Moisture and precipitation
What falls out of a cloud is decided by a temperature profile you cannot see from the ground — which is why the precipitation type in a report tells you about air thousands of feet up.
Weather Information — FI.III.C · CA.I.C · PA.I.C
Three ingredients
Precipitation needs water vapour, enough lift to condense it into cloud, and a growth process to make the droplets heavy enough to fall. Cloud alone is not enough — most clouds never rain, because the third ingredient never gets going.
Two ways a droplet gets big enough
Collision-coalescence, the warm rain process: droplets of different sizes fall at different speeds, so bigger ones catch smaller ones and merge. It needs no ice at all, which is why tropical clouds rain heavily without ever freezing.
The ice crystal process: water vapour deposits directly onto ice crystals, which grow until they are heavy enough to fall. In middle latitudes this is how most precipitation starts — including a lot of summer rain, which began as snow and melted on the way down.
The vertical temperature profile decides the type
This is the part worth being able to reason through rather than memorise, because it works in both directions: given the profile you can predict the type, and given the type you can infer the profile.
| At the surface | What the air column looks like |
|---|---|
| Rain | A deep above-freezing layer based at the surface. |
| Snow | Below freezing through the entire depth — it never melts on the way down. |
| Ice pellets | A shallow above-freezing layer aloft over a deep below-freezing layer at the surface. The flake partly melts, then refreezes solid before it lands. |
| Freezing rain | A deep above-freezing layer aloft over a shallow below-freezing layer at the surface. It melts completely, then becomes supercooled in the shallow cold layer and freezes on contact. |
Ice pellets and freezing rain are the same two layers in opposite proportions. That is the whole distinction, and it is why they so often change into one another as a warm layer thickens or thins overhead.
Ice pellets are a warning about the air above you.
They mean there is a layer up there warm enough to melt snow — and therefore a layer
that will produce freezing rain if it deepens, or if you climb into the supercooled
water it contains. A report of PL is a reason to think hard before
climbing, not simply a note about the runway. See
structural icing.
Dewpoint, and why it is the number to watch
The dewpoint is the temperature to which air must be cooled to become saturated. It is a measure of how much moisture is actually in the air — unlike relative humidity, which changes when you only change the temperature.
Temperature and dewpoint converging means the air is approaching saturation, which is why the spread predicts fog and why the cloud base calculator is built from it: an unsaturated parcel lifted from the surface cools about 2.5 °C per 1,000 ft faster than its dewpoint falls, so the spread closes with height at a predictable rate.
Where you meet it next
- Clouds — where the moisture becomes visible
- Structural icing — supercooled water, and what it does
- Fog — saturation at the surface
- Reading the precipitation groups in a METAR
Sources
Growth processes and precipitation types 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.