From AMS Glossary
- In British usage, a freezing condition injurious to vegetation, which is considered to have occurred when a minimum thermometer exposed to the sky at a point just above a grass surface records a temperature (grass temperature) of -0.9°C (30.4°F) or below.
Since 1961 in Britain the statistics refer to the "number of days with grass minimum temperature below 0°C" rather than to ground frost. A fuller discussion is given in McIntosh (1963).
See frozen ground.
McIntosh, D. H. 1963. The Meteorological Glossary. Her Majesty's Stationery Office, London, . p. 12.