The air above us has weight. At sea level, this weight exerts a pressure of 1kg per cm.sq. This is known as atmospheric pressure and we measure it in millibars, just as temperature is measured in degrees centigrade.
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Atmospheric pressure varies with altitude and temperature. The higher we go, the lighter the air and the less the remaining column of the atmosphere above, and therefore the less the pressure the atmosphere exerts. It has been found that pressure decreases at the rate of 100 millibar for every 1000m of ascent.
Also, when air is heated, it expands, becomes less dense and exerts less pressure on the surface. Therefore pressure decreases as the temperature increases, and vice versa.
Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.
The difference in pressure give rise to winds, and winds blow from region of high pressure to region of low pressure. In places where the pressure is high, more force is exerted by the atmosphere from above than in the surrounding areas where it is low. As a result, air escapes laterally from the high pressure belt to the low pressure belt. High pressure systems are “domes of density” that press down, while low pressure systems are similar to “atmospheric valleys” where the density of air is less. Since cool air has low capacity to hold water vapor as against warm air, clouds and precipitation are caused by cooling the air.
Reference
https://www.livescience.com/39315-atmospheric-pressure.html
https://www.nationalgeographic.org/encyclopedia/atmospheric-pressure/
http://people.umass.edu/~aciz000/Lecture%20Notes/lecture_notes_1.htm
