The pressure at any level in the atmosphere may be interpreted as the total weight of the air above a unit area at any elevation. At higher elevations, there are fewer air molecules above a given surface than a similar surface at lower levels. Since most of the atmosphere's molecules are held close to the earth's surface by the force of gravity, air pressure decreases rapidly at first, then more slowly at higher levels.

Barometric pressure is the amount of pressure exerted by the weight of the air above . At sea level, the atmosphere is about 24 km, which is equal to the barometric pressure of 760 mm of mercury. At the top of Everest, the air exerts very little pressure compared to the sea level, and the barometric pressure decreases to about 250 mm of mercury, or about one third of the barometric pressure of the sea level.

However, at a given altitude, the barometric pressure is not exactly the same everywhere. Since the weight of warm air is less than the weight of the head air, the barometric pressure depends on the air temperature. On the peak of Everest, the barometric pressure in June is 255 mm Hg. is, while in January it is 243 mm of mercury

Although there is an inverse relationship between barometric pressure and altitude, the composition of air gases does not change. The distribution percentage of oxygen, carbon dioxide and nitrogen in the air is always 20.93, 0.03 and 79.04%, respectively. What changes is only the partial pressure of the gases, which It is caused by the function of barometric pressure.

Dr Fatem Janbozorgi thank you for your contribution to the discussion

As altitude increases, the amount of air over a unit area decreases. Therefore, the atmospheric pressure will reduce due to lower air molecules. In most circumstances, atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. As elevation increases, there is less overlying atmospheric mass, so atmospheric pressure decreases with increasing elevation.The atmospheric pressure changes with altitude. As the altitude increases, the pressure decreases. Back on Earth, as elevation increases, the number of molecules decreases and the density of air therefore is less, which means there is a decrease in air pressure. With an increase in altitude, air becomes thinner and less dense. Therefore, the weight of the air above the denser air below is smaller, which causes a decrease in the air pressure at high altitude.As altitude increases, the amount of gas molecules in the air decreases the air becomes less dense than air nearer to sea level. This is what meteorologists and mountaineers mean by "thin air." Thin air exerts less pressure than air at a lower altitude. Near Earth's surface the pressure decreases with height at a rate of about 1 millibars for every 10 metres. As altitude increases, atmospheric pressure decreases. One can calculate the atmospheric pressure at a given altitude. Temperature and humidity also affect the atmospheric pressure. Pressure is proportional to temperature and inversely proportional to humidity. The relationship between the atmospheric pressure and the temperature of a place is directly proportional to each other. The temperature of a place increases as the atmospheric pressure of that place rises. On the other hand, the temperature of a place decreases as the atmospheric pressure of the place falls. Thus the air temperature is highest near the surface and decreases as altitude increases. The speed of sound depends on the temperature and also decreases with increasing altitude. The pressure of the air can be related to the weight of the air over a given location. Since the number of air molecules above a surface decreases with height, pressure likewise decreases with height. Most of the atmosphere's molecules are held close to the earth's surface by gravity. Because of this, air pressure decreases rapidly at first, then more slowly at higher levels. Temperature normally decreases as elevation or height increases, making locations at higher elevations colder. For every 100-meter increase in elevation, the average temperature decreases by 0.7°C. Even in areas located near the equator, the temperature at higher elevations is cooler. The density of the air does not change. If the temperature of the air increases, the atmospheric pressure must decrease in order to keep the density of the air constant. Thus, the atmospheric pressure decreases due to increase in the air temperature.