Precipitable water

Under precipitable rainwater (English precipitable water, abbreviation PW) is understood in meteorology and climatology that amount of liquid water in a column with a defined area over the entire height of the atmosphere as the (integrated ) water vapor (english integrated water vapor, abbreviation DP) is available. Specifying this maximum achievable rainfall, or water column height is done in millimeters ( mm). If we translate the term precipitable water directly from English into German, this is the preliminary (not quite correct ) result " rainwater ". This is but that amount of water that provides rain in a defined period of time, regardless of the available rainwater.

Application

The determination of precipitable water precipitation, ie of possible achievable precipitation values ​​is always important. In particular, weather services and climate scientists to make predictions therefore require this information to models and (examples):

• Mountain rescue and alpine rescue - snow reports and avalanche warnings
• Civil protection - flood warnings etc.
• General - heavy rain warnings, and other threats (eg hail)

Example

In a cube with a volume of one cubic meter and an area of ​​one square meter to "wet" air close to saturation at 20 ° C, but still no clouds, or droplet located. Since this is only a small cube we may assume a constant density. The present in the air in the cube water vapor has a weight of around 18 g Well stacked one above the other 1,000 such cubes (assuming that this is the thickness of a normal cloud, and thereby neglecting other variables such as pressure, temperature, etc. ). Being in this tower with 1,000 m altitude water vapor present is thus 18 kg. Could you " squeeze " the tower as a citrus fruit, the result is 18 kg or about 18 L of water. These 18 liters of water in a tank with 1 m² area would result in a water level of 18 mm. Therefore, our cloud with 1000 m height at the saturation would result in a precipitation water of 1 mm or 1 l / m².

In real terms, the amount to be included to extend over the entire atmosphere, and have all the essential parameters such as pressure, temperature, etc. into the density function are considered.

Calculation

It integrates with the aid of a density function of the water vapor over a defined area the entire water vapor in the atmosphere above ( column ) and thus receives pulse dialing

If you have pulse dialing calculated, so you need the density of water, to go to rainwater PW

Measurement

There are different methods:

• Measuring the intensity of radiation of two different wavelengths of the sun, one of which is absorbed by water. This is calculated using Lambert- Beerschem law.
• Radiosonde measurement (relative humidity, air pressure and temperature) and unification of all parameters in a density function and integrating over the entire height of the atmosphere ( see above).