Cold and warm fronts are not separate creatures: they are the two arms of one storm, a mid-latitude cyclone (a low). These storms are born under the jet stream and bring most of the rain and snow outside the tropics. Each has a warm front and a cold front.
Key idea
A mid-latitude cyclone grows, matures and dies over a few days. It starts as a kink on a stationary front, deepens into an open wave with a warm front ahead and a cold front behind, matures as its pressure drops and winds strengthen, then occludes when the faster cold front catches the warm front and lifts the warm air off the ground. Cut off from warm, humid air, the low then fills and fades.
Why it works
The storm feeds on the contrast between cold and warm air. As the low deepens, warm air flows north ahead of it (the warm front) and cold air flows south behind it (the cold front), turning stored temperature contrast into wind. A trough in the jet stream helps: air spreading apart aloft above the low pulls more air up from below, deepening it. Cold fronts usually move faster than warm fronts, so the cold front closes the gap. Once the warm air is lifted off the ground everywhere near the center, the fuel is cut off: occlusion marks the beginning of the end.
Worked example: Put the stages in order
Order these stages: occluded front, kink on a stationary front, open wave with a warm sector, the low fills and fades.
- Birth: a kink on a stationary front.
- Growth: an open wave with a warm sector between the fronts.
- Late life: the cold front catches the warm front: occlusion.
- Death: the low fills and fades.
Answer: Kink → open wave → occlusion → decay
Worked example: When will it occlude?
A storm's cold front moves 40 km/h and its warm front 25 km/h in the same direction, 450 km apart. About how long until the cold front catches up?
- The gap closes at $40 - 25 = 15$ km/h.
- Time: $450 \div 15 = 30$ hours.
Answer: About 30 hours
Common mistake
Thinking an occluded front means the storm is just forming. Occlusion comes late in a storm's life, when the cold front has caught the warm front; the low is usually at its deepest then and soon weakens. Check: the map symbol for an occluded front (purple, with triangles and half circles on the same side) sits near the center of a mature low, never at the start of a new one.
Map symbols: a stationary front alternates red half circles and blue triangles on opposite sides; an occluded front is purple with both shapes on the same side.