Guide
How much is two millimetres of rain?
Rainfall is measured in millimetres, but few people have a feel for what the number means in practice. Here's the translation.
One millimetre of precipitation means the rain would form a one-millimetre-deep layer of water if nothing ran off or evaporated. That's exactly one litre of water per square metre. Two millimetres is therefore two litres per square metre – over a typical house plot of 700 square metres that's 1,400 litres, roughly seven bathtubs. That sounds like a lot, but spread over the whole area it's just a damp afternoon.
The time period is crucial
The most common misunderstanding is confusing total amount with intensity. Twenty millimetres in a day is a perfectly ordinary rainy autumn day in Sweden. Twenty millimetres in an hour is a cloudburst that overwhelms stormwater systems and floods basements. Same number, completely different consequence. Always check the unit: mm/h describes how hard it's pouring right now, mm/day how much accumulates in total.
| Intensity | Name | How it feels |
|---|---|---|
| < 0.5 mm/h | Drizzle | Barely wets the streets. An umbrella is rarely needed. |
| 0.5–2 mm/h | Light rain | The odd puddle. You get wet after a while outside. |
| 2–6 mm/h | Moderate rain | Audible against the window. Gutters start to flow. |
| 6–15 mm/h | Heavy rain | Visibility worsens. Puddles form quickly. |
| 15–30 mm/h | Downpour | Soaked within a minute. Stormwater can't keep up. |
| > 30 mm/h | Cloudburst | Flooded streets, a white haze of water. Unusual. |
Droplet size and fall speed
Drizzle drops are about half a millimetre in diameter and fall at a couple of metres per second – slow enough that the wind easily carries them sideways. A heavy raindrop is three to five millimetres and reaches nine metres per second. Drops can't get much bigger than that; air resistance breaks them apart on the way down. That's why a cloudburst isn't made of giant drops but of an enormous number of ordinary drops packed tightly together, which gives it the characteristic white haze.
Why the city gets wetter than the forest
In a forest, the tree canopy catches the first few millimetres entirely, and the ground then slowly absorbs the rest. The same amount of rain on asphalt has nowhere to go: everything runs straight to the drains. Swedish stormwater systems are often built for around 10 to 20 millimetres per hour. Once that level is exceeded, water stands on the street within minutes, no matter how well the system otherwise works.
What the forecast actually promises
Precipitation forecasts normally give an average amount over an area and an hour. A thunderstorm shower can therefore show 3 mm in the forecast while simultaneously delivering 15 mm on a single street while the neighbouring area stays dry. The more convective the weather – that is, shower-driven rather than a widespread rain area – the less precise the number becomes for your specific address.
Try it yourself
Drag the rain slider in the visualiser and see the difference between drizzle and a cloudburst, in a city setting or in the forest.
Visualise rain