VIL was a "fad" and was most popular when the WSR-88D radar was first installed. VIL is the integration of reflectivity within a column of air. A higher VIL means there is more precipitation in a column of air. At one time it was thought a VIL value is associated with a certain hail size. The term "VIL of the day" was a term used by the NWS to describe the VIL value that associates with severe hail (3/4" of an inch in diameter or greater). If was found that the relationship between the VIL value and hail size is much more complex than previously thought. The VIL values correlation to hail size depends on season, synoptic environment, elevation, storm speed, storm structure, hail reflectivity characteristics etc. The relationship is too complex to issue a severe thunderstorm warning with respect to hail size from the VIL value alone, although the VIL value can be used as a guide and is especially useful if the VIL is very high. The VIL value has the following limitations:
1. The values are seasonally and regionally dependent. The value is also dependent upon the cloud physics and synoptic considerations for that particular day.
2. 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.
3. VILs are less accurate for highly tilted storms. VIL is underestimated for tilted updrafts since the hail core tilts with respect to a vertical column of reflectivity.
4. VILs are higher for wet hail than dry hail
5. VIL value may be contaminated by non-precipitation echoes
VIL can be used as a "guide" for assessing hail size and intensity of precipitation but as mentioned has many limitations. VIL has units of mass divided by area (kg/m^2). The mass is estimated by the radar reflectivity factor. VIL has a display similar to reflectivity products: heavier precipitation and hail will show a higher VIL (higher VIL value colors).