Air Pressure and Frontal Systems
Air pressure and fronts define arctic atmospheric circulation patterns. Air pressure is a measure of the force exerted by air on all objects. This force decreases rapidly with altitude: at sea level air pressure registers at about 1000 millibars, at altitudes of 5.5 kilometers air pressure registers at about 500 millibars. Air pressure is related to temperature. Given equal volumes, warm air is lighter (less dense) than cold air and consequently exerts less pressure.
Changes in air pressure often signify weather changes. Rising air pressure usually means fair weather, whereas falling air pressure generally signals stormy weather. This is because a drop in air pressure often indicates that a cyclone, or low pressure system, is moving into an area, bringing clouds and precipitation. A rise in air pressure frequently means that an anticyclone, or high pressure system is moving into an area, bringing fair weather.
Measuring Air Pressure
Air pressure is a force per unit area. Meteorologists measure air pressure in millibars (mb). Pressure readings are usually made with a mercury barometer or with an aneroid barometer.
Mercury barometers measure the height of a mercury column in a vertical glass tube. The closed upper end of the tube has a vacuum and its lower end opens into a small container of mercury. (When air pressure changes, the height of the mercury column changes, dropping as air pressure falls, and rising as air pressure increases.)
The aneroid barometer is less precise than the mercury barometer. It consists of a coil of tubing which has had most of the air evacuated. As air pressure changes, the coil flexes, bending in when pressure rises, and bowing out when pressure drops. The measurement is translated into equivalent millimeters or inches of pressure and displayed on a dial.
A more modern form of barometers employ a quartz oscillator circuit that changes frequency depending on atmospheric pressure. These barometers are used in unattended weather stations, such as the data buoys deployed by parachute from aircraft.
The Relationship of Air Pressure to Air Masses and Fronts
Air Masses
An air mass is a large volume of air that is relatively uniform (horizontally) in temperature and water vapor concentration over hundreds of kilometers. Air masses are generally identified with the regions over which they develop. Two examples are continental polar and maritime tropical air masses. While air masses can persist over their formative regions for a considerable length of time, they often move across regions. As air masses move from one region to another, the air mass characteristics are modified by the underlying surface. For instance, as cold, dry "arctic air" moves over an ocean surface it gains heat and moisture.
Fronts
A change in air pressure signals the advance or passage of a front. Fronts are the narrow transition-zone boundaries separating two air masses of contrasting properties (temperature and humidity). Low pressure systems often develop along frontal boundaries and cloud cover and precipitation often occurs in the vicinity of fronts. Four main frontal types are described below:
A cold front occurs when a cold air mass advances and replaces a warm air mass. Advancing cold fronts force warm moist air to rise sharply, producing showers and thunderstorms during the warm season, and snow during the cold season. As a cold front passes, temperature and humidity drops and air pressure rises.
A warm front occurs when a cold air mass retreats and is replaced by a warmer, generally more humid air mass. As a warm front passes, temperature and humidity rise. The passage of a warm front often implies that a cyclone is approaching and pressures may fall.
A stationary front occurs when a cold air mass and warm air mass meet, but neither moves much in any direction. Cloudiness and light to moderate precipitation may persist for days on the cold side of a stationary front as the warm air gradually rises over the cold air.
An occluded front occurs as a cold front overtakes a warm front, and forces the warm air to rise. The cool air mass remains at the surface. Low clouds and light precipitation usually accompany the passing of an occluded front.
The Synoptic Meteorology section contains an example of a synoptic chart. The following diagram shows symbols used on weather maps for different frontal types.
Air Pressure and Frontal Systems in the Arctic
Several fronts and semipermanent high and low pressure systems characterize the Arctic. The "polar front" marks the boundary between cold polar air masses and warm tropical air masses. The polar front is intermittent rather than continuous around the globe. The strength of the polar front depends on the magnitude of the horizontal temperature gradient across the front. Where the temperature gradient is steep, the front is strong and is a potential site for cyclone or low pressure system development. Where temperature contrast is small, the polar front is weak.
Like the polar front, the "arctic front" is discontinuous and depends on the temperature contrast between two air masses. The arctic front is the boundary between polar and arctic air masses and lies to the north of the polar front. The arctic front can be as strong as the polar front. It is particularly prominent during summer in northern Eurasia.
Semipermanent high and low pressure systems ("highs" and "lows") are identified with particular regions and have seasonal characteristics. In winter, the Icelandic Low extends from near Iceland north into the Barents Sea, and is associated with frequent cyclone activity. The Aleutian Low is present in the Gulf of Alaska. The Beaufort-Chukchi Sea region is dominated by a ridge of high pressure linking the Siberian High and high pressure over the Yukon of Canada. In April and May arctic pressure gradients decrease. The Icelandic and Aleutian lows weaken. The Siberian High disappears, and is replaced by a wide but shallow low. The Arctic High is centered over the Canadian Arctic Archipelago. In summer, pressure gradients are generally weak. Intermittently, however, cyclones enter the Arctic from northern Eurasia and the north Atlantic, and tend to persist over the Canadian Basin. By October the pattern has almost returned to the winter configuration. The Icelandic and Aleutian lows strengthen, as does the Siberian High.
These highs and lows are treated more fully in:
Other arctic atmospheric circulation patterns are:
A description of the seasonal cycle of air pressure based on gridded sea level pressure fields has been drawn from the data section of the Atlas.