Learning hub · Meteorology · Lesson 2
Clouds, stability and fronts
Most of the weather that decides whether you fly comes from two things: whether the air is stable, and where the fronts are. Learn both and a weather chart starts to read like a story.
About 14 minutes, plus the quiz. Part of Meteorology. It follows Pressure, QNH and the altimeter.
1. Stable and unstable air
Rising air cools as it expands. Dry (unsaturated) air cools by about 3°C per 1,000 ft. Once it has cooled to its dew point, the water vapour condenses into cloud and gives out heat, so the rising air then cools more slowly, by between about 1.2 and 3°C per 1,000 ft, depending on temperature and pressure (2°C is a common figure for examples).
Whether a bubble of air keeps rising depends on the surrounding air. If the bubble stays warmer than its surroundings it keeps rising: the air is unstable. If it becomes cooler than its surroundings it sinks back: the air is stable.
- Stable air: layer cloud (stratus), drizzle, poor visibility in haze and fog, but a smooth flight.
- Unstable air: heaped cloud (cumulus and cumulonimbus), showers, turbulence and gusts, but good visibility between the showers.
2. Estimating the cloud base
Air that rises from the ground cools by about 3°C per 1,000 ft, while its dew point falls by only about 0.5°C per 1,000 ft. The gap closes by roughly 2.5°C per 1,000 ft, so cloud forms where the gap reaches zero. This gives a rule for the height of the base, above the ground:
Cloud base (ft) ≈ (temperature − dew point) × 400
With a METAR that says 20/12, the gap is 8°C, so the base of the convective cloud is about 8 × 400 = 3,200 ft above the airfield. A small gap of 1 to 3°C can mean low cloud or fog is near.
3. Fronts
A front is the boundary between two air masses. The three you will meet most often are these, described for the northern hemisphere.
Warm front. Warm air rides up over cold air along a gentle slope (often quoted as about 1 in 150, but it varies). The cloud thickens and lowers over many hours: cirrus first, then cirrostratus and altostratus, then nimbostratus with steady rain and poor visibility. After it passes, the temperature rises, the wind veers (turns clockwise, for example from south-east to south-west) and the steady rain eases.
Cold front. Cold air pushes under warm air along a steeper slope (often quoted as about 1 in 50), and moves faster. It brings a narrow band of heavy cloud, showers and sometimes thunderstorms, gusts and a sudden drop in visibility. Afterwards the temperature falls, the pressure rises, the wind veers (south-west to north-west), and the air is clear with scattered showers.
Occlusion. The faster cold front catches the warm front and lifts the warm air off the ground. The weather combines the features of both fronts.
4. Putting it to use
- Before a warm front, expect a long period of lowering cloud and worse weather. Fly early, or stay on the ground.
- A cold front is a short, sharp event. The air behind it is often excellent for flying, but showers and gusty winds remain.
- Keep well clear of cumulonimbus clouds. They can contain severe turbulence, icing, hail and lightning, and nothing about them is predictable close up.
Use the official forecasts, the charts and a proper briefing before a flight. Fronts move and change, and a forecast front position is an estimate.
5. Try it yourself
Work each one out on paper first, then open the answer to compare. The numbers are invented.
- A METAR gives a temperature of 24°C and a dew point of 13°C. The airfield is at 600 ft. Estimate the base of the convective cloud above the airfield and above sea level.
Show the answer
The gap is 24 − 13 = 11°C. Cloud base ≈ gap × 400 = 11 × 400 = 4,400 ft above the airfield.
Add the elevation of the airfield: 4,400 + 600 = about 5,000 ft above sea level. Check with the other form of the rule: the gap closes by about 2.5°C per 1,000 ft, so 11 ÷ 2.5 = 4.4 thousand feet. Both agree.
- The wind at your airfield is from 200° before a cold front arrives and from 290° after it has passed. Has the wind veered or backed, and by how much? What else do you expect behind the front?
Show the answer
Going from 200° to 290° is a clockwise change, so the wind has veered, by 290 − 200 = 90°.
Behind a cold front expect a falling temperature, rising pressure, clearer air with good visibility, and scattered showers and gusty winds. The front itself brings a narrow band of heavy cloud, showers and sometimes thunderstorms.
- On a summer morning the surface temperature is 21°C and the dew point is 16°C. Find the cloud base. Then show that a bubble of air rising from the ground is at its dew point at that height.
Show the answer
The gap is 21 − 16 = 5°C, so the base is about 5 × 400 = 2,000 ft above the ground.
Rising unsaturated air cools by about 3°C per 1,000 ft: 21 − 2 × 3 = 15°C at 2,000 ft. The dew point falls by about 0.5°C per 1,000 ft: 16 − 2 × 0.5 = 15°C.
At 2,000 ft the temperature and the dew point are both 15°C, so the water vapour condenses and cloud forms. The gap closes by 2.5°C per 1,000 ft, which is why 5°C takes 2,000 ft.
6. Watch
A visual walk through the four types of front and the weather each brings. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.
Open on YouTube if the player does not load. Not made by VectorEFB; check what you learn against your flying school’s material and your instructor.
More to watch
A longer walk through the four types of front and what each means for a pilot. This video is American, so wording and some rules differ from European (ICAO/EASA) practice.
Open on YouTube if the player does not load. Not made by VectorEFB.
7. Check yourself
Five original practice questions. Answer them, press Check answers and read each explanation.
Next: loading and the centre of gravity. Back to the learning hub.