Atmospheric pressure, or air pressure, is a force that is exerted on you by the air molecules around and above you. These molecules cannot be seen, so you might assume that they have no detectable mass. Individually, the mass of each molecule is indeed negligible, but there are so many in the air around us that it adds up to a considerable level of atmospheric pressure.
However, there is also some space between the molecules of a gas, which allows them to be compressed into a smaller space, which increases the pressure.
Atmospheric pressure can be used to help with weather predictions, and it is one of the most useful measurements that can be made in this field. A barometer is used to measure atmospheric pressure, but as the values can vary according to altitude, the readings will be most accurate when used in conjunction with an altimeter. An area of high atmospheric pressure generally means clear skies and cooler temperatures, while a low atmospheric temperature will mean warm but wet weather, with a risk of storms.
Altitude has an effect on atmospheric pressure – the higher you are, the lower the pressure will be. This is because there are fewer air molecules above you that are pressing down. The concept is similar to hydrostatic pressure that is exerted by fluids; deep sea divers experience a greater pressure as they descend to deeper levels. At altitude, however, a reduced pressure means that you will need to breathe more rapidly. Changes in the atmospheric pressure, such as those you experience in an aeroplane when it takes off or lands, lead to your ears popping. This balances the pressure in your ears with that of the outside environment; this equalisation is necessary for you to be able to balance while standing and moving.