Precipitation amount
Precipitation & snow precipitation total accumulated precipitation rainfall amount
Precipitation amount describes how much water falls on an area. One millimetre of precipitation equals exactly one litre per square metre.
Millimetres you do not need to find with a ruler
Imagine spreading the precipitation evenly across a level, watertight surface, with no runoff or evaporation. The depth of that water layer is the precipitation amount. 1 mm = 1 l/m². Ten millimetres delivers 100 litres onto a ten-square-metre patio.
An amount needs a time period
Ten millimetres in a day can arrive over many hours or in one brief downpour. That makes a considerable difference to runoff and puddles. Intensity describes the rate, for example in mm/h; a total counts water over the stated period.
Melting snow makes it comparable
In a precipitation total, snow is expressed as the water left after melting. Ten millimetres of water is not ten millimetres of snow depth. Loose snow occupies much more space than wet snow containing the same water.
An average is not a rain quota
Monthly and annual means combine measurements over many years. A single shower can bring plenty of water to one place and miss a neighbouring district. The station and reference period are therefore essential for local comparisons.
In Hanover
How much falls in Hannover?
For the DWD station Hannover, the 1991–2020 average is 626.7 mm per year. April averaged 35.5 mm and July 68.2 mm. These are averages across many years, not monthly rain quotas. One shower may deliver plenty of water to one place and miss the neighbouring district.
A station value describes its measuring site. How much soaks into the soil or runs off also depends on the surface, existing soil moisture and rainfall intensity.