An occluded weather front occurs when a cold front overtakes a warm front. A cold front moves more quickly than a warm front does, as the cold air rushes to fill in the space left behind by a rising warm air mass. When a cold air mass catches up to a warm front, an occluded front is formed. These fronts usually occur at areas of low atmospheric pressure.
An occluded front is shown on a weather map as a purple line with alternating half circles and triangles. The half circles represent the warm front, and will “point” in the direction that the warm front is moving. The triangles represent the cold front and are moving in the same direction as the warm front, but coming up from behind. The color purple is used because blue (cold front) and red (warm front) make purple when mixed together.
Many different kinds of weather may be associated with an occluded front. Precipitation (rain or snow) is likely. Cumulonimbus or nimbostratus clouds are also likely to form. Winds are likely to change directions as the front passes and the temperature warms or cools. After the front has passed, the weather is likely to be drier and the sky clearer.
An occluded front would likely bring rain or snow, depending on the air temperature. The intensity of the precipitation depends on which of the two air masses is colder, the one in front or the one behind.
Cold Occluded Front: If the cold front that is approaching (the left side of the graphic above) is colder than the cold air in the front of the air mass (the right side of the graphic above), it will act more like a cold front and bring violent thunderstorms, and even may include hail or tornadoes.
Warm Occluded Front: If the cold front that is approaching (the left side of the graphic above) is warmer than the cold air in the front of the air mass (the right side of the graphic above), it will act more like a warm front and bring light rain.
If you’re a teacher, student, or parent who is interested in learning more about weather fronts and the weather that they bring, check out some of these time saving resources!
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