Two forecasts can both show 20 mm of rain yet describe very different situations. Spread across a day, that amount may cause little disruption; falling in twenty minutes, it may overwhelm drainage and produce rapid surface flooding. The key is to separate rainfall intensity from accumulated precipitation.
What one millimetre means
One millimetre of rain equals one litre of water over each square metre, assuming none evaporates, infiltrates or runs away. A rain gauge measures the depth of that theoretical layer on a level surface.
Accumulation: how much water falls
An accumulation adds all precipitation during a stated interval—one hour, six hours, a day or an entire event. The time window is essential: “30 mm” without start and end times is incomplete information.
Totals help assess soil saturation, river response and water resources, but cannot show whether the rain arrived as a short downpour or a long moderate spell.
Intensity: how fast it falls
Intensity is the precipitation rate, commonly expressed in millimetres per hour (mm/h). A rate of 12 mm/h means 12 mm would accumulate if that rate continued for an hour. It does not mean it will last that long; a shower may reach a high rate for only a few minutes.
Check whether an app displays an instantaneous rate, a ten-minute average or an hourly average, because different averaging periods can produce different-looking values.
Why equal totals can have unequal impacts
- Duration: concentrated rainfall gives drains less time to clear water.
- Ground conditions: compacted, burnt or already saturated soil produces more runoff.
- Urban layout: paving, slopes, underpasses and blocked drains concentrate water.
- Storm movement: slow or stationary cells repeatedly affect the same place.
- Local variation: one gauge may receive far less than a nearby district beneath the heaviest core.
A practical way to read the forecast
- Identify the accumulation period: 1 h, 6 h, 24 h or whole event.
- Check the maximum rainfall rate, not only the daily total.
- Inspect the hourly distribution; a modest daily total may be packed into a short window.
- Use official warnings, radar and observations as the event approaches.
- Never cross flooded roads or channels after the rain eases; runoff can still arrive from upstream.
The useful rule is simple: accumulation tells you how much; intensity tells you how quickly. Real impacts depend on both, plus duration, terrain and local vulnerability.