Precipitation

Precipitation

Precipitation is water falling from clouds — liquid (rain, drizzle), solid (snow, hail, ice), or mixed.

Source: WMO International Cloud Atlas; AMS Glossary; ICAO Annex 3.

How do precipitation drops form?

Cloud droplets are too small (~10 µm) to fall. They must grow to ~100 µm – 1 mm:

1. Coalescence: drops collide and merge (warm clouds, T > 0 °C).
2. Bergeron-Findeisen process: in mixed clouds (water + ice), ice crystals grow at the expense of water drops → fall as snow or melt to rain.

Precipitation from stratiform vs convective clouds

Stratiform clouds produce smaller precipitation particles due to weaker updrafts, while convective clouds generate larger ones through stronger vertical air movements.

Precipitation producers — Cb and Ns

Nimbostratus (Ns) and Cumulonimbus (Cb) generally produce precipitation:

• Ns: steady widespread rain or snow.
• Cb: heavy showers, hail, thunderstorms.

Cb clouds generally produce showers due to intense convection, while Ns yields steady rain from widespread stable ascent.

Precipitation types — detail

Rain (RA)

Drops ≥ 0.5 mm. Large-droplet rain reaching the ground typically develops in mixed clouds with a freezing level above 5000 ft AGL — layer between freezing level and ground gives time for droplet growth.

Drizzle (DZ)

Very fine drops 0.1-0.5 mm. Drizzle reaching the ground develops in a layer of stratus (e.g. high fog) at positive or negative temperatures:

• Typically from low stratus layers.
• Reduces visibility (see visibility lesson).

Snow (SN)

Ice crystals forming in medium-high clouds at < 0 °C and reaching ground without melting. Snowflakes reaching the ground develop in a mixed cloud with a freezing level below 500 ft AGL:

• At freezing level > 500 ft AGL flakes would melt en route → rain.

Hail (GR)

Ice balls 5 mm to > 10 cm. Larger solid particles of precipitation (hail, soft grain) develop in deep convective clouds (Cb) with strong updrafts:

• Updrafts keep ice grains aloft until too large to suspend.
• Special hazard in thunderstorms.

Freezing rain (FZRA)

Liquid rain that freezes on contact. Freezing rain (FZRA) is a weather phenomenon usually related to warm fronts in winter:

• Formation: warm rain from upper warm layer falls through lower cold layer → supercooled → freezes on contact.
• Most dangerous icing condition (see icing).
• Icing due to freezing rain has to be expected in front of warm fronts or warm-front occlusion after winterly cold spells.

Freezing level — key value

The height of the freezing level is one of the most important prerequisites to avoiding aircraft icing:

• Pilot checks freezing level on SIGWX and WINTEM before every flight.
• Icing risk in cloud around the 0 °C isotherm.

Practical VFR consequence

• Steady rain (Ns) → warm front or low.
• Showers (Cb, TCu) → unstable air, cold-front backside, summer.
• Freezing rain → exit the layer immediately (see icing).
• Hail → Cb present → 20 km spacing (see thunderstorms).

International Cloud Atlas* (precipitation); AMS Glossary; ICAO Annex 3; FAA-H-8083-25B PHAK Ch. 12.*