A boundary between advancing warm air that is displacing 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 as the cold air ahead of the front gives way to the air in the warm sector behind the front, clearing of precipitation, and (in the northern hemisphere) the wind veering typically from south or south-easterly to south-westerly.