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.
Climatology
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.
Weather
Low pressure area is commonly associated with bad weather, while
high pressure area is associated with plenty of sunlight or good weather. On the land or on the sea surface, after getting too much heat from the sunlight, water evaporation becomes consequently more intense and a formation of a localized low pressure area can be expected. A
rainstorm or a
tropical cyclone (if on the sea) can well be formed in such conditions. Wind intensity can be approximately measured by the atmospheric pressure difference between two relatively nearby locations. If the pressure difference is large between two nearby cities (e.g.
Paris and
London), then strong winds are expected between these two locations.
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'.
See also
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Cyclone
★
High pressure area
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Surface weather analysis
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Weather map
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