From AMS Glossary
(Also called frost point technique.) A technique for using the radiosonde observation to determine the presence of liquid water droplets in supercooled clouds in saturated or nearly saturated layers of air.
For each reported level in the sounding, the negative value of eight times the dewpoint spread (-8D) is plotted on the pseudoadiabatic chart (or equivalent chart). Where the temperature sounding lies to the left of the -8D curve, liquid droplet clouds are considered to be present, and icing is possible on aircraft flying in the cloud layer.