Low pressure area

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A large low-pressure system swirls off the southwestern coast of Iceland, illustrating the maxim that "nature abhors a vacuum." The vacuum in this case is a region of low atmospheric pressure. In order to fill this void, air from a nearby high-pressure system moves in, pulling in clouds along for the ride. And because this low-pressure system occurred in the Northern Hemisphere, the winds spun in toward the center of the low-pressure system in a counter-clockwise direction; a phenomenon known as the Coriolis effect (in the Southern Hemisphere, the Coriolis effect would be manifested in a clockwise direction of movement). September 4, 2003

A low pressure area, or a low for short, is a region where the atmospheric pressure is lowest with relation to the surrounding area. Tropical storms, extratropical cyclones, subpolar cyclones, and subarctic cyclones are called low-pressure cells in some English-speaking communities.

Lows are frequently associated with stronger winds and atmospheric lift. This lift will generally produce cloud cover, due to adiabatic cooling, once the air becomes saturated as it rises. Thus, low pressure typically brings cloudy or overcast skies, which may minimize diurnal temperature extremes in both summer and winter, due to the significant cloud cover. This is due to less incoming shortwave solar radiation and lower temperatures during the day, since the clouds reflect sunlight. At night, the absorptive effect of clouds on outgoing longwave radiation, such as heat energy from the surface, allows for warmer diurnal low temperatures in all seasons.

Climatologically, low pressure forms at the Intertropical convergence zone (ITCZ), as part of the Hadley cell circulation. Many of the world's rainforests are associated with these climatological low pressure systems. Frontal lows are temperate zone phenomena, and develop along the polar front as a result of the interaction between cold and warm surface air masses. Thermal lows also form over areas such as Death Valley as the result of intense ground heating; they are much smaller in geographic extent than either convergence lows or frontal lows.

Surface low pressure systems will tend to be smaller in area and have stronger surface winds than a given high pressure system, due to the addition of surface friction to the pressure gradient force, centrifugal force and coriolis effect that drive the circulation.

[edit] Thermal low

In deserts, lack of ground and plant moisture that would normally provide evaporative cooling can lead to intense, rapid solar heating of the lower layers of air. The hot air is less dense than surrounding cooler air. This, combined with the rising of the hot air, results in an isolated low pressure area called a thermal low.

[edit] See also

• High pressure area
• Cyclone