The most complex weather situation occurs at an occluded front. This is where a warm air mass is caught between two cooler air masses. the denser cool air masses snub underneath the less dense warm air mass, displacing it upwards. The two cooler air masses meet in the middle and may mix together. The temperature near the ground becomes cooler. the warm air mass is cut off, or 'occluded', from the ground. As the rising warm air mass cools, and it's water vapour condenses, the weather may turn cloudy and rainy/snowy.
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