Vertically integrated liquid

From Glossary of Meteorology

vertically integrated liquid

(Abbreviated VIL.) Vertical integral of liquid water content obtained from radar observations at different elevation angles within a precipitation echo; has dimensions of mass per unit area.

Liquid water content M is computed from the equivalent reflectivity factor Ze using the Marshall–Palmer drop-size distribution and is expressed as
where the units of M are grams per cubic meter and Ze has its conventional units (mm6 m-3). The presence of hail in a storm produces larger VIL values than otherwise would be expected. VIL values greater than a threshold value are used to indicate the presence of large hail or other types of severe weather. The threshold value varies geographically, seasonally, and daily with changing environmental conditions.