Atmospheric pressure refers to a mass of air in the atmosphere, exerting weight over a unit of area. The pressure of air at any level in the atmosphere is measured by the total mass of air above any point. The density of air is greatest at the land's surface and decreases with altitude, thus, atmospheric pressure decreases with increasing height, rapidly at first, and then gradually slows with higher altitude. Meteorologists often measure air pressure in millibars (Mbar), thus the air pressure at sea level is 1013.25 Mbar. Another unit of measurement for air pressure, used in aviation and weather broadcasts, is inches of mercury (inHg). At sea level, the pressure of air, in inches of mercury is 29.92 inHg.
The Earth's atmosphere
The atmosphere extends for many hundreds of kilometers (km), gradually becoming thinner with greater altitude. Air pressure decreases gradually with altitude because air is most concentrated at the Earth's surface. The air molecules in the atmosphere are continuously moving and colliding with each other. Their density decreases rapidly in the first 100 km (62 miles), and then slower upwards up to 500 km (310 miles). All of these air molecules are attracted towards the Earth by the force of gravity; hence, the greatest air pressure is at the surface. The summit of Mount Everest, at 8,848 meters (29,029 feet) of altitude, lies over 70% of all the atmosphere's air content. The air pressure at the summit is about 300 Mbar.
Measuring air pressure
Atmospheric pressure is usually measured using a barometer. The barometer measures in bars, which is the force of 100,000 newtons exerting a force over an area of one square meter; however, since the bar is a very large unit, the unit of measurement most commonly used in weather is the millibar. One millibar is the same as one thousandth of a bar. The hectopascal, which is another unit is measurement, is gradually replacing the millibar as the unit of pressure on weather maps. Another unit of pressure used in aviation is inches of mercury. Thus, at sea level, the standard atmospheric pressure is 1013.25 millibars, which is the same as 1013.25 hectopascals or 29.92 inches of mercury.
High pressure and low pressure regions
A high pressure area is a region where atmospheric pressure is greater than the air surrounding it. Winds inside a high pressure system tend to flow towards lower pressure systems. The strongest high pressure systems are associated with cold air masses from the poles. Low pressure area is a region where atmospheric pressure is lower than the air surrounding it. Low pressure systems develop under regions of wind divergence, where the upward direction of the wind lowers surface temperatures. Low pressure systems form when localized heating warms the air, causing it to rise and lower atmospheric pressure at that region of the Earth's surface.
Local pressure systems
Pressure systems cause the daily weather that is experienced locally. Low pressure systems most commonly cause the development of clouds and rainfall, while high pressure systems are associated with dry weather and clear skies with greater nighttime temperature changes due to infrared radiation and significant amounts of sunshine during the day. Atmospheric pressure varies significantly and affects the weather experienced around the world. Atmospheric pressure variations affect strongly the tropical zones, where low pressure systems develop, causing warm air to rise. The high pressure systems from the poles eventually replace these low pressure systems, forming air circulation patterns.
Low pressure areas are characterized by having less air mass above them, while high pressure areas differ, from low pressure areas, by having more air mass above them. Differences in temperature cause these two masses to flow into either direction from each other, producing winds. Air pressure is the force that is exerted on a given area as a measurement of weight. Air has weight and takes up space, but it can be compressed to take up space into a smaller volume.