Precipitable water (PW) measures total water vapour in the atmospheric column. Typical values range from 5–10 mm in dry polar air to 50–70 mm in moist tropical air. It is measured by radiosondes, GPS, and satellites.
Heavy rain prediction
High PW does not guarantee rain (a lifting mechanism is needed), but PW above 40 mm combined with convergence indicates high potential for torrential rain. In the Mediterranean, PW values >45 mm in autumn with an upper-level DANA are the classic combination for extreme rainfall.