Every coloured map of rain, heat or drought is built from numbers, but not all of them are the same kind of number. Some were measured, some were estimated and some are predictions. On screen they look identical. What you can safely do with each one is very different.
Measured: what an instrument recorded
A measured value comes from an instrument at a place: a rain gauge, a thermometer in a ventilated screen, a water-level sensor on a river bank. It describes that spot at that time, and it is the closest thing to a fact a map can show.
Its limits are about coverage and delivery. A gauge describes its own location, not the whole district around it. Readings can also arrive late or not at all, so a measured value should always carry the time it was taken.
Estimated: what a model filled in
Most places have no instrument close by, so the gaps are filled by models. Satellites and physical models of the atmosphere, guided by the measurements that do exist, produce values for every grid cell. Reanalysis datasets such as ERA5, which we use for our drought maps, work this way.
An estimate can be very good, especially for broad patterns over weeks or months. But it is still an estimate, with uncertainty that grows where measurements are sparse and over small areas.
Forecast: what a model expects next
A forecast is a statement about something that has not happened yet. It can be built from the same models as an estimate, but it is checked in a different way: by comparing past forecasts with what actually followed.
That test is why we do not show our flood forecasts yet. Until a forecast has been scored against past events, nobody, including us, knows how far to trust it.
Why the difference disappears on screen
A map turns all three into the same coloured shapes. A province shaded dark red looks equally certain whether the colour came from a gauge reading this morning, a model estimate for last month or a prediction for next week.
Indices make it harder still. Once a value is converted into a category such as "severe", the reader no longer sees whether it started as a measurement, an estimate or a forecast.
Three questions to ask
First, was this number measured, estimated or forecast? If the page does not say, assume it was estimated.
Second, when was it produced, and what period does it describe? A value shown today may describe last month.
Third, what has it been checked against? A measurement can be checked against a nearby instrument and a forecast against past events. If nothing has been checked, treat the number with that in mind.
How we label our own numbers
We try to say which kind of number each of our maps shows, the date it covers and what it does not measure. Where we have not yet tested something, we say so rather than leaving a blank.
For warnings and evacuation orders in Thailand, follow the Department of Disaster Prevention and Mitigation (DDPM), the Thai Meteorological Department and your local authorities. The DDPM hotline is 1784; for medical emergencies, call 1669.