Low-pressure area
A low pressure area, or "low", is a region where the atmospheric pressure is lower in relation to the surrounding area. Tropical storms, extratropical cyclones, and polar cyclones are called low-pressure cells.
Lows are frequently associated with stronger winds and atmospheric lift. This lift will generally produce cloud cover through 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. Since the clouds reflect sunlight, incoming shortwave solar radiation is less which causes lower temperatures during the day. 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. In Europe (especially in the UK), reoccurring low pressure weather systems are typically known as depressions. These tend to bring wet weather throughout the year.
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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 convegence 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 because of the addition of surface friction to the pressure gradient force, centrifugal force and coriolis effect that drive the circulation
A 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 heat from the sunlight, water evaporation becomes 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.
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.
