Weather fronts tend to form around low pressure systems and are commonly associated with adverse weather. They represent the boundary between two different air masses. An air mass is a large body of air with nearly uniform temperature and humidity. The temperature and humidity of an air mass is determined by its origin. The coldest air masses originate near the poles and are called Arctic air masses, cold polar air masses originate in high latitude regions, and warm/hot tropical air masses originate at lower latitudes. The humidity of an air mass is dependent on whether it originated over the land or sea. For example, a continental polar air mass is cold and dry, while a tropical maritime air mass is warm and humid.
Air masses are moved around by winds in the upper atmosphere. The boundary where two air masses converge is a front. The convergence of two air masses of different temperature and humidity leads to atmospheric instability, which is why fronts are almost always associated with clouds and rain. Fronts can be cold, warm, stationary, or occluded.
Cold Front
A cold front occurs when a cold air mass moves into an area occupied by a warmer air mass. Because the colder air is more dense than the warm air it is replacing, it forces the warmer air aloft. This creates a region of unstable rising air that produces cumulonimbus clouds, and the associated rain, squall, high winds, hail, and thunderstorms. The intensity of a cold front is determined by many factors: the magnitude of the temperature gradient between the two air masses, the speed at which the cold air mass is traveling, the humidity of the warmer air mass, and the behavior of the upper level winds. For example, a fast moving cold air mass is likely to produce more severe conditions compared to a slow moving one. Furthermore, if the warm air mass being replaced is maritime in origin, then more severe conditions are likely compared to a continental air mass because the maritime air mass is more humid. Cold fronts are represented in synoptic charts by a solid blue line with blue triangles.
Warm Front
A warm front occurs when a warmer air mass moves into a region occupied by a colder air mass. Warm fronts tend to move more slowly than cold fronts, and the boundary between the two air masses is less distinct. Because the encroaching warm air is less dense than the cold air it is displacing, it gets pushed upwards. The passage of a warm front is still associated with rain; however, instead of towering cumulonimbus, warm fronts are associated with nimbostratus which produce light rain over an extended period. Warm fronts are represented on synoptic charts by a solid red line with solid red semicircles.
Stationary Front
Air masses are not always in motion. A stationary front depicts the boundary between two stationary air masses. Stationary fronts are often associated with prolonged periods of dreary and overcast weather. Occasionally, stationary fronts can produce more extreme weather. For example, derechos are straight line wind storms that can produce destructive hurricane strength winds and extensive bands of cumulonimbus storm cells. Derechos travel parallel to stationary fronts, can extend for many hundreds of kilometers, and persist for many hours. Stationary fronts are represented on synoptic charts by a striped red and blue line with alternating solid red semicircles and solid blue arrows.
Occluded Front
An occluded front occurs when a cold front overtakes a warm front. They are common around mature low pressure systems. As the cold front overtakes the warm front, the warmer air is displaced aloft. Occluded fronts are most commonly associated with drier conditions; however, these conditions can vary depending on the temperature gradients between three converging air masses. Occluded fronts are represented on synoptic charts by a solid purple line with alternating solid purple triangles and semicircles.
It is difficult to determine the location of frontal systems from model data alone. Meteorologists usually interpret the location of fronts from the appearance of cloud bands on satellite images and other observations. Because it is difficult for a weather model to pinpoint the location of a frontal system. Synoptic surface charts that have been interpreted by a meteorologist are the best source of information on the location, trajectory, and speed of a frontal system.
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