Atmospheric pressure definition
Atmospheric pressure or barometric pressure, by definition, is the ratio of the force at which the atmospheric air column pushes the surface of the Earth (or other planet) to the surface on which this pole presses. In the mountains, the atmospheric pressure is lower and in the lowlands it is higher because the air column has different heights.
On the basis of the average value of atmospheric pressure on the Earth at sea level, the unit of pressure – atmosphere – was determined to be 1013,25 hPa. Atmospheric pressure can, however, change under the influence of weather phenomena.
Atmospheric pressure at sea level
Air pressure varies with altitude, this relationship is approximately decreasing exponentially and is determined by the approximate relationship referred to as the barometric formula. For small altitudes (up to 1000 m) above sea level it is assumed that the pressure drops linearly by 11.3 Pa per meter of height (11.3 hPa / 100 m). The pressure drop makes the air with altitude less and less cool. For example, the pressure at the top of Mount Everest (8,848 m above sea level) is about 310 hPa. However, half of the pressure from the sea level, or 500 hPa, occurs at a height of approx. 5600 m above sea level. The actual pressure converted to the sea level is called normalized pressure.
For atmospheric pressure measurements a mercury barometer is used.
Atmospheric pressure standard
The daily course of pressure is very diverse. Only during stabilized high-pressure weather the daily pressure fluctuations are regular. Within 24 hours, the atmospheric pressure reaches the highest values between 10:00 and 22:00, the lowest pressure is recorded between 4:00 and 16:00. Pressure changes during the year are analyzed on the basis of measurement data from many years. In the interior of the continents, we observe the highest pressure values in the winter due to the strong cooling of the land and the deposition of heavy air. In summer, however, the lowest pressure values occur in these regions. This is due to the strong heating of the land and the rising of warm air. Near the water reservoirs, the maximum pressure is observed at the beginning of the summer, because the air above the colder waters is heavier. However, the minimum pressure values in these areas occur in winter. The air lying over the warmer waters is then lighter than over land. A wobbly balance is created over heated areas, which triggers vertical, rising air movements. For this reason, the pressure is lowered over these areas, and its alignment is made by supplying air from colder, lower pressure areas. Uneven distribution of air temperature has a significant influence on the pressure distribution. In the temperate zone and especially in Western Europe, the highest pressure values in winter and in summer are observed twice a year. We observe the lowest pressure in spring and autumn. Atmospheric pressure values vary depending on latitude.