How cold and warm fronts differ
A weather front is the boundary between two air masses of different temperature and density. At a cold front, advancing dense cold air wedges underneath warmer air and forces it upward steeply. The steep lift produces tall cumulonimbus clouds, brief but intense rain or thunderstorms, and a sharp temperature drop with often gusty wind shifts as the front passes.
At a warm front, lighter warm air glides up and over a retreating cold air mass along a gentle slope. The gradual ascent spreads wide layers of stratiform cloud ahead of the front, giving long periods of lighter, steadier rain and a slow warming once it arrives.
Reading fronts on a weather map
On a surface chart a cold front is drawn as a blue line with triangles pointing in the direction of travel, and a warm front as a red line with semicircles pointing the way it moves. Cold fronts typically move faster than warm fronts, which is why a cold front can overtake a warm one to form an occluded front. The point where they meet on a mid-latitude low is the warm sector.