Cloud formation
Clouds form when moist air cools to its dew point and water vapour condenses on condensation nuclei.
Source: WMO International Cloud Atlas; AMS Glossary; ICAO Annex 3.
Prerequisites
- Water vapour in the air (humidity).
- Condensation nuclei (dust, salt, aerosols).
- Cooling (via lift, radiation, mixing) to the dew point.
Four main cooling mechanisms
- Lift (adiabatic cooling): thermal, orographic, frontal, convergent.
- Radiative cooling: near surface at night (→ ground fog).
- Advective cooling: warm air over cold surface (→ advection fog).
- Mixing cooling: two different air masses mix (→ mixing fog).
Cloud classification — by formation
Clouds are differentiated into convective (intense, localized ascent forming cumuliform types), stratiform (gentle, widespread ascent creating layered clouds), and orographic (terrain-induced uplift over mountains or hills):
| Formation | Condition | Cloud types |
|---|---|---|
| Convective | intense local lift | Cu, Cb, TCu, Ac, Cc |
| Stratiform | gentle widespread lift → stable layered clouds over large areas | St, Sc, As, Ns, Cs, Ci |
| Orographic | mountain-induced lift | lenticularis, cap cloud, hill fog |
Stratiform clouds — main cause
The most common cause for stratiform cloud formation is slow but widespread lifting of an entire air mass, leading to stable, layered clouds over large areas — e.g. at a warm front where warm air glides gently over cold.
Convective clouds
Cumulus clouds (Cu) can develop into a cumulonimbus (Cb) — stages:
- Cu humilis (small, fair-weather Cu).
- Cu mediocris (medium).
- TCu — towering cumulus (strongly developed vertically, anvil hint).
- Cb — cumulonimbus (fully developed thunderstorm cloud).
Cloud-base calculation
Cloud base ≈ 400 ft × spread [°C] (see Lapse Rates lesson).
Condensation conditions
Supersaturation and condensation cores are preconditions that have to be met for condensation to occur (see humidity lesson).
Foehn — at what altitude do clouds form?
Warm, dry catabatic wind (Foehn): an ascending parcel of air reaches exactly the altitude where the formation of cumuliform clouds starts — the lifted condensation level (LCL) is the cumuliform cloud base.
International Cloud Atlas* (online); AMS Glossary; ICAO Annex 3; FAA-H-8083-25B PHAK Ch. 12.*