Rain maps look alike, but a rain radar and a rain forecast answer different questions. A radar shows where rain is falling right now. A forecast shows where a computer model expects rain to fall over the next hours. Knowing which one you are looking at, and how to read its units, makes both far more useful.
Rain radar: what is falling now
A weather radar sends out pulses and measures how much of the signal bounces back off raindrops. More and bigger drops send back more, so heavier rain shows as stronger colours. Radar images update every few minutes, which makes them the best tool for the next half hour: you can watch a storm move and judge whether it will reach you.
Radar has limits. It can miss light drizzle, it can show "rain" that evaporates before reaching the ground, and it does not know what the storm will do next. Use it for now and the next short while.
Rain forecast: what is expected
A forecast comes from a weather model: a computer simulation of the atmosphere, run several times a day. The Bangkok flood map uses the ICON model from Germany's national weather service, via Open-Meteo, and plays it hour by hour on its "Rain & radar" tab. Forecasts are good at the broad picture, such as whether a wet evening or a big band of rain is on the way, but less sure about exactly which district a single thunderstorm will hit.
Millimetres per hour: what the numbers mean
Rain rate is measured in millimetres per hour (mm/h): how deep the water would get in an hour on flat ground that did not drain. As a rough guide:
- Under 2.5 mm/h: light rain. An umbrella is enough and roads stay mostly clear.
- 2.5 to 7.6 mm/h: moderate rain. Steady rain; low spots start to collect water.
- 7.6 to 50 mm/h: heavy rain. Drains struggle; flooded roads become likely, especially if it lasts.
- Over 50 mm/h: violent rain. Rare and short, but it can flood streets within minutes.
Duration matters as much as rate. Ten minutes at 20 mm/h adds a few millimetres of water; three hours at 8 mm/h adds more than 20 and is far more likely to flood roads.
Reading the colours
On the flood map's forecast layer, light blue is light rain, deeper blue and purple are moderate, pink and red are heavy, and orange to yellow is very heavy. Point at or tap any spot to see the rate in mm/h. Blurry edges are normal: the model works on a grid, so small storms appear as soft blobs rather than sharp shapes.
"Chance of rain" is not "how much rain"
A 90% chance of rain means rain is very likely at some point in that hour. It says nothing about how much. A 90% chance of 0.5 mm is a light shower; a 40% chance of 15 mm is a much bigger worry. Look at the rain chance and the amount together. The map's hourly chart shows both: bars for millimetres and a dashed line for the chance.
Using both before a trip
- Check the hourly forecast for the part of the city you are going to.
- If heavy rain is expected, look at the radar to see where it is now and which way it is moving.
- Check the flooded-road reports along your route.
- If you can, travel before the heavy band arrives or an hour or two after it passes.
All of this is on one page: open the Bangkok flood map and switch to "Rain & radar". For Thai readers, the same page is available in Thai at แผนที่น้ำท่วมกรุงเทพ.
