Column of Heavy air that pushes down on Earth. Heaviest at sea level (1 atm of pressure) because more air above.
This column of heavy air is known as the atmosphere. The atmosphere is composed of several layers, with the lowest layer being the troposphere where most weather events occur. The weight of the air above exerts pressure on the surface of the Earth, with the pressure being highest at sea level where there is more air above pushing down.
Atmospheric pressure is typically measured in units of atmospheres (atm), with one atmosphere of pressure being equivalent to the pressure exerted by a column of mercury 760 millimeters high at 0 degrees Celsius at sea level. The average atmospheric pressure at sea level is about 1 atm.
Changes in atmospheric pressure can affect weather patterns, as areas of high pressure typically bring fair weather while areas of low pressure are associated with stormy conditions. Monitoring changes in atmospheric pressure is an important aspect of weather forecasting.