Precipitation is everything that falls from a cloud and reaches the ground: rain, drizzle, snow, hail. It feeds rivers, aquifers and crops, but also floods and landslides. To make sense of a forecast you need to know the words, the units and the typical amounts.
Drizzle, rain, snow, hail: the words
Drizzle is made of very fine drops, under 0.5 mm across, that seem to float; it falls from stratus. Rain is made of drops of 0.5 mm or more. A shower is rain that starts and stops abruptly, with rapid changes of intensity: it comes from heaped clouds (cumulus, cumulonimbus). Steady rain, by contrast, comes from an extensive layer, the nimbostratus.
Snow is made of ice crystals, single or clumped into flakes. Hail is made of balls or lumps of ice at least 5 mm across, formed in the most active cumulonimbus. Between the two, snow pellets, or graupel, are small, softer grains of ice. Freezing rain, finally, is liquid rain that freezes on contact with the ground: the most dangerous kind for roads.
How a raindrop forms
A cloud droplet measures about one hundredth of a millimetre: far too small to fall. To become a raindrop it has to grow a millionfold in volume. Two mechanisms make this possible. In warm clouds, droplets of different sizes collide and merge as they fall: this is coalescence. In cold clouds, ice crystals sit alongside supercooled droplets and grow at their expense, because vapour deposits more easily on ice: this is the Bergeron process.
The crystals formed this way fall. If they pass through a layer warmer than 0 °C they melt and arrive as rain; if the air stays cold down to the ground they arrive as snow. That is why winter rain is often snow that has melted on the way.
Millimetres, litres and centimetres of snow
One millimetre of precipitation corresponds to one litre of water per square metre. Ten millimetres of rain therefore means ten litres on every square metre, or a hundred thousand litres on a hectare. A rain gauge measures this depth of water directly in a graduated container.
For snow, depth is sometimes given in centimetres, but the water equivalent varies a great deal. A rule of thumb says that ten centimetres of fresh snow yield one centimetre of water, a ratio of ten to one; the ratio is lower for wet snow and higher for powder.
Intensity classes
Intensity is measured in millimetres per hour. The American Meteorological Society glossary distinguishes light rain (under 2.5 mm/h), moderate (2.5 to 7.6 mm/h) and heavy (over 7.6 mm/h), and calls rain of more than 50 mm/h violent. Light rain barely wets you; heavy rain cuts visibility and sends water running; violent rain quickly causes flooding.
Intensity matters as much as the total. Ten millimetres over a day of fine rain are absorbed by the soil; ten millimetres in ten minutes are not. That is why rain and flood warnings take account of totals over one, three or six hours.
A worked example: Toronto
For Toronto, using the forecast of 2026-11-02 08:13: 2.9 mm of rain are expected over the next 48 hours, with a peak of 2.5 mm in a single hour, which corresponds to rain of the "moderate" class.
Frequently asked questions
What does 10 mm of rain mean?
Ten litres of water per square metre, a depth of ten millimetres if the water did not run off. It is a notable amount over a day, but for run-off what counts is how fast it falls.
What is the difference between drizzle, rain and a shower?
Drizzle is very fine drops under 0.5 mm. Rain has bigger drops. A shower is sudden rain that starts and stops quickly, from heaped clouds.
What does "40% chance of rain" mean?
In the Open-Meteo forecasts behind this site, the probability of precipitation is the share of ensemble scenarios that give more than 0.1 mm in the hour. It says nothing about intensity.
Sources
- World Meteorological Organization (WMO), International Cloud Atlas, WMO-No. 407: definitions of drizzle, rain, snow and hail (hydrometeors).
- American Meteorological Society, Glossary of Meteorology, entry "rain": intensity classes in millimetres per hour.
- Today's example: hourly precipitation from the Open-Meteo forecast over the 48-hour window.