Precipitation

Clouds, Fog and Precipitation · Lesson 3 of 3•8 min read

A cloud droplet is about a fiftieth of a millimetre across and falls so slowly that the gentlest updraught keeps it aloft forever. Precipitation is the story of how droplets get a million times heavier (there are exactly two mechanisms) and of what arrives at the surface once they do: the eleven precipitation types of the reports and forecasts, the drop sizes that separate them, the freezing rain that ices an aircraft faster than any cloud, and what each cloud genus of this module's first lesson actually delivers.

The two formation processes

Check you understood

Three questions from this lesson — one straightforward, one that applies it, one that pushes a little further. Nothing is recorded, and you can draw a new set as often as you like.

1. Warm-up
What is freezing rain (FZRA)?
2. Applying it
By what mechanism does freezing rain usually form?
3. Pushing it
Two winter snowfalls are compared: one at −1 °C with large, wet flakes, and one at −20 °C with fine, dry snow. How does visibility compare, and what is the main caveat?

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The ice-crystal process (Wegener-Bergeron-Findeisen) — works only in mixed cloud, where ice crystals and supercooled droplets coexist. The key is a fact of vapour physics: the saturation vapour pressure over ice is lower than over water; air that is merely saturated with respect to the droplets is supersaturated with respect to the crystals. Vapour therefore deposits steadily onto the crystals, the droplets evaporate to replace what the air lost, and the crystals grow rapidly at the droplets' expense until they are heavy enough to fall, aggregating into snowflakes on the way, melting into rain if the lower air is warm. This is the engine of most mid-latitude precipitation.
The coalescence process — needs no ice at all. Droplets of different sizes fall at different speeds, so a larger drop overtakes, collides with and absorbs the smaller droplets in its path, growing as it falls. It rules warm clouds (tropical rain from clouds that never reach the freezing level), and it is the maker of drizzle in shallow stratus everywhere.

Snow, rain, drizzle and hail

Snow — Bergeron crystals aggregated into flakes, reaching the ground unmelted when the air beneath the cloud stays near or below freezing.
Rain — either melted snow (the mid-latitude norm) or drops grown warm by coalescence; drops larger than half a millimetre.
Drizzle — coalescence in shallow stratus with updraughts too weak to hold anything: droplets of 0.2 to 0.5 mm, falling slowly and close together.
Hail — a cumulonimbus specialty: a growing stone rides the updraught through the supercooled levels again and again, collecting a fresh layer of ice on each journey, and falls when it finally outweighs the strongest updraught the storm can offer.

The precipitation types of the codes

Aerodrome reports (METAR) and aerodrome forecasts (TAF) draw from a fixed vocabulary of eleven precipitation types, each with its code:

Drizzle (DZ) — fine, dense droplets of 0.2 to 0.5 mm from stratus.
Rain (RA) — liquid drops larger than 0.5 mm.
Snow (SN) — branched ice crystals, usually aggregated into flakes.
Snow grains (SG) — very small, white, opaque grains; the frozen counterpart of drizzle, from stratus.
Ice pellets (PL) — transparent pellets of refrozen raindrops; their arrival announces melting and refreezing layers overhead.
Hail (GR) — hard, layered stones of 5 mm and more, from cumulonimbus.
Small hail (GS) — stones smaller than 5 mm: snow pellets encased in a glaze of ice.
Snow pellets (GS) — white, opaque, crisp pellets of rimed snow, 2 to 5 mm, sharing the small-hail code.
Ice crystals (IC) — tiny unbranched crystals glittering in very cold, often clear air: diamond dust.
Freezing drizzle (FZDZ) — supercooled drizzle that glazes whatever it touches.
Freezing rain (FZRA) — supercooled rain: the heaviest clear-ice hazard in the list.

Drop sizes and hail damage

The ICAO (International Civil Aviation Organization) and WMO (World Meteorological Organization) approximate diameters are exam currency: a cloud droplet is about 0.02 mm, drizzle runs from 0.2 to 0.5 mm, and raindrops are larger than 0.5 mm. A raindrop holds around a million cloud droplets, which is why growth needs one of the two processes rather than condensation alone. At the far end of the scale sits hail: a stone's mass, and with it the energy of impact, grows with the cube of its size, so hailstones can cause significant damage to an aircraft (battered radomes, cracked windshields, dented leading edges) in an encounter lasting seconds. Large hail is one of the standing reasons the thunderstorm avoidance rules of the flight-hazards module exist.

Freezing precipitation

The mechanism behind freezing rain is a temperature profile with a warm nose: a layer of above-freezing air sandwiched between cold air aloft and a sub-zero layer at the surface — the signature of a winter warm front, where mild air has slid up over the cold wedge ahead of it. Snow falls from the cold air aloft, melts to rain in the warm layer, then supercools in the cold surface layer and freezes on impact, sheeting runways, wings and windshields in clear ice. Freezing drizzle manages the same trick without any ice phase: supercooled droplets grown by coalescence inside sub-zero stratus.

The weather conditions to expect it in follow from the profile: winter precipitation ahead of a warm front or occlusion, surface temperature at or below 0 °C, with milder air advancing above the inversion (occlusions are met properly in the fronts module). The give-away from inside the cockpit or on the ground is ice pellets: rain that had time to refreeze on the way down means the same warm nose is overhead, and ice pellets at the surface warn that freezing rain is nearby, often just ahead, where the cold surface layer is shallower.

Convective and stratiform precipitation

How precipitation falls tells you what made it:

Convective cloud — gives showers: sudden start, sudden end, large drops, intensity that can jump from nothing to torrential, all from the compact, vigorous cells of cumulus and cumulonimbus.
Stratiform cloud — gives continuous precipitation: steady, prolonged, smaller drops, spread over hundreds of kilometres from the deep layered sheets of a frontal system.

Genus by genus:

Stratus — at most light drizzle, or snow grains as its frozen counterpart.
Stratocumulus — occasional light drizzle, often nothing.
Altostratus — light continuous rain or snow.
Nimbostratus — prolonged, moderate to heavy continuous rain or snow: the workhorse of frontal weather.
Altocumulus and the three high-etage clouds — nothing to the ground; at most virga, evaporating below the base.
Cumulus — moderate rain or snow showers.
Cumulonimbus — heavy showers, hail and thunderstorms.

Snow and visibility

Winter visibility rides on the moisture content of the snowing air:

Air near 0 °C — holds relatively much vapour: its snow comes as large, wet, clinging flakes, densely packed, and visibility in such snowfall is poor, at its worst dropping below the 1,000 m fog threshold of the previous lesson.
Very cold air — holds little vapour: its snow is fine, dry and sparse, and visibility usually stays better, until wind lifts the loose, dry crystals back off the surface as blowing snow, which can undo the advantage in minutes.

The rule of thumb: the milder and moister the snow, the worse the obscuration while it falls; the colder and drier, the better, but the more vulnerable to the wind afterwards.