Air Pressure is one of the important characteristics of the Earth's Atmosphere which regulates the wind and weather pattern across the world. The Pressure and gravity have very close affinity because atmospheric pressure depends upon several factors such as height, gravity and absolute temperature, etc. Gravity exerts a pull on the planet's atmosphere just as it keeps us attached to its surface. Therefore, this gravitational force pulls everything its surrounds, the pressure rising and falling as the Earth turns.
The weight of air molecules pressing down on the Earth is known as Air Pressure. The pressure of the air molecules changes as we move upward from sea level into the atmosphere. The highest pressure is at sea level where the density of the air molecules is the greatest. In other words, air pressure is the force per unit of area exerted on the Earth's surface by the weight of the air above the surface.
The molecules presented in the air with their respective size, motion and number exerted force. These factors are the characteristics of the air, which determine the temperature and density of the air and finally pressure.
Hence, we can say that the number of air molecules above the particular surface determines the air pressure. If number of molecules increases, they exert more pressure and as a result, we experience high atmospheric pressure. Similarly, if the number of molecules decreases, air pressure decreases.
Air pressure is measured by a barometer. It is not uniform at every part of the Earth. The normal range of air pressure on the Earth varies from the 980 millibars (mb) to 1050 millibars (mb). This is because Earth is heated by Sun unequally across the globe. These differences are the result of low and high air pressure systems, which are caused by unequal heating across the Earth's surface and the pressure gradient force.
Note: As we know that pressure is the extended force on a surface per unit area. The SI unit of pressure is Pascal (Pa) whic is equivalent to one newton per meter square.
It is an area where the atmospheric pressure is lower than that of surrounding locations which causes high winds, warm air and atmospheric lifting produces cloud, precipitation and sometime turbulent weather such as tropical storms and cyclone. It is also called a depression.
The low pressure areas have moderate temperature or moderate seasonal temperature because cloud present over it reflects the incoming solar radiation back to the atmosphere. So, areas prone to low pressure cannot warm as during the day and at night these cloud act like blanket, trapping heat below.
An area where the atmospheric pressure is greater than surrounding area is known as high-pressure system. This system moves clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere due to Coriolis Effect.
It is associated with light winds at the surface and subsidence through the lower portion of the troposphere. When pressure increases, more air fills the space left from the low and subsidence evaporates most of the atmosphere's water vapour. Hence, areas with high pressure systems are usually with clear skies and calm weather.
Unlike areas of low pressure, the areas with high-pressure experience extremes in diurnal and seasonal temperatures due to the absence of cloud. Since there are no clouds to block incoming solar radiation or trap outgoing longwave radiation at night.
Hence, we can say that the Atmospheric pressure is an indicator of weather. The movement of the wind is caused by air, trying to balance the difference between high and low pressure areas by moving from high pressure areas to low pressure areas. It carries moisture and heat horizontally. Because of Earth’s rotation, wind blowing north/south will deflect to the right. It gives twirl effect to the air, causing the flow around pressure centres, storm, hurricanes, and tornadoes.