What Is Precipitation?
Precipitation is any form of water — liquid or frozen — that falls from clouds to the ground. Rain, snow, sleet, hail, drizzle, and freezing rain are all types of precipitation.
It happens when water vapour in the atmosphere condenses into droplets or ice crystals inside clouds. Once these particles grow heavy enough, gravity pulls them down. What form they take when they reach you depends on the temperature of the air between the cloud and the ground.
Types of Precipitation
Rain
The most common type. Water droplets fall when the air temperature between the cloud and the ground stays above 0°C the whole way down. Droplet size ranges from 0.5mm to around 5mm — anything smaller is drizzle.
Drizzle
Very fine, closely spaced droplets smaller than 0.5mm. Falls from low stratus clouds and often comes with fog or overcast skies. Light but persistent.
Snow
Ice crystals that form when the atmosphere is at or below freezing for most of the journey from cloud to ground. Snow can still reach the surface when ground temperatures are a few degrees above 0°C — if the warm layer near the surface is shallow, the flakes don’t have time to melt completely. Snowflakes are clusters of individual ice crystals — their shape depends on temperature and humidity. The “fluffiest” snow tends to fall in very cold, dry conditions.
Sleet (Ice Pellets)
Starts as snow, melts into rain as it passes through a warm layer of air, then refreezes into small ice pellets before hitting the ground. Common during transitional weather when temperatures hover around freezing.
Freezing Rain
Starts as snow, melts completely in a warm layer aloft, then falls through a shallow subfreezing layer near the surface. The droplets become supercooled — still liquid but below 0°C — and freeze instantly on contact with roads, pavements, power lines, and trees. This is the most dangerous type of precipitation — even a thin coating of ice can make surfaces treacherous.
Hail
Ice balls that form inside strong thunderstorm updrafts. Water droplets get carried upward into freezing air, freeze, fall, get caught by the updraft again, accumulate another layer of ice, and repeat. Hailstones start at around 5mm in diameter and can grow to golf ball-sized (~4.5cm) or larger. Unlike other precipitation types, hail can fall during warm weather because it forms high in the atmosphere where temperatures are well below freezing.
Graupel (Snow Pellets)
Soft, small balls of ice that form when supercooled water droplets freeze onto falling snowflakes. Looks like tiny styrofoam balls. Often a sign that a thunderstorm or heavy shower is nearby. Sometimes mistaken for hail, but graupel is softer and crumbles when squeezed.
How Precipitation Is Measured
Precipitation is measured in millimetres (mm) or inches (in) — the depth of water that would accumulate on a flat surface.
- 1mm of rain = 1 litre of water per square metre of ground
- 7.5mm of rain or more in an hour is a heavy downpour
- 1mm of rain roughly equals 10mm of snow (though this ratio varies with temperature and snow density)
Weather stations use rain gauges — simple containers that collect rainfall over a set period. Modern automated stations use tipping bucket gauges that record each small increment electronically, providing real-time data.
What Precipitation Numbers Mean in a Forecast
When you see precipitation in a forecast, there are typically two numbers:
Amount — how much rain or snow is expected, in mm or inches. This tells you the intensity:
| Rainfall | Intensity |
|---|---|
| Under 2.5mm/hr | Light rain |
| 2.5–7.5mm/hr | Moderate rain |
| 7.5–50mm/hr | Heavy rain |
| Over 50mm/hr | Violent rain |
Probability — the chance of precipitation occurring, shown as a percentage. A 40% chance of rain means there’s a 4-in-10 likelihood that rain will fall at your location during that period. It doesn’t mean it’ll rain 40% of the time — it’s about likelihood, not duration.
Reading Precipitation in Context
The same amount of precipitation can mean very different things depending on how it falls.
30mm of rain over 12 hours is a steady, manageable rain. 30mm in one hour can cause flash flooding. Intensity matters more than the total.
With snow, the amount that accumulates depends on temperature. Cold, dry snow packs loosely — 1cm of water can produce up to 15cm of snow. Warmer, wetter snow is denser, so the same 1cm of water might only give you 5cm on the ground.
Freezing rain is the most deceptive. Even a few millimetres can coat roads and power lines in ice, creating dangerous conditions that a much heavier rainfall wouldn’t.
How Airpult Shows Precipitation
On Airpult, the hourly and daily forecast shows expected precipitation amounts and probability for your location. The hourly breakdown helps you plan around the heaviest periods — so you know not just whether it’ll rain, but when the worst of it is expected. Find a location to see it in your forecast.