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The Challenge: Predict the Weather

What weather is associated with warm and cold fronts?

A warm mass of air will generally hold moisture picked up as it passes over large areas of water through the process of evaporation. As this warm moist air rises it cools and the water vapor condenses into rain. So a warm air mass tends to bring with it plenty of rain and drizzle. Since a warm front extends further forward at higher altitudes, this rain frequently starts to fall before the front reaches you at ground level. Once the front passes over, the rain often eases off but occasional drizzly patches are likely to follow on behind in the warm sector.

A cold air mass has a much smaller moisture capacity, so there isn't the same extensive rain as in a warm air mass. However, as a cold front comes in and drives under the mass of warm air, the warmer and moist air will be forced upwards. As the warm air is pushed higher, the moisture it carries condenses and falls as rain. This is why a lot of heavy rain is produced along a cold front but once the cold air mass has come in this often abruptly changes to a clear spell of weather.

As an occlusion is essentially a warm front with a cold front merged into it the weather ahead of it behaves as if a warm front is moving in. Behind the occlusion, however, it acts as a cold front. So you might see drizzly rain before the occlusion, suddenly becoming a heavy downpour as the occlusion passes over, then becoming clear weather punctuated by the odd patch of showers.

Weather map showing cold and warm fronts