Air masses
An air mass is a large body of air with broadly uniform properties (temperature, humidity, stability) acquired by lingering over a source region.
Source: WMO; AMS Glossary; DWD Klimatologie.
Definition
"Air mass" means a body of air with the same properties concerning air temperature, humidity and atmospheric layering (stable or unstable):
- Horizontal extent: several 100 to 1000 km.
- Vertical: to the tropopause.
- Properties stable over long distances.
Formation — source regions
The source areas and the path it travels give air mass its properties:
- Air mass "rests" over a source region (ocean, pole, desert, continent) and takes on its T and humidity.
- When moving away it retains these properties and affects distant regions.
Classification by source
| Code | Name | Source | Properties |
|---|---|---|---|
| mP | Maritime polar | N. Atlantic, N. Pacific | Moist, cold-cool |
| cP | Continental polar | Siberia, Canada | Dry, cold |
| mT | Maritime tropical | Subtropical oceans (Biscay, Med) | Moist, warm |
| cT | Continental tropical | Sahara, Middle East | Dry, hot |
| mA | Maritime arctic | Polar seas | Very cold, moist |
| cA | Continental arctic | Polar landmasses | Very cold, very dry |
Maritime vs continental
Maritime
Maritime air is always moist, in summer cool, in winter mild:
- Oceans have smaller temperature swings than land.
- Water evaporation saturates the air.
- Result: maritime air = clouds, precipitation more frequent.
Continental
When continental air reaches Central Europe, the weather is warm and dry in summer, cold and dry in winter:
- Land has strong T swings.
- Dry (little evaporation).
Continental polar air usually has the lowest humidity often providing excellent visibility with minimal haze or fog.
Transformation
"Transformation of an air mass" means a change to the properties of an air mass while it travels:
- Maritime air over land → drier.
- Cold polar air warms as it moves south.
- Warm tropical air cools as it moves north.
Advection
Advection processes = horizontal transport of an air mass with its properties (see "density and temperature" lesson):
- Example: supply of mild ocean air from the Atlantic to Central Europe in winter with westerly winds.
- Warm-air advection = bringing warmer air.
- Cold-air advection = bringing colder air.
Central Europe — typical air masses
| Wind | Typical inflow | Weather |
|---|---|---|
| SW | mT — maritime tropical (Biscay/Med) | Warm, moist, thunderstorms in summer |
| W | mP — maritime polar (N. Atlantic) | Mild, moist, frequent clouds/precip |
| NW / N | mA — maritime arctic | Cold, moist, showers |
| E | cP — continental polar (Russia) | Cold-dry winter, warm-dry summer |
Visibility in different air masses
- Continental polar: dry → excellent visibility.
- Maritime tropical: humid → haze, reduced visibility.
- Maritime polar: frequent precipitation → variable visibility.
Klimatologie*; FAA-H-8083-25B PHAK Ch. 12.*