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Given a scientific passage and an answer, generate a question for the given answer.

Passage: A cold front occurs when a cold air mass runs into a warm air mass. This is shown in Figure below . The cold air mass moves faster than the warm air mass and lifts the warm air mass out of its way. As the warm air rises, its water vapor condenses. Clouds form, and precipitation falls. If the warm air is very humid, precipitation can be heavy. Temperature and pressure differences between the two air masses cause winds. Winds may be very strong along a cold front. Answer: a cold front
Output:
What occurs when a cold air mass runs into a warm air mass?