Cold, dense air squeezes its way through the warmer, less-dense air, and lifts the warm air. Because air is lifted instead of being pressed down, the movement of a cold front through a warm front is usually called a low-pressure system. Low-pressure systems often cause severe rainfall or thunderstorms. Warm fronts usually show up on the tail end of precipitation and fog. As they overtake cold air mass es, warm fronts move slowly, usually from north to south. Because warm fronts aren't as dense or powerful as cold fronts, they bring more moderate and long-lasting weather patterns.
Warm fronts are often associated with high-pressure system s, where warm air is pressed close to the ground.
High-pressure systems usually indicate calm, clear weather. Photograph by Tina Magas , MyShot. Fronts on Weather Maps On weather maps, cold fronts are illustrated by blue lines with sharp "teeth" pointing in the direction of the wind. Warm fronts are illustrated by red lines with rounded bumps pointing in the direction of the wind. High-pressure systems are usually associated with clear weather.
Low-pressure systems are often associated with storms. The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Areas of high and low pressure are caused by ascending and descending air. As air warms it ascends, leading to low pressure at the surface. As air cools it descends, leading to high pressure at the surface. In general, low pressure leads to unsettled weather conditions and high pressure leads to settled weather conditions. In an anticyclone high pressure the winds tend to be light and blow in a clockwise direction in the northern hemisphere.
Also, the air is descending, which reduces the formation of cloud and leads to light winds and settled weather conditions. In a depression low pressure , air is rising and blows in an anticlockwise direction around the low in the northern hemisphere. The land cools quickly, having little stored heat. Consequently high-pressure regions form over the land, while low-pressure regions dominate the ocean. With the clear atmosphere of the subsident region, the land surface can continue cooling.
The loss of heat is compensated for by an increase of energy that flows into the system, as a warm airflow, from the oceanic low-pressure region. When the amount of energy radiated to space matches the inflow, an equilibrium is reached, but by that time a very deep high-pressure region has developed. Transient Systems. The second cyclonic group consists of transient cyclones and anticyclones associated with weather systems. Located in the equatorial and middle latitudes, they may grow, mature, and decay within a few days.
Depressions in middle latitudes are cyclonic systems that develop rapidly and move eastward against the basic westerly flow, over distances from to 2, km 30 to 1, mi. Central pressures often fall below millibars mb.
Inclement weather, strong winds connected to the high-pressure gradient , and squalls are associated with such mid-latitude systems, which result from basic instabilities of a heated and rotating atmosphere. Because of the Coriolis effect, the upper tropospheric flow toward the pole in the Hadley cell is forced eastward, developing strong westerlies.
The air accelerates as it moves progressively poleward. Because the winds are produced by pressure gradients, which in turn are functions of the temperature distributions, zones of strong winds ought to be associated with strong temperature gradients.
Were this situation to continue, the wind and temperature gradients would build up an infinite potential-energy reservoir. If such a system is perturbed, however, so that cold air moves equatorward across the gradient and warm air moves poleward, rapid changes will ensue.
As the light warm air overrides dense cold air and the latter undercuts warm air, a thermal circulation develops that taps the potential-energy store. The perturbation continues to grow, effectively relaxing the north-south temperature gradient and reducing the speed of the intense westerlies. This process, called a baroclinic instability, is the cause of most middle-latitude depressions.
Subsequent development continues to move warm air poleward and cold air equatorward, producing adjacent pools of warm and cold air. The resultant large east-west temperature gradient produces a pressure distribution that causes a cyclonic circulation around the low-pressure center and an anticyclonic flow around the high.
In the tropics, cyclonic systems known as tropical depressions may develop with central pressures less than 2 mb lower than the environment.
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