Precipitations probability

The probability of precipitation , or rain probability for rain , incorrectly also called rain risk , is the probability that there will be at least one precipitation event in the forecast period for a specific location .

The information is formally independent of the amount and duration of the precipitation, which are usually communicated separately. If a local broadcaster reports that showers can be expected for tomorrow and the probability of precipitation for the period from 6 a.m. to 12 p.m. is 50%, then there is a good chance of staying dry during a short stay outdoors, because showers are far shorter than six Hours.

In weather forecasts, the probability of rain for an area is usually given. This is the mean value of the rain probability for the individual points in the area. A holidaymaker who wants to climb the Harzer Brocken the next day , with a rain probability of 50% specified for the whole of northern Germany, is much more likely to have rain at least once, because the local climate is more humid than in the surrounding area.

For forecast periods that are significantly longer than a few days in the future, the probability of precipitation is the long-term mean value of the precipitation frequency for this time of year and day. For very short-term forecasts, a look at the sky, taking the wind direction into account, helps. In between, various methods help: With knowledge of the current weather situation , it is possible to state on the basis of weather records which probabilities the weather situation will change and how, and from this to calculate the probability of precipitation. The much more complex numerical weather forecast is more meaningful for medium forecast periods and does not lose spatial resolution as quickly. Several slightly different simulation runs are started: Inaccurate and incomplete observation data can be spatially and temporally compensated and interpolated in various ways. There are also adjusting screws for the general consideration of physical processes below the grid resolution. For the different forecast periods, e.g. B. in the 6-h grid, and a tight grid of points is determined for each of the solutions whether there are precipitation events. Then the family of solutions is averaged in order to obtain the precipitation probabilities for the points, and averaged over the points of an area for the information on the area.

The utility of the precipitation probability depends on the application. It is useful, for example, when a short rain shortly before the grain harvest would be a problem. On the other hand, if the farmer asks himself whether he should irrigate his field, he would be better off with the expected value of the amount of precipitation .

Many weather forecasts intentionally indicate low precipitation probabilities higher than they actually are. See Wet bias (English for roughly “bias / distortion towards the wet”).

example

A predicted rain probability of 100% for the city of Wuppertal for November 1st means that under comparable weather conditions (air pressure, wind direction, air masses, etc.) it has always rained somewhere in Wuppertal in the past , so it is almost certainly also will rain on November 1st in the urban area. It cannot therefore be deduced from this that it will rain all day (100% of the period) or that it will rain everywhere in Wuppertal (100% of the forecast area).

Definition in the USA

In the USA, the national weather service defines the probability of precipitation ( English Probability of Precipitation ) by multiplying the probability that it will rain somewhere in a certain area by the proportion of the total area in which it will rain, if it rains at all (precipitation amount greater than 0, 01 ") This definition describes a combination of the reliability of the forecast and the expected amount of precipitation.

literature

Christoph C. Raible et al .: Short-term predictions of precipitation probability and temperature . promet 27, 2001, pp. 80-86, ( online ).

Web links

Antonia Rötger: Confusing information . Press release, September 12, 2003.

Probability of precipitation in the weather dictionary of the German Weather Service

Individual evidence

↑ Motherboard Staff: What It Really Means When The Chance Of Rain Is 50 Percent. In: Vice. August 22, 2016, accessed May 11, 2019 .
^ NOAA National Weather Service: FAQ - What is the Meaning of PoP. Retrieved May 11, 2019 (American English).

[1] Motherboard Staff: What It Really Means When The Chance Of Rain Is 50 Percent. In: Vice. August 22, 2016, accessed May 11, 2019 .

[2] NOAA National Weather Service: FAQ - What is the Meaning of PoP. Retrieved May 11, 2019 (American English).