What are cold warm and stationary fronts

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Fronts

By a front we mean the inclined boundary between two air masses at which temperature and humidity change abruptly. They can't mix right away because their densities are too different. Instead, the lighter, warm air begins to rise over the colder, denser one. A front forms in the boundary layer of this transition.

 

  

Fronts are typical of the mid-latitudes and move relatively often from west to east as westerly winds prevail. However, since they move around the highs and lows as they migrate to the east, they can sometimes come from other directions. Often their course is also influenced by landforms such as mountains or large bodies of water. This is especially true in the lower layers of the air.

 

Fronts are accompanied by all sorts of cloud types and often bring rain with them as well. If a front crosses an area, the wind speed and the wind direction change. The air pressure rises or falls and the humidity changes. There are four types of fronts: cold fronts, warm fronts, occlusal fronts and stationary fronts. The type of front that is present depends on the direction and properties of the air mass.

 

1st front on the coast of Findon (UK)
Source: Univ. of Aberdeen, Dept. of Physics
 

Cold front

We speak of a cold front when a colder air mass replaces a warmer one. In the cold front, the cold air follows the warm air. Since cold air is denser than warm, the cold air pushes the warmer air in front of it and forces it to move upwards.

 

2. Cold front. Source:
http://www.ckkc.co.uk/html/stories/metrolgy.html
Please click to enlarge! (100 K)

 

The warm air mass escaping upwards cools down and clouds begin to form. The precipitation is usually more intense when a cold front passes through, but it is spread over a relatively short distance (50 - 70 km) and is therefore rarely long-lasting. This is due to the fact that the hot air, which is strongly pushed upwards, forms powerful cloud towers. These bring heavy rain, sometimes thunderstorms, hail and in extreme cases even tornadoes. The air that follows the cold front is significantly cooler and also drier than the one that precedes it. If the front is pulled through, the temperature can drop by up to 15 ° C within an hour.

 

3rd picture on the right:
In the weather symbols, a cold front is represented by a solid blue line with triangles. The tips of the triangles point in the direction of the warmer air mass and in the direction of movement of the front.
Source: University of Illinois
 

Warm front

We speak of a warm front when a warm air mass approaches a colder air mass. The warmer air rises and slides onto the cold one. Warm fronts are usually gentler than cold fronts. They move more slowly and gradually push themselves onto the cold air until it is out of the way. The precipitation is usually less severe than with the cold front, but often extensive (typical front width: 300 - 400 km).

 

4. Warm front
Source: http://www.ckkc.co.uk/html/stories/metrolgy.html
Please click to enlarge! (100 K)

 

The air that follows a warm front is warmer and more humid than the displaced air mass. The front brings steady, light rain or even snow. This can last a few hours, but sometimes several days.

 

The first signs of a warm front are the cirrus clouds, which are followed by Cirrostratus and Altostratus clouds. Eventually nimbostratus and stratocumulus clouds appear. The cirrus clouds can be up to 1000 km ahead of the main front. All these types of clouds have a more or less horizontally aligned structure due to the slight incline of the front surface. This is completely different with cold fronts, in which the clouds are oriented vertically.

 

5. Radar image of the clouds on a warm front
Source: Univ. of Aberdeen, Dept. of Physics
Please click on the picture to enlarge it!

 

6. Picture to the left:
The weather symbolism uses a solid red line with semicircles for a warm front. These point in the direction of the colder air and in the direction of movement of the front.
Source: University of Illinois

 

Occlusion front

An occlusion front occurs when warm and cold air together form a mixed front. There are two types of occlusion fronts: cold and warm. In both cases, a cold front overtakes a warm front.

In case of a cold occlusion front, a cold front with a very cold air mass overtakes a warm front. The cold air pushes both air masses ahead: the warm air that was in the process of sliding up and also the cooler air lying on the ground. As a result, the warm front becomes the front on top, so to speak. The behavior of the weather resembles a warm front at the beginning, but changes to that of a cold front towards the end. Heavy precipitation can set in. The cloud cover is mixed and not as clearly defined as on other fronts.

 

Fig. 7a: Formation of a cold occlusion front
Source: University of Illinois
http://ww2010.atmos.uiuc.edu/(Gh)/home.rxml

 

Fig. 7b: Formation of an occlusion front

A warm occlusion front occurs when a cold front with moderately cold air overtakes a warm front. In the warm front, the temperature contrasts are high because very cold air lies on the ground. In this case, the entire cold front slides with the warm air ahead and the moderately cold air in the wake onto the very cold air on the ground (see Fig. 9. b). The cold front becomes the front on top. In this case, the weather behaves similarly to a warm front.

From one stationary front we speak when a warm or cold front approaches, the air of which is neither really warm nor really cold. It behaves similarly to a warm front, but is more calm.

 

8. a) cold occlusion front
Image: Elmar Uherek
8. a) and b) please click to enlarge!
 
8. b) warm occlusion front
Image: Elmar Uherek
 

 

About this page:
author: Vera Schlanger - Hungarian Meteorological Service
scientific reviewing: Dr. Ildikó Dobi Wantuch / Dr. Elena Kalmár - Hungarian Meteorological Service, Budapest
Last revision: 2007-09-03

 

 
Further reading:
http://www.ckkc.co.uk/html/stories/metrolgy.html
http://www.cira.colostate.edu/RAMM/picoday/discussion.html
http://www.abdn.ac.uk/physics/meteo/
http://ww2010.atmos.uiuc.edu/(Gh)/indexlist.rxml
 

 

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last updated 03.09.2007 10:29:22 | © ESPERE-ENC 2003 - 2013