Air masses: where the air comes from
1The lesson in plain words
An air mass carries the weather of its source. Know where the air has come from and you can expect its cloud, visibility and showers.
What an air mass is
An air mass is a huge body of air, often over a thousand kilometres across, with broadly similar temperature and humidity across it at any given height. It gets them by staying over one source region, such as the Arctic, the subtropical ocean or a desert.
Names
Two letters describe it. Maritime (m) air formed over the sea and is moist; continental (c) air formed over land and is dry. Polar (P) air is cold; tropical (T) air is warm; Arctic (A) air is colder still. So mP is maritime polar air: cool and moist.
Changed by the journey
Air changes as it travels. Cold air moving over a warmer sea is heated from below and becomes unstable: heap clouds and showers. Warm air moving over a colder sea is cooled from below and becomes stable: low cloud, drizzle and sea fog. The Stability lesson in the Clouds unit explains why.
Air masses in Europe
mP from the north-west: cool, showery, very good visibility. mT from the south-west: mild, moist, low cloud, drizzle, sometimes fog. cP from the east in winter: very cold and dry, with snow showers where it crosses warmer sea. cT from North Africa: hot, dry, hazy and sometimes dusty. Arctic air from the north: very cold, with wintry showers.
Where they meet
Where two air masses meet, they do not mix easily. The boundary between them is a front. The polar front between cold polar air and warm tropical air is where most Atlantic depressions are born.
2Key data
- mP (maritime polar)
- Cool, moist; showers, gusts, very good visibility
- mT (maritime tropical)
- Mild, moist; stratus, drizzle, poor visibility, sea fog
- cP (continental polar)
- Very cold, dry in winter; snow showers over warm sea
- cT (continental tropical)
- Hot, dry; haze and dust, poor visibility
- Front
- Boundary between two air masses
3Key points
- 1
An air mass takes the temperature and humidity of its source.
- 2
m = maritime (moist), c = continental (dry), P = polar, T = tropical.
- 3
Cold air over warmer sea: unstable, showers.
- 4
Warm air over colder sea: stable, low cloud and fog.
- 5
The boundary between air masses is a front.
4Frequent mistakes
Wrong: Polar air always brings poor visibility.
Right: Maritime polar air over a warmer sea is unstable: visibility is usually very good between showers.
Wrong: Tropical air always means fine sunny weather.
Right: Maritime tropical air over a cooler sea often brings low cloud, drizzle and fog.
5Practice case & exam keys
In spring off north-west Spain, the forecast says a mild, moist south-westerly flow will replace a cool north-westerly. The sea is still cool. Would you keep a landfall plan that depends on seeing shore marks?
Show the answer
Cool, showery mP air with good visibility gives way to mild, moist mT air. Over the cool sea, the new air cools from below and becomes stable. Expect low cloud, drizzle and possible sea fog. Do not rely on seeing shore marks. Replan for poor visibility; postpone if you cannot make the landfall safely.
Exam keys
- Air masses are classed maritime/continental and polar/tropical/Arctic by source.
- Heating from below makes an air mass unstable; cooling from below makes it stable.
- A front is the boundary between two air masses.
Finish this unit
When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.
My notes for this unit
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