Pressure system meaning




Pressure system meaning


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Pressure system meaning


Pressure system

A pressure system is a region of the Earth's atmosphere where air pressure is unusually high or low. High and low pressures form (and die) constantly due to thermodynamic interactions of the atmosphere and water in oceans, lakes, and other bodies of water.


Low pressure system

The components of storms are attracted to regions of low pressure. For this reason, heavy precipitation and overcast conditions are often associated with low-pressure systems. Due to the Coriolis Effect, low-pressure systems often develop cyclonic properties: in the northern hemisphere, winds around the system move counter clockwise, and in the southern hemisphere they move clockwise. Low pressure systems, additionally, often become junctures of fronts. You may have seen a weather map with a red L on it. This red L means there is a low pressure system over that area of the map.

Most of history's most powerful storms, such as the 1993 North American storm complex and all tropical cyclones have been low-pressure cyclonic systems. Tornadoes invariantly have very strong local low-pressure systems at their vortices.


Air moves into a Low pressure system. It pushes any air that was there upwards.


High-pressure system

High pressure systems are associated with clear, cool weather. Around high-pressure systems, winds flow clockwise in the northern hemisphere, counter clockwise in the southern hemisphere. You may have seen a weather map with a blue H on it. This blue H means there is a high pressure system over that area of the map.

In the northern winter, high-pressure systems (called Canadian highs or Arctic air masses) often migrate to mid-latitude regions such as the North American upper Midwest, New England, and northern Europe. These create cold snaps where unseasonably cold and sunny weather are observed. Cold snaps often follow winter warm spells, where temperatures may be as high as 10°C to 20°C (50-68 °F), and often happen suddenly. The most dramatic Arctic cold snaps, observed in the central regions of North America involve temperature drops of 25°C (45°F) or more in a few hours.

Arctic highs, alone, rarely trigger precipitation because of the cloudless weather they produce. However, in combination with other weather-making systems, the cold air they bring can produce massive snowstorms.

High pressure usually means good weather.


The air moving away from the High pressure system leaves a "hole" to be filled, so air from above sinks into that "hole".



The cloudy rainy weather of low-pressure depressions is due to rising air, which is most pronounced near frontal regions. The anticyclone on the other hand is produced by a large mass of descending air. This subsidence takes place throughout a depth of the atmosphere up to 12km. Such subsidence means that the air is very stable and atmospheric pressure is high. In addition, winds associated with an anticyclone are usually very light if present at all, especially close to the centre of the high-pressure system.

Subsidence warms the air by compression. Any clouds present quickly evaporate as the temperature of the air rises above its dew point. For this reason, anticyclones usually bring fine, dry and settled weather, particularly in the summer.

Sometimes, subsidence and compression of the air can produce a temperature inversion at one or two thousand metres above the ground. Such phenomena act as caps to rising air heated by the ground under the influence of the Sun, preventing extensive air cooling and cloud formation. Unfortunately, if the air is moist below the temperature inversion, a dreary formless layer of cloud can form which becomes difficult to disperse owing to the light winds. Such debilitating weather is common in winter when the Sun’s radiation is too weak to burn off the cloud layer.

Winter anticyclones, if clear of cloud, bring with them further problems. A short cloudless day is the forerunner of a long night with more radiation cooling than a low-angle Sun can counteract the next day. The second night of cooling therefore starts with a lower air temperature than the first. Such conditions, if persistent, can lead to successive nights of frost, which become progressively harder. When the air is particularly moist, cooling at night soon results in fog. Britain in particular can experience episodes of anticyclonic fog from late September through to May.

Anticyclones move, but not quite in the same purposeful way as travelling depressions. They nudge their way into position and can be incredibly stubborn about leaving, perhaps persisting for weeks, diverting depressions to different routes. Such persistent anticyclones are known as "blocking highs". In winter they can lead to long spells of very cold weather, especially if their airflow comes from Russia and Siberia. In summer they can lead to long hot spells and sometimes drought.

A ridge of high pressure is a wedge-shaped extension of an anticyclone or belt of high pressure. The weather associated with ridges is similar to that in an anticyclone. In temperate latitudes as in the British Isles, ridges of high pressure often occur between two depressions and move with them. They give rise to intervals of fair weather between the cloud and rain of the low-pressure systems.




Depressions, sometimes called mid-latitude cyclones, are areas of low pressure located between 30° and 60° latitude. Depressions develop when warm air from the sub-tropics meets cold air from the Polar Regions. There is a favourite meeting place in the mid-Atlantic for cold polar air and warm sub-tropical air. Depressions usually have well defined warm and cold fronts, as the warm air is forced to rise above the cold air. Fronts and depressions have a birth, lifetime and death; and according to the stage at which they are encountered, so does the weather intensity vary.

A depression appears on a synoptic (weather) chart as a set of closed curved isobars with winds circulating anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere due to the rotation of the Earth. The warm and cold fronts associated with depressions bring with them characteristically unsettled weather. Depressions vary from between 200 and 2,000 miles in diameter; they may be deep when pressure at their centre is very low and the isobars are tightly packed, or shallow when less well developed.

A depression develops like the propagation of a wave in water. Initially, a uniform boundary or front exists between cold air pushing southwards and warm air pushing northwards (in the Northern Hemisphere). A wave-shaped distortion may appear on the front, and a small low-pressure centre develops at the crest of the wave. In the immediately surrounding area the pressure begins to fall. A disturbance of this kind is called a wave depression. As the "wave" develops, a warm sector of air forms bounded by the warm and cold fronts, which begins to tie over the engulfing cold air. Both the warm and cold fronts originate from the centre of the depression. On the ground, sudden changes in the wind direction may be experienced when fronts pass by.

Wave depressions can grow off the tail ends of primary cold fronts. The depression so formed is then called a secondary depression. New centres may also develop from occluded fronts within the primary depression. The secondary system can then become the main system, and the primary occluded front becomes caught up in the developing circulation, effectively becoming a third front.





Hurricanes have two main parts. The first is the eye of the hurricane, which is a calm area in the centre of the storm. Usually, the eye of a hurricane measures about 20 miles in diameter, and has very few clouds. The second part is the wall of clouds that surrounds the calm eye. This is where the hurricane's strongest winds and heaviest rain occur.

Hurricanes are born over warm, tropical oceans. The top 50 meters of the ocean surface needs to be 26.5o C. The air above the ocean must be cooler than the water temperature, allowing thunderstorms to form. Hurricanes are fuelled by water vapour that is pushed up from the warm ocean surface, so they can last longer and sometimes move much further over water than over land. The combination of heat and moisture, along with the right wind conditions, can create a hurricane.

Hurricanes are enormous, and they can range in size from 300-600 miles wide and about 10 miles high. They typically have a lifespan of about 10 days. The wind speed of a hurricane is 75 miles per hour or more. Between 40 mph and 74 mph winds, the storm is called a tropical storm.

How a Hurricane Forms

Hurricanes form in tropical regions where there is warm water (at least 80 degrees Fahrenheit / 27 degrees Celsius), moist air and converging equatorial winds. Most Atlantic hurricanes begin off the west coast of Africa, starting as thunderstorms that move out over the warm, tropical ocean waters. A thunderstorm reaches hurricane status in three stages:

  • Tropical depression - swirling clouds and rain with wind speeds of less than 38 mph (61.15 kph)
  • Tropical storm - wind speeds of 39 to 73 mph (54.7 to 117.5 kph)
  • Hurricane - wind speeds greater than 74 mph (119 kph)


It can take anywhere from hours to several days for a thunderstorm to develop into a hurricane. Although the whole process of hurricane formation is not entirely understood, three events must happen for hurricanes to form:

  • A continuing evaporation-condensation cycle of warm, humid ocean air
  • Patterns of wind characterized by converging winds at the surface and strong, uniform-speed winds at higher altitudes
  • A difference in air pressure (pressure gradient) between the surface and high altitude

Warm, moist air from the ocean surface begins to rise rapidly. As this warm air rises, its water vapour condenses to form storm clouds and droplets of rain. The condensation releases heat called latent heat of condensation. This latent heat warms the cool air aloft, thereby causing it to rise. This rising air is replaced by more warm, humid air from the ocean below. This cycle continues, drawing warmer, moist air into the developing storm and continuously moving heat from the surface to the atmosphere. This exchange of heat from the surface creates a pattern of wind that circulates around a centre. This circulation is similar to that of water going down a drain.

"Converging winds" are winds moving in different directions that run into each other. Converging winds at the surface collide and push warm, moist air upward. This rising air reinforces the air that is already rising from the surface, so the circulation and wind speeds of the storm increase. In the meantime, strong winds blowing at uniform speeds at higher altitudes (up to 30,000 ft / 9,000 m) help to remove the rising hot air from the storm's centre, maintaining a continual movement of warm air from the surface and keeping the storm organized. If the high-altitude winds do not blow at the same speed at all levels -- if wind shears are present -- the storm loses organization and weakens.

High-pressure air in the upper atmosphere (above 30,000 ft / 9,000 m) over the storm's centre also removes heat from the rising air, further driving the air cycle and the hurricane's growth. As high-pressure air is sucked into the low-pressure centre of the storm, wind speeds increase.





Near the equator, from about 5° north and 5° south, the northeast trade winds and southeast trade winds converge in a low pressure zone known as the Intertropical Convergence Zone or ITCZ. Solar heating in the region forces air to rise through convection which results in an over-supply of precipitation. The ITCZ is a key component of the global circulation system.

Weather stations in the equatorial region experience precipitation up to 200 days each year, making the equatorial and ITC zones the wettest on the planet. The equatorial region lacks a dry season and is constantly hot and humid.

The location of the ITCZ varies throughout the year and while it remains near the equator, the ITCZ over land ventures farther north or south than the ITCZ over the oceans due to the variation in land temperatures. The location of the ITCZ can vary as much as 40° to 45° of latitude north or south of the equator based on the pattern of land and ocean.

In Africa, the ITCZ is located just south of the Sahel at about 10°, dumping rain on the region to the south of the desert. The Intertropical Convergence Zone has been called the doldrums by sailors due to the lack of horizontal air movement (the air simply rises with convection). The ITCZ is also known as the Equatorial Convergence Zone or Intertropical Front.

There's a diurnal cycle to the precipitation in the ITCZ. Clouds form in the late morning and early afternoon hours and then by 3 to 4 p.m., the hottest time of the day, convectional thunderstorms form and precipitation begins.


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