Since warm air is lighter than cold air, the warm air will ride over the top of the colder air, where it will cool, condense, and squeeze out moisture.
Warm fronts are generally weak weather-makers, bringing in a steady light rain or drizzle. It can also create fog as the warm air condenses as it meets up with the colder surface air. Since cold air is heavy and tough to budge, warm fronts can be slow-movers, keeping the rains around for dozens of hours.
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