050 Meteorology topic guide
Warm and Cold Fronts
A mid-latitude depression forms when a wave develops on the polar front, the boundary between cold polar air and warmer air further south. Ahead of the low's centre, a warm front pushes into the retreating cold air; behind it, a cold front undercuts the warm air the low is displacing. Both fronts share the same warm sector between them, yet they behave completely differently at the surface, because a warm front rides up and over the cold wedge ahead of it while a cold front bulldozes underneath the warm air it is removing.
That single geometric difference explains almost everything the exam tests: why warm-front cloud arrives hours before any rain, why cold-front weather is short and violent, and why the faster-moving cold front eventually catches the warm front to form an occlusion. This page works through the slope and speed of each front, the cloud and weather sequence they produce, and the pressure, wind and temperature changes that tell an observer which front has just gone through.
Slope, speed and the cloud sequence
A warm front has a shallow slope, commonly quoted as around 1 in 150, because the light, buoyant warm air rides gently up over the retreating cold wedge. That shallow slope spreads the frontal cloud far ahead of the surface position, which is why the classic sequence, cirrus, then cirrostratus with a halo, then altostratus, then nimbostratus, can unfold over many hours before any rain reaches the ground.
A cold front is steeper, commonly quoted as around 1 in 50 to 1 in 100, because the dense cold air drives forward close to the surface and forces the warm air ahead of it to rise sharply rather than gently. Cloud therefore arrives quickly and close to the surface position itself, often as a narrow line of cumulus building into cumulonimbus, producing heavy, sometimes squally showers followed by rapid clearance once the line has passed.
What changes at the moment of passage
At a warm front, cloud continues lowering as the rain becomes steadier, then eases toward drizzle or stops as the front itself passes overhead. Temperature rises on entering the warm sector, the wind veers, and pressure, which had been falling steadily, levels off. The warm sector itself is often unremarkable weather, mild, but frequently hazy or holding low stratus, with visibility reduced more by moisture than by any dramatic feature.
At a cold front, the narrow band of cumulonimbus brings a short, heavy shower or squall, the wind veers sharply and freshens, temperature falls as cold air reasserts itself, and pressure begins rising immediately behind the front. Visibility improves quickly once the line has cleared, and the sky settles into scattered cumulus in the cooler air behind.
Occlusion: when the cold front wins the race
Because the cold front moves faster over the ground than the warm front ahead of it, it eventually catches up and lifts the warm sector off the surface, forming an occlusion. If the air behind the cold front is colder than the air ahead of the warm front, the result is a cold occlusion, where the cold front still undercuts everything in its path. If the air behind is comparatively less cold, the result is a warm occlusion, where the advancing air rides up over the older cold air ahead instead.
Weather at an occlusion tends to combine features of both parent fronts, and the point of occlusion, where the warm sector has just been lifted clear of the ground, is often where the depression's heaviest and most persistent precipitation is found.
Worked example
Worked example: time to occlusion
A cold front lies 180 NM behind the warm front of the same depression. The cold front is advancing at 45 kt and the warm front at 15 kt. Assuming both speeds stay constant, in how many hours will the cold front catch the warm front and form an occlusion?
- A4 hours
- B6 hours
- C12 hours
- D3 hours
Show the answer and walkthrough
Correct answer: B
- A. This divides the distance by the cold front's speed alone (180 / 45), which would only be correct if the warm front stayed still. It does not: it keeps moving away from the cold front at 15 kt.
- B. Correct. The gap closes at the difference between the two speeds, 45 minus 15, which is 30 kt. 180 NM at a 30 kt closing speed takes exactly 6 hours.
- C. This divides the distance by the warm front's speed alone (180 / 15), a number with no physical meaning in this problem.
- D. This adds the two speeds together (45 plus 15 is 60 kt) as if the fronts were approaching from opposite directions. They are not: the cold front is chasing the warm front in the same direction, so the speeds subtract rather than add.
Step by step
- Both fronts move in the same direction, so the gap between them closes at the difference of their speeds, not the sum.
- Closing speed: 45 kt minus 15 kt gives 30 kt.
- Time to close a 180 NM gap at 30 kt: 180 divided by 30 gives exactly 6 hours.
- Sanity check: in 6 hours the cold front covers 45 x 6 = 270 NM, and the warm front covers 15 x 6 = 90 NM. The warm front's original 180 NM head start plus its own 90 NM gives 270 NM, matching the cold front exactly.
Common mistakes
Assuming both fronts share the same cloud sequence
Reciting cirrus through nimbostratus for a cold front as well as a warm front costs the mark, because cold front weather is convective, cumulus and cumulonimbus in a narrow band, not the long layered sequence that only the shallow warm front slope produces.
Expecting pressure to rise ahead of an approaching front
Pressure falls steadily ahead of both the warm front and the cold front as the depression approaches. Reading a pressure rise ahead of the front points to the wrong front, or the wrong side of the low, on synoptic-chart questions.
Forgetting that the cold front is faster, not just steeper
A steep slope alone does not explain rapid, violent weather. It is the cold front's greater speed over the ground that lets it catch the warm front and force the occlusion, and missing that speed difference unravels any question about the depression's later life cycle.
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Last reviewed July 2026