Total column water vapour
Temperature & humidity precipitable water integrated water vapour TCWV PW
Precipitable water expresses the water vapour in a column of air as an equivalent depth of liquid water. It describes the moisture supply, not the expected rainfall total.
Imagine collecting the whole column
Picture a vertical column of air above one square metre of ground. If all its water vapour condensed, it would form a certain depth of water. 30 mm of precipitable water equals 30 kg of water per square metre. For a particular dataset, check how high the column extends.
What is counted?
The quantity counts gaseous water vapour. Existing cloud droplets, ice crystals and raindrops are not included. It therefore differs from total water content that also includes cloud water.
Plenty of moisture does not guarantee rain
A high value provides a large moisture supply. Rain generally also needs processes that lift and cool the air. Without that trigger, even a very moist column can accompany dry weather.
Why more rain can fall than the value suggests
Thirty millimetres in the column does not impose a thirty-millimetre rainfall limit. Wind can continually bring fresh vapour into a storm. A cell lingering over one area, or several passing in succession, can produce much more rain. The atmosphere is not a sealed bucket.