Precipitation is the deposition of water to the Earth's surface, in the form of rain, snow, ice or hail.
All precipitation quantities are expressed in millimetres (mm) of liquid water equivalent for the preceeding time interval (or in inches). One millimetre of rain corresponds to 1 litre per square metre of water on the surface, or approximately 10 millimetres of snow.
We present our best high resolution precipitation forecast in the point meteograms, pictocast, rainSPOT and on the maps.
Other presentations and scales can be supplied on request.

Types

meteoblue forecasts precipitation types in the form of:

• rain (liquid),
• snow (crystals),
• ice (frozen water), ice rain, freezing rain,
• dew (condensation on surfaces), for Agro services.

Other precipitation types, such as hail (solid ice) can be calculated for special services.

Convective and stratiform precipitation

Precipitation falls in 3 forms:

• Convective precipitation falls as showers with rapidly changing intensity, and over a certain area for a relatively short time, as convective clouds have limited horizontal extent.
• Orographic precipitation falls when masses of air pushed by wind are forced up the side of elevated land formations, such as large mountains.
• Stratiform precipitation caused by frontal systems (mainly of cold air) that usually bring rain distributed in a uniform way over a larger area.

meteoblue shows precipitation quantities as a total. In the meteograms, precipitation is divided into convective (includes both convective and orographic) in light blue (showers) and total (by adding stratiform precipitation) in dark blue. Convective precipitation is likely to be more erratic, and unevenly distributed than stratiform precipitation, so actual quantities, spatial distribution and probabilities are likely to vary much more compared to average display values than for stratiform precipitation.

Snowfall is shown with "***" and freezing rain as "!". Hail is only displayed for special services. If precipitation falls as snow, the height of the snow cover can be indicated separately - otherwise, multiply the quantity of water by the factor 10 to obtain fresh snow cover.

If the temperatures are above 3°C, precipitation will usually be rainfall. This limit of 3°C is an indication. For snowfall to take place, the atmosphere must be below zero degrees in the range of 0.5-5 km above ground. Temperatures at the ground level are almost always higher than above, and sometimes above 0°C, when snowflakes fall. If the temperatures are too high, then the snow flakes will melt before reaching the ground.
In some cases (e.g. warm fronts), warm moist air may over-flow above cold air. The precipitation then is originally rain, which turns into snow on the way to the ground, when it reaches the cold air layer. In same cases (with a very cold air layer above ground), the rainfall can freeze, causing "freezing rain", which is made entirely of liquid droplets. The raindrops are supercooled while passing through a sub-freezing layer of air above the ground with a few hundreds of meters of thickness, and then freeze, when they hit any surface, including the ground, trees, electrical wires, aircraft, and automobiles. The resulting ice is called glaze ice "or "Blitzeis"), and can accumulate to a thickness of several centimetres. The METAR code for freezing rain is FZRA.

Precipitation quantity

Precipitation is measured in quantity for a certain time interval, e.g. millimetres per hour. The measurement is always valid for the period preceeding the given time. For example, the 3 hours precipitation shown for 12:00 UTC shows the total quantity in mm (= l/m²) between 9:00 UTC and 12:00 UTC.

Measurement of precipitation is the attempt to represent a large scale non-uniform process by sampling methods, large scale raster images or modeling. meteoblue compares precipitation forecasts to measurements based on WMO standards, unless otherwise indicated.

Precipitation quantity values from simulations are valid for the the area of a grid cell, e.g. 10x10 km. A quantity of 10 mm can therefore be:
= 10 mm distributed uniformly across 10x10 km, or
= 20 mm in 50% of the are of 10x10 km, or
= 100 mm in 10% of the are of 10x10 km, or
= 20 mm on one side and 0 mm on the other side of 10x10 km, or
= any other combination adding up to a quantity of 10 mm over 10x10 km.
For tropical climates, precipitations can be very local, and this will make it more difficult to interpret (and calculate) the predictions and measure the actual quantities, because precipitation quantity can change by 20 mm within 500 meters.

Methods and units

Precipitation can be measured using 3 methods:

1. Local weather stations: with pluviometre ("raingauges", "snowgauges").
2. Tele-detection: using reflection of radar, the distribution of precipitation in the atmosphere is calculated.
3. Local indirect observation: using bowls, surface cavities, surface runoff or flood gauges.

Measurement units are millimetre, centimetre or inches; 1 millimetre corresponds to 1 litre of water per square meter.
Snow precipitation is measured in centimetres or inches. Fresh cover thickness is reduced over time by deposition and compaction.
meteoblue uses metric units for presenting precipitation; Liquid precipitation is presented in mm for the displayed time period (day or hour). Solid precipitation (snow) is presented in mm of water equivalent (WE), or cm of height for a given time.

Probability

Precipitation probability (as well as some other parameters) are calculated from an ensemble of 20 model runs. The probability is the frequency with which precipitation occurs in these 20 different forecast calculations. This calculation is usually made for an area (grid cell) of 50x50 km. A given probability is therefore displayed for a larger area, whereas precipitation quantities are calculated in higher resolution; this may sometimes lead to inconsistencies, with precipitation being displayed for certain locations despite an overall low precipitation probability, or no precipitation being displayed for certain locations despite an overall high precipitation probability. Further details are provided under interpretation (below).

The precipitation probability is independent from the predicted precipitation quantity. For example, a drizzle can occur with a very high probability through a precipitation quantiy of less than 1 mm per event (e.g. an afternoon). A thunderstorm can occur at the forecast location with very low probability, but deposits a precipitation quantity of more than 10 mm per event in case of occurrence.
The presentation of precipitation in the forecast depends on the probability; if a rain icon is shown in the Pictogram, the probability of rain is above 15%.

Interpretation

The precision of the precipitation information depends very much on the type of precipitation, topography and the method of observation used. A single method is not able to perfectly describe precipitation quantities within an area, as the quantity can vary substantially within a few hundred metres of distance. The method of precipitation measurements serves as an indication of developments and must be checked by appropriate local reference methods.
More information about precipitation is shown in our meteoScool page under the sections about precipitation and thunderstorms.

Resolution in mountainous areas

In Europe, the superposition of the high resolution 3-day-forecast model with 3x3km grid cells and 12x12km grid cells 7-day-forecast model can induce an “over-estimation” effect for the precipitation forecast of day 4 to 7. Indeed, forecast precipitation in the mountains will be heavier from the 4th day to the 7th.

This is a special effect of our European high-resolution domain: precipitation of day 1 to 3 will be calculated on a 3x3 km grid. In mountainous regions, the grid permits a clear distinction between valleys and mountains for the first 3 days. From day 4 to 7, however, forecast is calculated on 12x12 km grid cells; therefore, part of the valley will show precipitations, even though they are only forecast in the mountains. For this reason, locations in the valleys will indicate precipitations for day 4 to 7, even if none is forecast for the first 3 days. Up to now, we have not yet found an “easy” way to eliminate this effect. Forecasts from day 4 to 7 will then be calculated on 12x12 km grid cells, as a longer-term forecast calculation on the high resolution model (3x3 km) would necessitate extra efforts of calculations; and diminishing quantities of precipitations in the valleys (more populated) would “artificially” reduce precipitation forecasts in the mountainous regions (less populated, but important nonetheless).

We have thus decided not to eliminate this effect through whatever calculation, but to rather provide an interpretation through this explanation.

Verification

Precipitation forecasts and actual precipitation amount are part of the most important things in meteorology. Short term verification shows the difference between weather forecasts and the actual weather. However it’s very difficult to get credible data. Wrong forecasts would be even worse, than none.
For the following reasons, we don’t do short time verifications for precipitation so far:

1. The amount of precipitation is measured only at a small part of weather stations.
2. The method of measurement differs strongly: Some stations measure hourly (e.g. Meteoswiss), some every 3 hours, some twice a day (morning and evening), some just once a day. Hourly comparisons are almost not possible. To do an uniform daily amount out of the different time intervals, doesn’t work well neither.
3. To get credible measures for precipitation is even harder: in many stations occur occasional errors. Conclusion: Measurements without verifying are worse than forecasts.
4. Losses are almost impossible to notice for precipitation amounts: Zero can mean no precipitation or the failure of the station.
5. We offer our services all over the world: Credible measurements in a station net (like e.g. in Switzerland or in Germany) do not allow us to get credible information about the precipitation amount e.g. in Italy or in Austria.

Therefore a verification of precipitation amounts needs a credible station net, a certain surface coverage, and an incoming quality control, which is time-consuming and therefore not automatically updatable.

However we have this theme on the list and we also will introduce such comparisons on monthly and yearly basis. As a first step, this year our service history+ will be supplemented with satellite data, where you can make such comparisons exhaustively and also download the data for comparisons with your own data.