Explanation of an Occluded Front

Three's Company
The occluded fronts are a result of three air masses coming in contact with one another.

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A large structure of air which is made up of water vapor and defined by its temperature is known as an air mass. When there is contact between two air masses which are at different temperatures, it leads to the formation of an air front. Such an air front can be one of the following types:

A Warm Front
A Cold Front
A Stationary Front
An Occluded Front

These are formed when an air mass does not directly pass through another air mass which is at a different temperature, instead it flows around the other air mass. This is a result of the winds that are caused by the rotation of the Earth. Given below are facts about an occluded front.

Occluded Front Symbol

The symbol of an occluded front is as shown in the image below. It is represented by a solid line that has scallops, or semi-circles, followed by triangles facing in the direction of the air mass that is less cold. The motion of this front is in the direction in which these scallops and triangles are facing. The presence of these scallops and triangles is alternate. In case of colored diagrams, an occluded front is represented by a solid purple line.

Occluded Front Symbol

How Does an Occluded Front Form?

Usually an occluded front is formed in areas of depression caused by low pressure. When a cyclone develops behind a warm front, the cold front that was formed behind the warm front moves towards it. As a result of the storm, its speed will be higher than that of the warm front. The air mass behind the cold front is colder when compared with the cool air mass that was ahead of the warm front. These two masses of air, cold and colder, will come in contact with one another as the warm air mass will move upwards.

Thus the front that is formed when the two cold air mass come in contact with one another is known as an occluded front. The direction of its movement will be towards the cold air mass from the colder air mass. The warm air that had moved upwards cools and expands which leads to the formation of clouds. An occluded front diagram is shown below.

Occluded Front

Characteristics of an Occluded Front

When an occluded front passes, the atmospheric pressure that was low slowly begins to rise. As a result there are changes in the direction of wind. The precipitation that could have been anywhere between "light to heavy" before the passing of the front, changes to "light to moderate" after passing of the occluded front. Similarly, the haze is reduced and there is characteristic improvement in visibility. The dew point is steady before the front in question passes over which may either drop or increase later (depending upon the type of occlusion, cold or warm respectively).

Occlusions are further classified into two types, warm occlusion and cold occlusion. When the mass of air that lies behind an occluded front is warmer than the air mass which lies in front of the occluded front, it is a case of warm occlusion. On the other hand, if the mass of air which lies behind the occluded front is colder than the air mass that lies in front of the occluded front, then it is known as cold occlusion. Formation of a warm occlusion is a rare phenomenon as compared with the chances of formation of a cold occlusion. These fronts have well-defined boundaries and their formation can be a sign of a weakening storm.

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