Another quantity that can be calculated from the large 14-elevation radar scan is the so-called Vertically Integrated Liquid (VIL). The VIL value at a certain location is the sum of all observed radar reflectivities (converted to liquid watercontent) in a vertical column above this location. The unit of VIL is kg/m2 or mm, and it can be regarded as a measure for the potential rainfall.

The rate at which precipitable water droplets are formed is proportional to the updraft speed, and therefore the VIL value is both a function of updraft speed and cloud thickness. It has been noted already by others that observations of high VIL values and the occurrences of severe thunderstorms correlate quite well. An example of a VIL image, recorded at the same date and time as the echo top product shown before, is given on this page.

In stratiform clouds a VIL value of 10 kg/m2 is rarely exceeded. In areas with strong convection, however, a VIL value of 10 kg/m2 is easily exceeded and VIL values of 25 kg/m2 or higher are not exceptional. In the Radar Severe Weather product that is currently under development, the VIL product will probably play an important role in identifying areas with strong updrafts.