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# Atmospheric Pressure

## Air Pressure At Sea Level

Mean sea level pressure (MSLP) is the pressure at sea level or (when measured at a given altitude on land) the center pressure reduced to sea level, assuming an isothermal layer at the center temperature.

This is the pressure commonly given in weather reports on radio, television, newspapers and the Internet.

Lowering to sea level means that the range of normal fluctuations in pressure is the same for everyone. The pressures that are considered high pressure or low pressure do not depend on geographic locations. This makes isometrics a meaningful and useful tool on a weather map.

The (QNH) or (QFE) set altimeter adjustment in the aviation industry is another atmospheric pressure lowered to sea level, but the way this decrease is characterized is somewhat different.

• QNH :- The altimeter will read the altitude when it is in airspace due to the barometric altimeter adjustment. Under ISA temperature conditions the altimeter will read the altitude above mean sea level in the surrounding airspace.
•  QFE :- Due to the barometric altimeter adjustment, the reference base of a specific airspace (usually the threshold of an airstrip) will read zero. Under ISA temperature conditions the altimeter will read the base altitude in the surrounding airspace.

QFE and QNH are an arbitrary Q code, not an abbreviation, but the mnemonics “nautical altitude” and “field altitude” are often used by pilots to differentiate the two.

The mean sea level pressure is 101.325 kPa (1013.25 millibar, or hPa) or 29.921 inches of mercury (inHg) or 760 millimeters (mmHg). The QNH in Aviation Weather Reports (METAR) is broadcast worldwide in millibars or hectopascals (1 millibar = 1 hectopascal), except in the United States, Canada, and Colombia, where it is measured in inches of mercury. is expressed in (up to two decimal places).

The United States and Canada also report sea level pressure (SLP), but which is reduced to sea level by a different method, in the comments section, in hectopascals or millibars, not the internationally circulated portion of the code.

However, Canada’s public weather reports have sea level pressure in kilopascals instead, while Environment Canada’s standard unit of pressure is the same Archived 2007-03-11 at the Wayback Machine.

The weather code requires three digits; The decimal point and one or two most significant digits are omitted; 1013.2 millibars or 101.32 kPa is transmitted as 132; 1000.0 millibars or 100.00 kPa is transmitted as 000; 998.7 millibars or 99.87 kPa is transmitted as 987.

The highest sea-surface pressure on Earth occurs in Siberia, where sea level pressure in the Siberian International Sea often exceeds 1050.0 millibar. Minimum measurable sea-surface pressure is found in the centers of tropical cyclones and tornadoes.

### Low Pressure System Weather

A low pressure system, also called a depression, is an area where the atmospheric pressure is lower than that of its surroundings.  Lows are usually associated with higher winds, warmer air, and atmospheric lifting. (High Pressure System)  Under these conditions, lows typically produce clouds, rainfall, and other turbulent weather, such as tropical storms and cyclones.

Low pressure areas have neither extreme integers (day versus night) nor extreme seasonal temperatures because clouds in such areas reflect incoming solar radiation back into the atmosphere. As a result, they cannot heat up during the day (or in the summer), and at night, they act as a blanket, trapping the heat below.

#### High Pressure System

A high pressure system, sometimes called an anticyclone, is a region where the atmospheric pressure is higher than that of the surrounding region. These systems move clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere due to the Coriolis effect.

High-pressure areas are normally caused by a phenomenon called subduction, which means that as the air in the high cools, it condenses and moves toward the ground.  Here the pressure increases as more air fills the space left with less.  Most of the water vapor in the atmosphere also evaporates, so high pressure systems are usually associated with clear skies and calm weather.

Unlike areas of low pressure, the absence of clouds means that areas experiencing high pressure are prone to extreme precipitation and seasonal temperature extremes because there are no clouds to block incoming solar radiation or because of long periods of nightfall. Trapping radiation for a long time.

[Air Pressure At Sea Level | Low Pressure System Weather]

Categories: Earth Science