The essentials in two minutes
Air travels in huge masses that keep the memory of their home region: polar or tropical, maritime (moist) or continental (dry). Two different masses mix no more than oil and vinegar: they push against each other along a boundary, the front.
When the warm air attacks, it is too light to drive the cold air out: it climbs over it up a very gentle ramp, spread over hundreds of kilometres. So the sky gives a day’s warning, from the highest cloud to the lowest: cirrus, halo, grey veil, then fine, steady rain. This is the warm front, the deterioration that announces itself. When the cold air attacks, it ploughs under the warm air like a snowplough and lifts it all at once: a wall of cloud, gusts, violent showers, a plunging thermometer. This is the cold front, the deterioration that strikes almost without warning.
Between the two lies the warm sector, grey and mild. Behind comes the post-frontal air: washed, crystal-clear, unstable polar air — often the best flying day of the week, the day after the cold front.
The whole system moves from west to east, and the wind turns to the right at each stage: south, south-west, west, north-west. With the sky, the wind, the barometer and the thermometer, you know where you are in the film, and so what is coming in the next six hours. For hiking, one rule sums it all up: warm-front rain lasts and means postponing the outing; a post-frontal shower passes and can be worked around.
Read on
The whole chapter is open to subscribers, with its animated figures: €3 a month or €25 a year. The book as an ePub, €6.99, comes with a month’s subscription. Contents:
- 4.1Air masses: air has a memory
- 4.2The polar front and the life of a frontal system
- 4.3The warm front: the deterioration that gives warning
- 4.4The warm sector
- 4.5The cold front: the deterioration that strikes
- 4.6Occlusion
- 4.7Post-frontal air: the pilot’s treasure
- 4.8The full timeline of a frontal system’s passage
- 4.9The Alps as a front passes: barrier effect and foehn
- 4.10Hiking: two kinds of rain, two kinds of fog
- 4.11Key figures
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Chapter glossary#
- Air mass
- A very large volume of air with almost uniform characteristics (temperature, humidity), acquired through a long stay over a source region. Named after its origin (Arctic, polar, tropical) and its track (maritime, continental).
- Front
- Narrow transition zone between two air masses of different densities. Its passage brings abrupt changes in temperature, humidity and wind.
- Polar front
- Semi-permanent boundary between polar air and tropical air in the mid-latitudes, with the jet stream above it. The cradle of frontal systems.
- Frontal system
- A wave on the polar front, wound up around a low, with a warm front and a cold front, moving from west to east.
- Pannus
- Ragged shreds of low cloud beneath the rain of a nimbostratus, clinging to the slopes: the hiker’s frontal “fog”.
- Warm sector
- Area of tropical maritime air between the warm front and the cold front: grey, mild, fairly stable weather, low bases.
- Squall line
- Organised line of thunderstorm cells, ahead of or along an active cold front: gusts, driving rain, hail.
- Occlusion
- The stage at which the cold front has caught up with the warm front, the warm air being pushed aloft. A sign of an ageing frontal system.
- Post-frontal air
- Polar maritime air behind the cold front: unstable, crystal clear, cumulus and isolated showers. Active (frequent showers, wind) or dry (the unstable fine weather after the front).
- Barrier effect
- Enhancement of cloud and rain on the windward side of a massif, where the air is forced to rise. In the Alpine region it is often called “Stau”.
- Foehn
- Warm, dry wind that blows down the lee side of a massif, after leaving its moisture on the other side. Detailed in chapter 6.