A front is the boundary between two air masses. At a cold front, cold air advances and the warm air ahead of it is shoved upward, one of the four ways air is lifted. On a map it is a blue line with triangles pointing the way it moves. A station in its path records a sudden change.
Key idea
A cold front is steep (rising roughly 1 km for every 50 to 100 km), so it lifts warm, humid air sharply into a narrow band of showers and thunderstorms. As it passes (Northern Hemisphere): the pressure falls, reaches its lowest and then jumps up; the wind shifts from south or southwest to west or northwest, often gusty; the temperature and dew point drop; and the skies clear behind it.
Why it works
Cold air is denser, so it hugs the ground and plows under the warm air like a wedge, and friction near the ground makes the leading edge bunch up and steepen. Forced up a steep slope, the warm, humid air rises fast, often in unstable towers: cumulonimbus. Behind the front, the cold, dry air brings a lower dew point and higher pressure (dense air weighs more). The wind shift follows from the pressure pattern: the front lies in a trough of low pressure, and the winds turn as it passes.
Worked example: When will it arrive?
A cold front is 180 km west of a town and moving east at 30 km/h. When will it reach the town?
- Time = distance ÷ speed = $180 \div 30 = 6$ hours.
Answer: In about 6 hours
Worked example: How deep is the cold air?
A cold front slopes 1 km up for every 80 km. How deep is the cold air 200 km behind the front at the ground?
- Height of the front above that point: $200 \div 80 = 2.5$ km.
- So the cold air is about 2.5 km deep there.
Answer: About 2.5 km
Worked example: Reading a weather record
A station records: 3 p.m.: 27 °C, dew point 20 °C, wind S, 1005 hPa. 5 p.m.: 26 °C, wind SW, 1003 hPa, thunderstorm. 7 p.m.: 18 °C, dew point 9 °C, wind NW gusting, 1009 hPa. When did the front pass?
- Before 5 p.m.: warm, humid, south winds, falling pressure: ahead of the front.
- By 7 p.m.: much colder and drier, northwest wind, pressure up: behind the front.
- It passed between 5 and 7 p.m., with the thunderstorm at its leading edge.
Answer: Between 5 and 7 p.m.
Common mistake
Expecting it to warm up after the storms pass. Behind a cold front comes the cold air mass: the temperature and dew point fall and the pressure rises, often with gusty northwest winds and clearing skies. Check: a falling dew point is the surest sign the front has passed.