Low-pressure area
A low-pressure system over Iceland demonstrates the core concept of atmospheric pressure differences. In meteorology, a low-pressure area is simply a region where air pressure sits lower than surrounding locations. This state stands in direct opposition to high-pressure areas that bring lighter winds and clear skies. Inclement weather such as cloudy conditions, strong winds, rain, or storms commonly accompanies these systems. Winds circle anti-clockwise around lows in the northern hemisphere while circling clockwise in the southern hemisphere due to opposing Coriolis forces.
The formation process of a low-pressure area is known as cyclogenesis. Meteorologists use this term for the development and strengthening of cyclonic circulations within the atmosphere. Cyclogenesis occurs under areas of wind divergence that happen in upper levels of the atmosphere aloft. The first type appears on the east side of upper troughs forming half of a Rossby wave within the Westerlies. A second type occurs ahead of embedded shortwave troughs which possess smaller wavelengths. Diverging winds aloft cause atmospheric lift within the troposphere below as air flows upwards away from the surface. This upward motion partially counteracts gravity packing air close to the ground thereby lowering surface pressures.
Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains. In Europe particularly in the British Isles and Netherlands recurring extratropical low-pressure weather systems are typically known as depressions. These tend to bring wet weather throughout the year. Thermal lows also occur during summer over continental areas across subtropics such as the Sonoran Desert or the Sahara. A study of extratropical cyclones in the Southern Hemisphere shows between 30th and 70th parallels there are an average of 37 cyclones existing during any six-hour period. Another study suggests approximately 234 significant extratropical cyclones form each winter in the Northern Hemisphere.
When a convective low acquires a well-hot circulation in the tropics it is termed a tropical cyclone. Tropical cyclones generally need to form more than five degrees north or south of the equator allowing Coriolis effect to deflect winds. Water temperatures of at least 26 degrees Celsius down to a depth of at least 50 meters are needed for formation. High humidity especially in lower-to-mid troposphere makes conditions favorable for disturbances to develop. Low amounts of wind shear are required since high shear disrupts storm circulation. A pre-existing system of disturbed weather is necessary though no cyclonic development occurs without circulation.
Wind initially accelerates from areas of high pressure toward areas of low pressure due to density differences between air masses. The stronger the pressure difference between systems the stronger the wind experienced in its vicinity. Atmospheric lift produces cloud cover through adiabatic cooling once temperature drops below dew point as air rises. Cloudy skies typical of low-pressure areas act to dampen diurnal temperature extremes by reflecting sunlight during day and trapping heat at night. Tornadoes often remain too small and short-lived to be influenced directly by Coriolis force but may be so-influenced when arising from a low-pressure system.
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Common questions
What is a low-pressure area?
A low-pressure area is simply a region where air pressure sits lower than surrounding locations. This state stands in direct opposition to high-pressure areas that bring lighter winds and clear skies.
How does cyclogenesis form a low-pressure system?
Cyclogenesis occurs under areas of wind divergence that happen in upper levels of the atmosphere aloft. Diverging winds aloft cause atmospheric lift within the troposphere below as air flows upwards away from the surface, thereby lowering surface pressures.
Where are low-pressure systems most frequently located globally?
Globally, low-pressure systems are most frequently located over the Tibetan Plateau and in the lee of the Rocky Mountains. In Europe particularly in the British Isles and Netherlands recurring extratropical low-pressure weather systems are typically known as depressions.
What conditions allow tropical cyclones to develop?
Tropical cyclones generally need to form more than five degrees north or south of the equator allowing Coriolis effect to deflect winds. Water temperatures of at least 26 degrees Celsius down to a depth of at least 50 meters are needed for formation along with high humidity and low amounts of wind shear.
Why do winds circle differently around low-pressure areas in each hemisphere?
Winds circle anti-clockwise around lows in the northern hemisphere while circling clockwise in the southern hemisphere due to opposing Coriolis forces. Wind initially accelerates from areas of high pressure toward areas of low pressure due to density differences between air masses.