VIL is an abbreviation for Vertically Integrated Liquid.

Unlike the VIL product, which determines vertically integrated liquid using grid boxes (also known as grid-based VIL), the VIL number used on the storm attribute table is cell-based. This allows for the vertically integrated liquid associated with composite reflectivity to read slightly higher than that of the VIL product, especially when a thunderstorm is tilted highly.

The reason for the difference is the way the reading is taken. With cell-based VIL, the radar's elevation scans slice the storm nearly horizontally, searching for the highest possible reading. With grid-based VIL, however, the radar's measurements are made in vertical grid VIL boxes. Therefore, with grid-based VIL, the highest levels of vertically integrated liquid are more apt to being overlooked.

Either way, vertically integrated liquid is the amount of liquid in a vertical column. The output is displayed in kilograms per square meter.

Radar operators often use VIL to identify storms with heavy rainfall and large hail. It can also be used to predict the onset of wind damage if it is combined with other products.

There is not a set VIL number which determines when storms are capable of producing large hail. The reason for this is due to the freezing level in the atmosphere and the height of the updraft. In general, the layer of freezing can be found higher in the atmosphere during summer, and lower, possibly at the surface, during winter. The lower the freezing layer is in the updraft, the better chance there is that hail can be expected to fall on the surface. As the freezing layer becomes higher in the atmosphere, a larger updraft tower is required for hail, and therefore a heavy rain event is more likely to occur.

A frontal boundary, such as a cold front, can also have an affect on how VIL numbers correlate to the size of hail. Behind a front, when cold air is aloft, a VIL of 10 to 25 is sufficient to sustain nickel-sized hail. However, warm air aloft in front of a cold front can cause storms with a VIL of 25 to 40 to only drop dime-sized hail.

During the summer, depending on your location, a high vertically integrated liquid number, one that favors heavy rainfall or large hail, is usually between 50 and 65. This number is typically less during the winter months.

A radar cannot detect vertically integrated liquid well, or possibly at all, if a thunderstorm is close to the radar site. Beams are unable to extend into the updraft at such a degree from the radar's transmitter.


Jordan Gerth, May 2007