warm front

warm front A surface where advancing warm air displaces colder air (e.g. in mid-latitude depressions) where, owing to the convergence of the air masses and the difference of density between them, the warm air tends to rise over the cold air. Slopes of warm fronts are typically less than 1:100 and the ascent of air is gradual. Stratiform cloud develops in the rising air. High cirrus cloud followed by lower and thickening altostratus indicate the approaching front. As the frontal contact with the ground approaches, heavy nimbostratus and much rain may occur. Passage of the front is marked by a rise of temperature, clearing of precipitation, and (in the northern hemisphere) the wind veering typically from south or south-easterly to south-westerly.