Low-pressure area

In meteorology, atmospheric divergence aloft occurs in two kinds of places: Diverging winds aloft, ahead of these troughs, cause atmospheric lift within the troposphere below as air flows upwards away from the surface, which lowers surface pressures as this upward motion partially counteracts the force of gravity packing the air close to the ground.

Thermal lows form due to localized heating caused by greater solar incidence over deserts and other land masses.

Tropical cyclones can form during any month of the year globally but can occur in either the northern or southern hemisphere during December.

Since clouds reflect sunlight, incoming shortwave solar radiation decreases, 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 night-time minimums in all seasons.

Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains.

Such upward motions decrease the mass of local atmospheric columns of air, which lowers surface pressure.

Polar lows can be difficult to detect using conventional weather reports and are a hazard to high-latitude operations, such as shipping and offshore platforms.

[19] Tropical cyclones form due to latent heat driven by significant thunderstorm activity, and are warm-core with well-defined circulations.

[21] High humidity is needed, especially in the lower-to-mid troposphere; when there is a great deal of moisture in the atmosphere, conditions are more favorable for disturbances to develop.

[24] 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.

[26] Monsoons resemble sea and land breezes, terms usually referring to the localized, diurnal (daily) cycle of circulation near coastlines everywhere, but they are much larger in scale - also stronger and seasonal.

[31] Large polar cyclones help determine the steering of systems moving through the mid-latitudes, south of the Arctic and north of the Antarctic.

Thermal lows also occur during the summer over continental areas across the subtropics - such as the Sonoran Desert, the Mexican Plateau, the Sahara, South America, and Southeast Asia.

[33] Elongated areas of low pressure form at the monsoon trough or Intertropical Convergence Zone as part of the Hadley cell circulation.

[39] Monsoon troughing in the western Pacific reaches its zenith in latitude during the late summer when the wintertime surface ridge in the opposite hemisphere is the strongest.

Its poleward progression is accelerated by the onset of the summer monsoon which is characterized by the development of lower air pressure over the warmest part of the various continents.

[40][41] The large-scale thermal lows over continents help create pressure gradients which drive monsoon circulations.

A low-pressure system over Iceland .
This depiction of the Hadley cell shows the process which sustains low-pressure areas. Diverging winds aloft allow for lower pressure and convergence at the Earth's surface, which leads to upward motion.
QuikSCAT image of typical extratropical cyclones over the ocean. Note the maximum winds on the poleward side of the occluded front .
February position of the ITCZ and monsoon trough in the Pacific Ocean, depicted by area of convergent streamlines offshore Australia and in the equatorial eastern Pacific
A visible image of Hurricane Dennis intensifying in the Jamaica Channel.
Schematic representation of flow (represented in black) around a low-pressure area in the Northern hemisphere. The pressure-gradient force is represented by blue arrows, the Coriolis acceleration (always perpendicular to the velocity) by red arrows.