At a warm front, warm air advances and cold air retreats. It looks like the mirror image of a cold front, but it behaves very differently. Its long, slow approach is written in the sky, in the clouds you can name. On a map it is a red line with half circles on the side it is moving toward.
Key idea
A warm front slopes gently, rising roughly 1 km for every 200 km or more. Warm air glides up over the cold air ahead, so the clouds arrive in order, high to low: cirrus, cirrostratus, altostratus, then nimbostratus, with a wide band of steady rain (or snow) ahead of the front. After it passes, the temperature and dew point rise, the wind shifts (in the north, from east or southeast to south or southwest) and the pressure stops falling.
Why it works
The warm air is less dense, so it can't push the heavy cold air out of the way along the ground. Instead it rides up over it, on a long, shallow ramp. Rising slowly over a huge area, the warm air cools gently and evenly, making layered clouds, not towers. Because the ramp is so shallow, the first clouds appear far ahead of the front at the ground: cirrus at 8 to 10 km may show up 1,000 km or more ahead, a day or two before the rain.
Worked example: How far ahead are the cirrus?
A warm front slopes 1 km up for every 200 km. How far ahead of the front at the ground is the frontal surface 9 km up, where cirrus form?
- Distance = height × 200 = $9 \times 200 = 1800$ km.
- The cirrus overhead can be 1,800 km ahead of the front at the ground.
Answer: About 1800 km
Worked example: When will it arrive?
Cirrostratus spread over a town when the warm front is 900 km away, moving at 25 km/h. How long until the front passes?
- Time = $900 \div 25 = 36$ hours: a day and a half of thickening clouds and then rain first.
Answer: About 36 hours
Common mistake
Thinking warm air shoves the cold air away along the ground. The cold air is denser; the warm air rides up over it on a gentle slope, and the cold air slowly retreats. That is why warm-front weather arrives aloft first (high clouds) and spreads slowly, rather than hitting at once like a cold front. Check: steady rain for many hours points to a warm front; a short, violent downpour to a cold front.
| Cold front | Warm front | |
|---|---|---|
| Slope | steep, about 1:50 to 1:100 | gentle, about 1:200 or flatter |
| Clouds | cumulonimbus in a narrow band | cirrus → cirrostratus → altostratus → nimbostratus |
| Precipitation | short, heavy showers and storms | steady, widespread rain or snow |
| After it passes | colder, drier, pressure rising, wind NW | warmer, more humid, wind S to SW |
| Map symbol | blue, triangles | red, half circles |