Weather front

In meteorology, a weather front is a boundary between two air masses with differing characteristics (e.g., air temperature or humidity).

When a weather front passes over an area, it is marked by changes in temperature, wind speed and direction, atmospheric pressure, and often a change in the precipitation pattern.

Weather fronts are often closely associated with atmospheric pressure systems. They are generally guided by the jet stream and travel from west to east. This movement is due to the Coriolis effect, caused by the Earth spinning about its axis. Weather fronts can also be affected by geographic features like mountains and large bodies of water, especially at the lower levels of the atmosphere.

There are four main types of weather fronts:

  • Cold front: cold fronts occur when a colder air mass approaches a warmer air mass. The colder air, being denser, cuts a wedge under the less dense warmer air, lifting it and finally overtaking it. Cold fronts move rapidly. Strong cold fronts can set off atmospheric disturbances such as thunderstorms, squall lines, tornadoes, high winds and short duration snowstorms ahead of the moving cold front and cooler, drier weather as the front passes. Depending on the time of year and geographic location, cold fronts can come in succession every 5 to 7 days. On weather maps, cold fronts are marked with the symbol of a blue line of triangles (icicles) pointing in the direction of travel.
A sea breeze is a form of a localized cold front.
  • Warm front: warm fronts occur when a warmer air mass approaches a colder air mass. The warmer air lifts up and over the colder air. Warm fronts move more slowly than cold fronts, and are associated with less severe weather. They gently settle over the cold front and move it out of the way. Warm fronts bring steadier, lighter rain or snow in front of them, which can last from a few hours to several days, and warmer, drier air as the warm front passes. They can sometimes cause a period of thunderstorms. On weather maps, warm fronts are marked with the symbol of a red line of half circles (suns) pointing in the direction of travel.
  • Stationary front: is a boundary between two different air masses, neither of which is strong enough to replace the other. A wide variety of weather can be found along a stationary front, but usually clouds and prolonged precipitation are found there. Stationary fronts will either dissipate after several days, or change into a cold or warm front. Stationary fronts are more numerous in the summer months. Stationary fronts are marked on weather maps with an alternating red/blue line of half circles and triangles pointing in opposite directions, symbolizing the dual nature of the front.
  • Occluded fronts: are formed where a slower moving warm front is followed by a more rapidly moving cold front. The wedge shaped cold front eventually overtakes the warm front and pushes it aloft. The two fronts continue to move in tandem, with the line between them being the occluded front. As with stationary fronts, a wide variety of weather can be found along an occluded front, but usually they are associated with stratus clouds and light precipitation. Occluded fronts are marked on weather maps with a black dotted line between closely placed cold and warm front markings pointing in the direction of travel. Occluded fronts usually form around low pressure areas and usually when the low pressure area is weakening. It is also known as a Trowal (Trough of Warm air Aloft).

A similar phenomenon is a dry line, which is the boundary between wet and dry air. The best-known dry line is the one that forms near the Gulf of Mexico. When a dry line passes an area, there is an associated decrease in humidity. The phenomenon is also associated with much of the instability that occurs in the Great Plains that tends to lead to powerful storms.

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