A cold front is defined as the leading edge of a cooler mass of air, replacing (at ground level) a warmer mass of air.
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The cooler and denser air wedges under the less-dense warmer air, lifting it. This upward motion causes lowered pressure along the cold front and can cause the formation of a narrow line of showers and thunderstorms when enough moisture is present. On weather maps, the surface position of the cold front is marked with the symbol of a blue line of triangles/spikes (pips) pointing in the direction of travel. A cold front's location is at the leading edge of the temperature drop off, which in an isotherm analysis would show up as the leading edge of the isotherm gradient, and it normally lies within a sharp surface trough. Cold fronts can move up to twice as fast as warm fronts and can produce sharper changes in weather. Since cold air is denser than warm air, it rapidly replaces the warm air preceding the boundary. Cold fronts are usually associated with low-pressure areas.
In the northern hemisphere, a cold front usually causes a shift of wind from southeast to northwest anticlockwise, also known as backing, and in the southern hemisphere a shift from northeast to southwest, clockwise, called veering. Common characteristics associated with cold fronts include:
| Weather phenomenon | Prior to the Passing of the Front | While the Front is Passing | After the Passing of the Front |
|---|---|---|---|
| Temperature | Warm | Cooling suddenly | Steadily cooling |
| Atmospheric pressure | Decreasing steadily | Lowest, then sudden increase | Increasing steadily |
| Winds |
|
Gusty; shifting |
|
| Precipitation/conditions* | Light patchy rain can be produced by stratocumulus or stratus in the warm sector. | Prolonged rain (nimbostratus) or Thunderstorms (cumulonimbus): depends on conditions. | Showers, then clearing |
| Clouds* | Increasing: Usually Cirrocumulus and/or cirrostratus then altocumulus and/or altostratus then stratocumulus, which joins together into stratus then thickens to nimbostratus. Often it only has some of these cloud types preceeding it, and in unstable conditions, a cold front may just take the form of a line of cumulonimbus with patchy altocumulus or stratocumulus or the usual cloud sequence ahead of it. | Cumulonimbus or nimbostratus | Patchy altocumulus or cirrocumulus then cumulus and cumulonimbus |
| Visibility* | Fair to poor in haze | Poor, but improving | Good, except in showers |
| Dew Point | High; steady | Sudden drop | Falling |
*Provided there is sufficient moisture.
A cold front commonly brings a narrow band of precipitation that follows along the leading edge of the cold front. These bands of precipitation are often very strong in nature, and can bring severe thunderstorms, hailstorms and/or tornadoes. In the spring, these cold fronts can be very strong, and can bring strong winds when the pressure gradient is tighter than normal. During the winter months, cold fronts sometimes come through an area with little or no precipitation. In the autumn months, cold fronts rarely bring severe thunderstorms, but are known for bringing heavy, and widespread rainstorms. These rainstorms sometimes bring flooding, and can move very slowly because cold fronts are more prone to slow movement in the fall. In the winter, cold fronts can bring severe cold spells, and heavy snowstorms. If moisture is not sufficient, cold fronts can pass without producing any precipitation at all, and the skies could be cloudless. Cold fronts do not produce the moisture, it will just condense against the cold air into cloud and rain droplets if there is enough water vapor in either airmass.
The idea that cold air wedges, or undercuts, the warm air is often used to depict how advancing cold fronts force warm air to rise along the sloping cold air, much like a snow shovel scooping up snow[1], creating instability. Provided with sufficient moisture, the rising air would thus condense, creating storms, clouds, and/or rain. While this concept is used to generally describe frontal precipitation patterns, it is technically incorrect.[2]
Frontogenesis is the process of creating or steepening the temperature gradient of a front. During this process the atmosphere reacts in an attempt to restore balance, the consequence is a circular motion along the front where air is being lifted up, along the cold front and dropping downward, behind the frontal boundary. This is the actual force of upward motion along a front that is responsible for clouds and precipitation.
As the temperature gradient steepens during frontogenesis, the thermal wind becomes imbalanced. To maintain balance, the geostrophic wind aloft and below adjust, such that regions of divergence/convergence form. Mass continuity would require a vertical transport of air along the cold front where there is divergence (lowered pressure). Although this circulation is described by a series of processes, they are actually occurring at the same time, observable along the front as a thermally direct circulation. There are several factors that influence the final shape and tilt of the circulation around the front, ultimately determining the type and location of clouds and precipitation. [3][4]
Cold fronts are the leading edge of a frigid air mass, hence the name "cold front". They can bring severe cold spells in the fall (autumn) and winter. Very often, cold fronts are associated with deadly cold weather. Sometimes, though, cold fronts have no significant effect on the weather. The cold fronts in the late fall become more polar in nature, and tend to bring very cold weather, and temperatures can drop by as much as 30°F. When cold fronts come through, there is usually a quick, yet strong gust of wind, that shows that the cold front is passing. The effects from a cold front can last only a few hours to several weeks, depending on when the next weather front comes through. The air behind the front is cooler than the air it is replacing and the warm air is forced to rise, so it cools. As the cooler air can not hold as much moisture as warm air, clouds form and rain occurs.
Cold fronts are very often associated with a warm front, squall line, or other weather front. Very commonly, cold fronts have an adjacent warm front that is ahead of the cold front. This is known as an occluded front. This forms an area where warm air is occurring and interacting with the cold front, an area known as a warm sector. In the warm sector, severe thunderstorms, tornadoes, and hailstorms are a common occurrence, due to the sharp difference between the warm air that is associated with the warm front and the cold air that is associated with the cold front. A cold front is considered a warm front if it retreats, and called a stationary front if it stalls.
Cold fronts form when a cooler air mass moves into an area of warmer air. The warmer air interacts with the cooler air mass along the boundary, and usually produces precipitation.