Occluded Fronts By the time a depression arrives

Occluded Fronts By the time a depression arrives over the UK its usually in the mature stage of its development and its warm and cold fronts may have merged to form an occlusion. What should we expect if this happened.

What is an Occluded Front As a depression develops and matures its cold front is travelling faster than its warm front. So as the cold front advances it cuts into the warm sector of the depression, a wedge of cold air pushes in underneath the warm sector forcing warm air upwards. The name comes from the occluding or shutting off of the warm air from the earths surface. There is a line or trough of warm air called a line of occlusion, seen as a band of cloud sometime with heavy rain, as the low is in decline the system will be moving slowly and is often stalled occluded front bringing heavy rain and floods.

Telling the Difference The best way to tell if it’s a warm or cold occlusion is from a weather chart, look at the shape of the front, if the cold front looks like an extension of the occlusion then it’s a cold occlusion and if the warm front looks like an extension it’s a warm occlusion. As an occlusion passes the wind will veer (clockwise). As with a normal warm front we see a gradual lowering of the cloud base until it becomes grey all over with layered clouds, eventually it will start to rain. We will not have a warm sector between the fronts. As the cumulonimbus cloud on the cold front arrives we will get pulses of heavier rain and gusty winds, not the dramatic change that we normally get. Again after the cold front it will not be as dramatic a clearing but will become showery with sunny spells. Often the depression is close to its end when the occluded front forms which makes it slow moving bringing prolonged significant rain, little wind change, . In the winter occluded fronts can bring heavy snowfall.

Different Occluded Fronts A cold occlusion is typical in summer when the air behind the front is colder than the air in front of it. The air temperature is colder than it was before the front in this front A warm occluded front is typical of winter conditions, if the air behind the front has had a long track over a relatively warm ocean, while the air ahead of the front may have become cooled over land. The air temperature will be warmer than before the front. As the occluded front is usually found in a low pressure system that is filling we often find that the front is running parallel to the isobars and this indicates a slow moving system. It then seems to hang around for a long time and is reluctant to clear.