure
The pressure decreases rapidly with height, in the lower atmosphere. For each 10m elevation in height, the air pressure decreases by 1 Mb. This decrease in air pressure is not always constant throughout the world.
| Level | Pressure in Mb | Temperature in Celsius |
| Sea Level | 1013.25 | 15.2 |
| 1 km | 898.76 | 8.7 |
| 5 km | 540.48 | -17.3 |
| 10 km | 265 | -49.7 |
The vertical pressure gradient force is greater than the horizontal pressure gradient. But, the pressure gradient force is balanced by an equal but opposite gravitation force. As a result, we dot experience any strong upward winds.
Horizontal Distribution of Pressure
The small difference in pressure is high, important in determining the wind direction and its velocity.
The horizontal distribution of pressure is studied by drawing isobars at constant levels.
Isobars
It is a line on the map that connects points have same atmospheric pressure at a given time or average period.
In order to eliminate the effect of height (altitude) on pressure, it is measured at any station after being reduced to sea level for comparison.
As seen in the above image, the patterns of the isobars are related to the pressure systems.
The Low-pressure system is enclosed by one or more isobars with the lowest pressure in the centre. The high-pressure system is also enclosed by one or more isobars with the highest pressure in the centre.
World Distribution of Sea Level Pressure
Near the equator sea level, the pressure is low. This area is called Equatorial Low. Along 30° N and 30°, S has high-pressure areas. This area is called Subtropical Highs.
Along the poles, 60° N and 60° S, these low-pressure belts area is called Sub Polar Lows. Near the Poles, the pressure is high and these areas are called Polar High.
These pressure belts are not permanent as it oscillates with respect to the movement of the Sun. The pressure belts move southwards during the winter and move northwards during the summer in the northern hemisphere.
Conclusion
In these notes we have discussed, Atmospheric pressure and its variation, such as Vertical variation and horizontal distribution, and also its worldwide distribution, with its pressure levels and belts.
References
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