Cold front
A cold front is the leading edge of a cooler mass of air at ground level that replaces a warmer mass of air and lies within a pronounced surface trough of low pressure. It often forms behind an extratropical cyclone (to the west in the Northern Hemisphere, to the east in the Southern), at the leading edge of its cold air advection pattern—known as the cyclone's dry "conveyor belt" flow. Temperature differences across the boundary can exceed 30 °C (54 °F) from one side to the other. When enough moisture is present, rain can occur along the boundary. If there is significant instability along the boundary, a narrow line of thunderstorms can form along the frontal zone. If instability is weak,[clarification needed] a broad shield of rain can move in behind the front, and evaporative cooling of the rain can increase the temperature difference across the front. Cold fronts are stronger in the fall and spring transition seasons and are weakest during the summer.
Development of cold fronts
A cold front occurs when a mass of comparatively colder air moves into where warmer air is present. The drier, colder air forms a steeply sloping boundary under the warmer, moister air at the surface and lifts that air. This often causes cloud formations with a strong vertical development, which may manifest as a line of showers and thunderstorms when enough moisture is present.[1] 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.[2] Cold fronts move faster than 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.[3]
In the northern hemisphere, a cold front usually causes a shift of wind from southwest to northwest clockwise, also known as veering, and in the southern hemisphere a shift from northwest to southwest (counterclockwise, backing). Atmospheric pressure steadily decreases with the approach of a cold front. With frontal passage, the pressure rises sharply and then stabilizes. Normally, cold fronts can be marked by these characteristics:[4][1]
| 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. In summer, sometimes thunderstorms if a preceding snow squalls or showers may occur.[5]
|
Prolonged rain (nimbostratus) or thunderstorms (cumulonimbus): depends on conditions.
|
Showers, then clearing |
| Clouds* | Often preceded by cirrus, cirrostratus then altostratus like a warm front (but usually with smaller amounts of these clouds). Areas of cirrocumulus and altocumulus within cirrostratus and altostratus more commonly seen than at a warm front. Larger cumulus clouds under the higher cloud types than at a warm front, where stratocumulus and cumulus humilis usually occur. Some of these cumulus clouds may produce showers ahead of the front.
|
Cumulonimbus and cumulus congestus producing frequent showers, with a sheet of upper altostratus, through which the sun can sometimes be seen. Less commonly nimbostratus occurs with continuous rain. | Patchy 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.
Clouds
If the cold front is highly
After the passage of the cold front, the sky usually clears as high pressure builds in behind the system, although significant amounts of cumulus or stratocumulus, often in the form of long bands called cloud streets, may persist if the airmass behind the front remains humid from a source of moisture.
Precipitation
A cold front commonly brings a narrow band of
Frontogenetical circulation
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 kind and location of clouds and precipitation.[7][12]
Temperature changes
Cold fronts are the leading edge of cooler air masses, hence the name "cold front". They have stronger temperature changes during the fall (autumn) and spring and during the middle of winter. Temperature changes associated with cold fronts can be as much as 30 °C (54 °F). When cold fronts come through, there is usually a quick, yet strong gust of wind, that shows that the cold front is passing. In surface weather observations, a remark known as FROPA is coded when this occurs.[13] The effects from a cold front can last from hours to days. 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 cannot hold as much moisture as warm air, clouds form and rain or snow occur.[4]
Characteristics of boundaries around an extratropical cyclone
Cold fronts form when a cooler air mass moves into an area of warmer air in the wake of a developing extratropical cyclone. The warmer air interacts with the cooler air mass along the boundary, and usually produces precipitation. Cold fronts often follow a
See also
References
- ^ OCLC 428735924.
- Hydrometeorological Prediction Center. Retrieved 2012-01-09.
- ISBN 978-1-119-96667-8. Retrieved 2012-01-09.
- ^ a b "Cold Front: transition zone from warm air to cold air". ww2010.atmos.uiuc.edu. Retrieved 2019-11-09.
- ^ )
- OCLC 51160155.
- ^ ISBN 978-0-12-354015-7.
- .
- ^ Glossary of Meteorology (2009). Prefrontal squall line. Archived 2007-08-17 at the Wayback Machine Retrieved on 2008-12-24.
- hdl:1956/12444.
- ^ K. A. Browning and Robert J. Gurney (1999). Global Energy and Water Cycles. Retrieved on 2008-12-26.
- ISBN 978-0-04-551115-0.
- ^ Nav Canada (January 2005). Aviation Weather Services Guide. p. 36.
- St. Louis University (2003-08-04). "What is a TROWAL? via the Internet Wayback Machine". Archived from the originalon 2006-09-16. Retrieved 2006-11-02.
