The main meteorological element that is the cause of the occurrence of air movements in the atmosphere is atmospheric pressure. The atmosphere with its mass exerts a certain pressure on the Earth’s surface. This weight is called atmospheric pressure.

Atmospheric pressure is the thrust exerted by a column of air above a unit of the Earth’s surface by its weight. Atmospheric pressure is expressed in hectopascals (hPa). Historically, the commonly used unit of pressure was millimeters of the column of mercury (mmHg). 1hPa is 0.75 mmHg. A mercury barometer is used to measure atmospheric pressure. It is an instrument in the shape of a glass tube closed on one side and filled with mercury. Above the mercury is a vacuum.

Atmospheric pressure
Atmospheric pressure

Atmospheric pressure acting on the mercury through the open end of the tube causes it to move up or down causing a consequent change in the height of the column of mercury. Mercury also expands when exposed to heat, so at different temperatures the same pressure will correspond to a different height of the column of mercury. Therefore, all atmospheric pressure readings using tables are reduced to an air temperature of 0°C. Due to the different values of the force of gravity acting on the mercury in different latitudes, barometer readings were also adopted to reduce to 45° of geographic res. Changes in pressure are recorded by an aneroid. The main part of this instrument is a resilient metal can in which air is contained. Under the influence of changes in atmospheric pressure, it is deformed.

The daily course of pressure varies greatly. Only during stabilized high weather are daily pressure fluctuations regular. Over the course of the day, atmospheric pressure reaches its highest values at 10:00 a.m. and 10:00 p.m., the lowest pressure is recorded at 4:00 a.m. and 4:00 p.m. Changes in pressure over the year are analyzed on the basis of measurement data from many years. In the depths of the continents, the highest pressure values are observed in winter due to the strong cooling of the land, and the settling of heavy air. In summer, on the other hand, these regions experience the lowest pressure values. This is due to the strong heating of the land, and the rising of warm air. In the vicinity of water reservoirs, pressure maxima are observed at the beginning of summer, as the air over the waters, which are cooler in relation to the land, is heavier.

Pressure minima in these areas, on the other hand, occur in winter. The air over warmer waters is then lighter than over land. An unstable equilibrium is created over the heated areas, which triggers vertical, upward air movements. For this reason, over these areas, the pressure decreases, and its equilibration is accomplished by the inflow of air from cooler areas with lower pressure. The uneven distribution of air temperature greatly affects the distribution of pressure. In the temperate zone and especially in Western Europe, the highest pressure values are observed twice a year in winter and summer. The lowest pressure is observed in spring and autumn.

Atmospheric pressure values vary according to latitude. At a latitude of 60°N in winter, the average pressure values for one month are 50hPa, while at a latitude of 10°N they are only 7hPa. Annual temperature fluctuations are also greater for higher latitudes than for circumpolar areas. The polar regions record the highest pressure values. Atmospheric pressure also varies with altitude. The number of air molecules changes at different altitudes. As altitude increases, the number of air molecules decreases, so air pressure also decreases with altitude.

As altitude increases, the weight and thickness of the atmospheric layer also changes. In order for pressure measurements at different altitudes to be comparable, they must be made independent of altitude, that is, brought to a single level. Therefore, pressure measurements used to be given in values reduced to sea level. Bringing the pressure to sea level (as is done when reducing the pressure value to 0°C) is based on a theoretical calculation of what the pressure would be if the barometer were at sea level. To calculate this value, the baric degree is used, which determines the altitude it takes for the pressure to decrease or increase by a unit (1hPa). Thus, this is the change in altitude corresponding to a 1hPa change in atmospheric pressure. The baric degree is inversely proportional to the value of pressure, and directly proportional to the value of temperature.

As the temperature increases, the baric degree increases. At 0°C, its value is about 8 m, while at an altitude of 5 km – about 16 m. In warm air masses it is necessary to overcome a higher altitude to achieve a pressure change of 1hPa. Therefore, in the upper layers of the troposphere, the pressure is higher over the equator than over the poles. Thus, an increase in air temperature causes a decrease in atmospheric pressure, while a decrease in air temperature causes an increase in atmospheric pressure. Atmospheric pressure decreases with altitude by an average of 11.5hPa for every 100m. In the lowest layer of the troposphere, pressure changes by an average of 1hPa for every 8 meters of altitude.

Lines with equal pressure values are isobars. The pressure values represented by these lines refer to sea level. Isobars are determined from average pressures and represent the horizontal distribution of pressure. If these lines run densely, it indicates a clear change in pressure over a short distance, while if they are distributed sparsely, then a small difference in pressure is observed. Isobaric maps are maps that show the distribution of pressure at sea level in winter, and in summer. They give a general idea of the formation of pressure, which has a very strong influence on the formation of weather processes.

The non-uniform heating of the Earth’s surface, and consequently of the air, causes differences to form in the distribution of atmospheric pressure. As a result, high and low pressure systems form in the atmosphere. A low-pressure system, also known as an atmospheric low, or cyclone, is an area where pressure decreases toward the center. Such a system on a synoptic map is formed by closed isobars surrounding an area of relatively low pressure. In the northern hemisphere, the wind blows counterclockwise around the low. A low-pressure system develops where warm and moist air rises from the Earth’s surface. Air near the center of a low-pressure system is unstable. Being warm and moist, it rises upward while cooling down. This leads to the formation of clouds, and often rain, or snow. In the northern hemisphere, winds blowing away from the center of the low bring cool air to the west and north, and warm air to the east and south of it.

A high-pressure system, also called a high, or anticyclone, is an area where pressure rises toward the center. It is an area where there are descending (falling) air movements. It is usually accompanied by stabilized, beautiful weather. Compared to low-pressure systems, wyes tend to occupy larger areas, are less mobile and last longer. Baric ovals are formed by huge masses of settling air. As the air settles and atmospheric pressure increases, air temperature increases, and humidity decreases. Warm, descending air causes the existence of a constant equilibrium in the atmosphere.

During the day, thanks to sunny weather, the land surface and lower layers of air heat up strongly, while at night there is a significant loss of heat, thanks to cloudless skies. High-pressure systems are characterized by high daily air amplitudes. Warm air near the Earth’s surface does not rise high. This inhibits the formation of high clouds. For this reason, in areas of anticyclones we usually have warm and cloudless weather lasting up to several weeks. Anticyclones are much larger in size than cyclones and can block the path of movement of lows. This causes a delay in the arrival of worse weather. In the northern hemisphere, the wind blows around the high clockwise from the center toward the periphery. As a result, it brings relatively cool air from the north on the eastern side of this system; while on the western side it brings relatively warm air from the south. There are no atmospheric fronts in high-pressure systems.

In addition to highs and lows, other atmospheric pressure systems are also encountered. One of these is the low-pressure gulf. It is the peripheral part of the low pressure system. It forms V-shaped elongated isobars that cut into the area of higher pressure. A low-pressure gulf is an elongated area of low atmospheric pressure in which a particularly low horizontal barometric gradient (a change in pressure over a certain unit of distance) is observed. A gulf can form in the lower layers of the atmosphere and at higher altitudes. It then has a significant impact on the course of weather near the earth’s surface. determines the formation of the low and its movement, as well as the location of clouds and precipitation, which form on the eastern side of the gulf axis. On the western side of the gulf axis, the air is usually cooler and drier and tends to fall. A baric low usually forms on the eastern side of a low-pressure gulf. When a low-pressure gulf occurs as an area not enclosed by reduced pressure isobars between two highs, then it takes the form of a furrow.

A high-pressure wedge is a system in which the high enters the lower pressure area and takes on a U-shape. This system is characterized by a very small barometric gradient. Sunny and dry weather usually occurs on the eastern side of the wedge’s axis, while cloudy and steamy weather occurs on its western side. This condition is caused by the fact that air tends to fall on the eastern side of the system in question, which inhibits the formation and development of clouds. A high-intensity high-pressure wedge often brings very warming weather in summer and mild weather in winter. An elongated transition area characterized by higher pressure between two areas of lower pressure is a dike, or ridge.

To define horizontal pressure changes over a certain distance, the concept of horizontal barometric gradient is used. It defines the value of the pressure change over a certain unit of distance, in the horizontal surface in the direction of the greatest pressure drop. Currently, this unit is 100 km. Thus, the horizontal pressure gradient tells us what is the pressure difference at a distance of 100 km on the horizontal surface. This magnitude on barometric maps is denoted by a vector, perpendicular to the isobars, pointing in the direction of lower pressure In temperate latitudes it is G= -2hPa, while in tropical cyclones its values are the highest and reach tens of hectopascals per 100 km. The vertical baric gradient, on the other hand, determines the pressure difference in the same vertical in hectopascals per 100 meters of altitude.

The world’s highest atmospheric pressure value was recorded on December 19, 2001 in Tosontsengel, Mongolia. At that time, the atmospheric pressure was 1086hPa. The lowest atmospheric pressure of 870hPa, caused by the passage of Typhoon Tip, was recorded on October 12, 1979 in the Pacific.

Changing atmospheric pressure values also adversely affect human health. A decrease or increase in pressure exceeding 8hPa from day to day significantly reduces well-being. These differences generally occur with the passage of atmospheric fronts. The most active in this regard are cold fronts, which, in addition to changes in pressure, are accompanied by a drop in air temperature. It adversely affects the functioning of the cardiovascular system. Thunderstorms have a similar effect because they strongly affect the human nervous system. Thunderstorms also often accompany cool fronts.