The definition for frost severity as laid down by the Meteorological Office in their Glossary of 1991, has remained unchanged for many years. Frost severity is dependent not just on the lowest minimum temperature, but also on the wind speed, so there are two sets of values for ‘slight’, ‘moderate’, ‘severe’ and ‘very severe’, one for when mean wind speeds are 10 knots or more and one for when they are less.
There are many euphemisms for frost used by today’s weathercasters that are simply misleading and bad practice. Common amongst these are ‘sharp’, ‘touch’, ‘pinch’, ‘hard’, and ‘light’, they’re all bollocks as far as I am concerned. If weathercasters stuck to the same rigid definition the general public might have a better clue to just how severe an upcoming frost might be, what a forlorn hope of mine that will ever happen.
It’s also interesting to note that the glossary mentions just two basic types of frost, an ‘air frostair frost An air frost occurs when the temperature of the air falls below 0.0°C’, and a ‘ground frost’, so forget about ‘grass frost’, even if the thermometer that’s used to measure it is referred to as a ‘grass minimum’ thermometer.