In Reigate: our first air frost of the Autumn was recorded last night at 3.00am: -0.3C.
An “air frost” is when the air temperature recorded by thermometers placed at 1.25m above the surface (a standard height) falls below freezing (0C). A “ground frost” is when the temperature recorded by thermometers placed on the ground, usually over grass, record 0C or lower. This is often quoted as “grass minimum”. Reigate Grammar weather station currently records air temperatures but we hope to record grass temperatures in the future.