WeatherWebb

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Frontal systems & troughs

on March 12, 2015

In previous weeks I have blogged about pressure systems and air masses, but this week I thought I’d discuss frontal systems.

There are three types of frontal system; occluded, warm and cold. A frontal system or weather front can be defined as a boundary that separates two different air masses and they are often associated with band of precipitation. They are typically found close to low pressure systems with the occlusion being close to the centre of the low.

A warm front is usually depicted on a weather map by a red line with semi circles on the side of which it’s travelling, as illustrated in the image below. A warm air mass will follow behind the warm front, and the area that is influenced by this air mass is often known as the “warm sector.” This is because, a cold front usually follows close behind, so the warm air does not linger for a long time. As the warm front approaches you will see an increase in cirrus cloud, followed by altocumulus clouds. A band of rain will then move in, and as the warm front moves through the winds will veer and the temperature will rise. Once in the warm sector it will remain rather cloudy (the cloud mainly stratus), mild and damp, until the cold front moves through.

Diagram to illustrate the positioning of frontal systems

Diagram to illustrate the positioning of frontal systems

A cold front is depicted by a blue line with triangles positioned on the side of which it’s travelling as illustrated above. Cold fronts tend to be more active than warm fronts and for this reason thunder and lightning may occur as it passes over. As the front moves through winds will veer once more and temperatures will fall. Additionally the pressure is likely to increase. It will turn sunnier/clearer once the front clears, but showers may follow on behind, particularly if there is a trough behind the cold front.

When the cold front catches up with the warm front, which it normally does as the cold front travels quicker than a warm front, it becomes an occluded front. An occluded front is usually depicted by a purple line on a weather map, with semi circles and triangles positioned on the side of which the front is moving. Occluded fronts often get caught up with the cyclonic low pressure system and wrap around the centre. This is often known as a “wrap-around occlusion” as shown on the synoptic map below. These areas can often lead to rather unsettled weather with long spells of rain as they are often slow moving in comparison to warm and cold fronts as they tend to move at the speed of the low pressure.

Synoptic map showing occluded front spiralling round a low pressure system (KNMI 2015).

Synoptic map showing occluded front spiralling round a low pressure system (KNMI 2015).

As mentioned previously, a trough may follow behind the cold front which can increase instability and produce showers and thunderstorms. A surface trough is normally associated with stormy weather as they have increased and instability and are more convective than frontal systems. They are identified by areas of elongated low pressure, this means the area of low pressure that stretches outwards. There is no universal symbol for a trough, but it is usually illustrated by a straight line, for example in the synoptic map above, a trough is positioned close to the low just south of Italy. Troughs do not need to be associated with frontal systems, but they will always be close to low pressure.


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