Vertically integrated liquid
Vertically integrated liquid is an estimate of the total mass of precipitation in the clouds. The measurement is obtained by observing the reflectivity of the air which is obtained with weather radar.
The VIL measurement is usually used in determining the size of prospective hail, the potential amount of rain under a thunderstorm, and the potential downdraft strength when combined with the height of the echo tops.
Multicells usually have alternating VIL values. Multicells can have high VIL values on one radar picture, yet much smaller values in the next radar picture.
When VIL values quickly fall, it might mean that a downburst is imminent. This is the result of the updraft within the cell weakening, thereby losing its ability to hold the copious amounts of moisture (including hail) within the storm's structure. Downbursts of this type are referred to as 'wet microbursts' by the National Weather Service for two reasons: (1) they contain heavy rainfall and (usually) hail; (2) they have damaging winds of greater than 58 mph (50 kn; 93 km/h). Microbursts are classified as being 'a swath of damaging winds not exceeding 2.5 miles (4.0 km) in diameter'. Wet microbursts have been mistaken for a tornado by the general public on multiple occasions, as the damage can be quick and hard hitting.
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