How precipitation grows

How can a million tiny cloud droplets become one raindrop?

Follow the mass and size change rather than imagining cloud droplets as already rain-sized.

Different fall speeds create collisions

Cloud droplets are typically only a few to a few tens of micrometres across and settle slowly relative to most atmospheric motions. Condensation forms and grows them initially, but precipitation-sized growth commonly requires a collection process. In collision–coalescence, larger droplets fall faster than smaller ones, encounter them and merge when the collision leads to coalescence. A broader size distribution helps start collection.

As a collector becomes larger it can intercept more droplets and fall faster, accelerating its growth, although collection efficiency, breakup and airflow around the drops complicate the actual process. Deep liquid clouds in warm moist air can efficiently produce rain without an ice phase. The name 'warm-rain process' refers to the ice-free pathway; collection can also occur in supercooled liquid clouds below 0 °C.

For ideal spherical droplets of equal density, volume is proportional to diameter cubed. Merging 1,000 droplets of 0.02 mm diameter gives one 0.2 mm drop; merging 1,000,000 gives one 2 mm drop if no water is lost. Real clouds do not merge every droplet into one perfect sphere, but the calculation shows the enormous size gap between cloud water and rain.

Why can an ice crystal grow while nearby droplets shrink?

Use the difference between saturation over ice and over supercooled water.

The Wegener–Bergeron–Findeisen process

In a mixed-phase cloud below freezing, ice crystals coexist with supercooled liquid droplets. Saturation vapour pressure over ice is lower than over liquid water at the same negative temperature. Air close to liquid-water saturation is therefore supersaturated with respect to ice. Vapour deposits onto crystals, and evaporation from liquid droplets can replace that vapour. The ice grows at the expense of the liquid-water reservoir through a vapour transfer.

This is the Wegener–Bergeron–Findeisen, or ice-crystal, precipitation-growth process. It requires the appropriate mixed-phase conditions, including ice nuclei or pre-existing ice and liquid water. An entirely liquid cloud lacks the ice collectors for this pathway, while an entirely glaciated cloud no longer has the same local droplet reservoir. Collision and collection can still occur in other forms within either cloud.

Growing crystals can aggregate into larger snowflakes or collect supercooled droplets that freeze onto them by riming. Once large enough to fall appreciably, they can reach the surface as snow, or melt into rain in a sufficiently warm lower layer. Much mid-latitude rainfall starts through ice-phase growth aloft even though the surface observer sees only liquid drops.

Must rain have started as liquid inside the cloud?

Separate growth in the cloud from phase changes below it.

Snow can become rain

Snow crystals grow by deposition and can join into aggregates; riming adds frozen droplets. If they fall through a sufficiently deep warm layer, melting changes them into raindrops. Surface rain can therefore originate from ice aloft. Snow can also reach a slightly positive surface temperature if the melting layer is shallow or evaporative cooling and the particle's thermal history prevent complete melting; a single surface thermometer does not define the whole precipitation path.

Liquid collision–coalescence can produce rain directly, especially in deep warm clouds. Smaller collected drops falling from a shallow stratiform cloud can reach the surface as drizzle. Drizzle commonly comes from low stratus, with many closely spaced small drops. A thin cloud with weak growth may remain non-precipitating, or its particles may evaporate before the ground as virga.

The particle at the surface records the cloud growth process, the layer temperatures and humidity, and the time spent falling through them. A surface weather code is therefore a useful clue but not a complete sounding. For flight, examine the layers aloft where melting, supercooling, freezing or substantial liquid water can exist, rather than assuming the surface phase applies throughout the route altitude range.

Why can hail become a major aircraft hazard?

Relate supercooled-water collection, strong convection and particle mass.

Accretion in a convective cloud

Hail develops in strong convective cloud, especially cumulonimbus, where ice embryos collect supercooled liquid droplets and grow by freezing accretion. Strong updraughts can support the growing stones and prolong their exposure to liquid water. Changes between wet and dry growth can create different layers; the layers should not be interpreted as an exact count of complete up-and-down trips through the cloud.

A stone eventually falls when its downward settling exceeds the supporting flow, although the storm's changing wind field and trajectory also matter. Hail may melt partly or completely below the freezing level, so its absence at a low warm surface does not exclude hail higher in the storm. Large stones can also be carried into regions outside the visually darkest core, including beneath parts of the anvil.

A larger diameter means a much larger mass: at equal density, doubling diameter multiplies mass by eight. Impact energy also depends on the square of relative speed. Hail can therefore severely damage windscreens, leading edges, radomes and engines. The 5 mm weather-code boundary does not establish a safe aircraft-impact threshold. Hail concern adds to, rather than replaces, the storm's turbulence, icing, lightning and outflow hazards.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.