Clouds
A cloud is the visible record of air that was forced to rise. Its shape tells you why it rose and how stable the air is — before you have read a word of forecast.
Weather Information — FI.III.C · CA.I.C · PA.I.C
Why rising air makes cloud
Pressure falls with height, so a parcel of air that rises expands. Expanding takes energy, and the parcel takes that energy from itself, so it cools — with no heat exchanged with its surroundings at all.
Keep lifting it and its temperature eventually reaches its dewpoint. That altitude is the lifted condensation level, and it is the height of the cloud base. Above the LCL, further lifting condenses vapour into droplets or deposits it as ice crystals, and the cloud grows upward as long as the parcel keeps rising.
Condensation also releases heat, which warms the parcel relative to its surroundings and helps it keep going. That feedback is why a cumulus cloud can grow so much faster once it has started than it did getting there.
The reverse runs the same way: descending air compresses, warms, and cloud evaporates. Sinking air is clear air.
Four ways air gets lifted
Orographic — wind blowing across rising terrain. Reliable enough that the same ridge produces cloud on the same side almost every time the wind is from the same direction.
Frictional / convergence — winds diverge away from high pressure, so air sinks, compresses, warms and cloud dissipates. Winds converge into low pressure, so air rises, expands, cools and cloud forms. This is the mechanism behind the shorthand everyone learns: high pressure, good weather; low pressure, bad. It is worth knowing it as a consequence rather than a rule.
Frontal — cold, denser air wedges under warm, less dense air and forces it up. See air masses and fronts.
Buoyancy — the ground heats unevenly, so some parcels become warmer and less dense than their neighbours and rise on their own. This is the one with a clock attached: it builds through the day and dies after sunset.
The three forms, and what each admits
Every cloud name you will meet is built from these, plus a prefix for height and nimbus for precipitation.
| Form | Looks like | Tells you |
|---|---|---|
| Cirriform | Thin, white, wispy, high | Ice crystals at altitude. Thickening and lowering cirrus is the classic first sign of an approaching warm front — often the earliest visual warning you get. |
| Cumuliform | Heaped, lumpy, vertical | Unstable air. Expect a rough ride, good visibility between the clouds, and showery precipitation if any. |
| Stratiform | Flat, layered, featureless | Stable air. Expect a smooth ride, poor visibility, steady precipitation, and conditions that will not clear themselves out quickly. |
Nimbus means it is raining. So nimbostratus is the steady rain of a stable layer, and cumulonimbus is a thunderstorm — a cumuliform cloud that has grown until it precipitates, which is the single most important word in cloud naming.
Height families, and what a METAR gives you
Clouds are grouped as low, middle and high, and the prefixes follow: alto-
for middle, cirro- for high. A METAR does not name any of this. It reports
coverage and height only — FEW, SCT, BKN,
OVC — because an automated station can measure a base but cannot identify a
genus.
Which means the naming is for you, looking out of the window or at a satellite image. The report tells you where the base is; the shape tells you what the air is doing. Neither substitutes for the other, and reading the report is only half the skill.
Where you meet it next
- Stability — the reason cloud has a shape at all
- Cloud base calculator — the LCL, worked out from a spread
- Moisture and precipitation
- Ceilings and flight categories
Sources
Formation, the sources of vertical motion and cloud classification are from the FAA Aviation Weather Handbook (FAA‑H‑8083‑28).
Study material, not a briefing and not an endorsement. Nothing here substitutes for an official weather briefing or for instruction from your CFI.