Rain probability and millimetres: two different measurements

A percentage does not measure the amount of rain

Precipitation probability describes the chance of measurable precipitation during the stated interval and at the point represented by the forecast. A high percentage does not automatically mean heavy rain.

The operational definition can vary between products and providers. Meteo Interactive therefore shows the source, update time and time interval and avoids comparing percentages that refer to different periods.

Millimetres describe an amount

One millimetre of rain is equivalent to one litre of water spread over one square metre. A forecast value is a model estimate, not a guarantee that every point in the area will receive exactly that amount.

Showers and thunderstorms can create large differences over short distances. Read millimetres alongside probability, the hourly sequence and, when available, recent radar imagery.

A practical way to read the forecast

First note when the probability rises, then check the forecast amount for the same hours. Finally, see whether the signal persists across updates or changes quickly.

For weather-sensitive activities, always use bulletins and warnings from the competent authorities. An automated forecast is informative and does not replace an official notice.

A comparison using invented numbers

Teaching example, not a forecast: imagine two intervals of equal length for the same location. Row A has 80% probability and 0.4 mm; row B has 40% and 3 mm. A indicates a greater chance of precipitation; B a larger forecast amount. Neither column alone answers both questions.

Do not multiply 3 mm by 40% to obtain a “corrected” amount. The displayed amount is already a forecast product and is not labelled as an amount conditional on rain occurring. These two numbers do not let you reconstruct the scenario distribution or promise how much rain will fall.

Which comparisons the site supports

Open a city page and record the location, source, data time and period represented by each value. The current summary, hourly probability and totals for the next days have different time scales: do not compare the current percentage with a whole day’s millimetres as though they covered the same interval.

Open-Meteo defines hourly probability and amount for the preceding hour; probability uses a threshold above 0.1 mm. Do not automatically apply that hourly definition to current or daily values.

If a comparable field is missing, keep the available value and mark the other unavailable: a dash does not mean zero. To compare updates, first save values for the same location and valid time; reading only the latest panel cannot reconstruct the previous one.

Why two percentages do not make a total

If two consecutive hours each show 40%, adding them does not yield an 80% chance over the whole period. Taking their average also answers a different question. The chance of rain at least once depends on how the scenarios for those hours are connected; the percentages alone do not tell you.

Millimetres can be added when they cover separate, non-overlapping intervals in the same product and update: 1 mm in one hour and 2 mm in the next give 3 mm over two hours. That forecast total does not specify the minute rain will fall and is not an observed measurement.