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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. This measurement is usually used in
determining the size of prospective hail. 

VIL is the integration of reflectivity within a column of air. A higher VIL means
there is more precipitation in a column of air. It was found that the relationship
between the VIL value and hail size is very complex. The VIL values correlation to
hail size depends on season, synoptic environment, elevation, storm speed, storm
structure, hail reflectivity characteristics etc. VIL values can be used as a guide
and is especially useful if the VIL is very high. 

The VIL value has the following limitations:
Values within 20 miles of radar will be underestimated since part of the storm will
be in the radar's cone of silence. Values at the edge of the radar display are also
underestimated since the higher elevation angles are overshooting the storms (lower
half of storm is not being sampled). VIL values will be higher where all tilt
angles are able to sample the entire precipitation / hail core of the storm.