Atmospheric pressure it is the weight (force) of the air exerted per unit area on the earth’s surface.
It is mathematically expressed as:
Pressure = Force/Area
The weight of air is the vertical column exerted from the upper limit of the atmosphere to the earth’s surface which is approximately 1.034kg/cm3 over the sea level.
It is measured and recorded in units known as Millibars. However, the average pressure or weight of the air on the earth’s surface is measured in Millibars per unit area.
There’s an inverse relationship between the temperature and atmospheric pressure of a place.
Hot temperatures lead to low pressure while cold temperatures lead to high atmospheric pressure. When air is heated, the air molecules expand and spread over a wide area resulting into low pressure while low or cold temperatures result into contraction and condensation of air molecules thereby exerting high pressure on the earth’s surface.
For that matter, Polar regions are high pressure zones while equatorial areas are low pressure belts.
Atmospheric pressure decreases with an increase in altitude. This is because air at a high altitude spreads over a wide area which reduces its weight thereby causing low pressure.
So, high altitude areas like highlands / mountain tops have low pressure due to the gravitational force towards the low altitudes.
Low altitude areas such as foot hills and sea level on the other hand experience high pressure because the air near the ground supports the weight of air above it hence, the underlying or bottom air molecules constantly push downwards onto the earth’s surface. This is also due to the high concentration of air impurities like dust particles and carbon dioxide at the low altitudes.
As the earth rotates, air at the poles (North and South poles) is blown away towards the equator. It crosses parallels which are getting longer. The cold dense air at the poles crosses from the high latitudes towards the equator spreading over a wide area leading to low pressure. This accounts for the low pressure at the equator.
Air rising at the equator spreads out as it moves towards the poles. It crosses parallels that are getting shorter and contracts to occupy a small space. Its pressure therefore rises. This accounts for the high pressure at the horse latitudes.
Air pressure tends to increase away from the equator towards the Polar Regions.
The equatorial region experiences low air pressure because of the intense or high insolation (heat) from the overhead sun while Polar Regions that experience a low intensity of insolation experience high pressure
Land and water surfaces experience varying air pressure because of differences in the rate of heating and heat loss. This however affects atmospheric pressure at a local scale. During the day, land surfaces absorb heat faster than water surfaces leading to low pressure over the land and high pressure over the sea. Conversely at night, low pressure develops over the sea and high pressure over the land because the land surface cools faster than the water surface
Moist air is cold and dense hence it exerts high pressure on the earth’s surface while dry air with little or no moisture is warm and light (less dense) hence exerting low pressure on the earth’s surface
Low-pressure belts shift with the apparent movement of the overhead sun. When the sun is overhead in the Northern Hemisphere (Tropic of Cancer) in June – July, high temperatures are experienced in the north leading to low pressure and high pressure over the Southern Hemisphere. In December – January when the sun is overhead in the Southern Hemisphere (Tropic of Capricorn), temperatures rise and low pressure develops over the south while the north develops high pressure.