Rising air, cooling and stability
1The lesson in plain words
Rising air cools. Whether it keeps rising or sinks back decides if you get flat layer cloud or towering showers. This is stability.
Rising air cools
When a small body of air, called a parcel, rises, pressure falls and it expands and cools. Unsaturated air cools about 1 °C per 100 m: the dry adiabatic lapse rate. After saturation, condensation releases heat, reducing the cooling rate: the saturated adiabatic lapse rate. It is often about 0.4–0.7 °C per 100 m in warmer lower-level air, approaching the dry rate in very cold air.
Cloud base
Rising air cools towards its dew point. Where saturation is reached, cloud can form: its lowest level is the cloud base. For heap clouds, estimate base height above the surface observation level as 125 × (temperature − dew point), using °C and giving metres. With 20 °C and a dew point of 12 °C, that is about 1,000 m above that level.
Stable and unstable
A lifted parcel that becomes colder than its surroundings tends to sink back: stable air. A warmer parcel tends to keep rising: unstable air. Air is conditionally unstable when the surrounding temperature falls with height faster than the saturated rate but slower than the dry rate. After saturation, a parcel may need more lifting before it becomes warmer than its surroundings. An inversion, where temperature rises with height, is very stable and acts as a lid.
What the sea does
A sea warmer than the air heats it from below, favouring instability near the surface. A colder sea cools the lowest air and favours stability. Cold air over warm water can develop showers if enough moisture is present and rising air is not capped. Warm, moist air over cool water may develop low cloud or fog.
Signs for a sailor
Stable air: flat layer cloud, haze or poor visibility, drizzle, a steady wind. Unstable air: heap clouds, showers, very good visibility between them, and a gusty wind with sudden shifts. Reduce sail early so you can handle the gusts.
2Key data
- Dry adiabatic lapse rate
- About 1 °C per 100 m (9.8 °C per km)
- Saturated adiabatic lapse rate
- Often 0.4–0.7 °C/100 m; nearer the dry rate in very cold air
- Standard atmosphere
- Temperature falls 0.65 °C per 100 m on average
- Cloud base (heap cloud)
- ≈ 125 × spread in °C: metres above surface observation level
- Inversion
- Temperature rises with height: very stable lid
3Key points
- 1
Rising air expands and cools: about 1 °C per 100 m until saturated.
- 2
Cloud forms where rising air reaches its dew point.
- 3
Stable air sinks back: layer cloud, haze, steady wind.
- 4
Unstable air keeps rising: heap cloud, showers, gusts.
- 5
A warmer sea favours instability near the surface; a colder sea favours stability.
4Frequent mistakes
Wrong: Rising air cools because it gets closer to cold space.
Right: It cools because it expands as the pressure around it falls.
Wrong: Unstable air means bad visibility.
Right: Unstable air usually gives very good visibility between showers. Stable air can trap haze or moisture and reduce visibility.
Wrong: Cold air over a warm sea gives calm, settled weather.
Right: Heating from below favours instability near the surface. Showers can develop if there is enough moisture and no stable lid.
5Practice case & exam keys
Behind a cold front, 12 °C air with a 6 °C dew point, measured at sea level, flows over warmer water. Heap clouds are growing, but the wind eases between them. Would you judge your sail area by those quieter spells? Explain, and estimate the cloud base.
Show the answer
No. Heating from below favours instability near the surface. The growing heap clouds warn of possible showers, gusts and sudden wind shifts. The cloud base is about 125 × (12 − 6) = 750 m above sea level. Choose a sail area you can handle in the strongest gusts.
Exam keys
- Dry air cools about 1 °C per 100 m when lifted; saturated air cools more slowly.
- Stable air resists rising (layer cloud); unstable air keeps rising (heap cloud, showers).
- A warmer sea favours instability near the surface; a colder sea favours stability and may bring fog or stratus.
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.
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