Some facts about air pressure are the barometer is the instrument used for its measurement and these measurement readings are in units called millibars. There is a change in air pressure relative to altitude. At higher elevations, the air pressure is lower due to the presence of fewer air molecules.
Additionally, at high altitudes, there is also a decrease in oxygen levels. For this reason, mountain climbers at high altitudes can require supplemental oxygen. Ascending too quickly can also cause people to become sick with an illness called decompression sickness. Conversely, air pressure increases as one moves from a higher to a lower elevation.
Air pressure is the force exerted by air's weight on a particular surface. At sea level, the average atmospheric air pressure is given as 101.325 kPa, or 1 atmosphere is equal to 101.325 kPa. This is also equivalent to 101,325 pascals. One atmosphere is also equivalent to 1,013 millibars. This unit of measurement was named after the scientist Blaise Pascal. Barometers contain mercury in them and one atmosphere is equivalent to 760 millimeters, or approximately 29.92 inches, of mercury.
When the mercury in a barometer does not move more than 0.5 of an inch under the 29.92 mark, this can indicate calm weather. In meteorology, high pressure systems are associated with fair weather and cooler temperatures and low pressure systems with stormy weather.