The Grass Minimum Temperature is the temperature recorded in open air ground on short turf, with the bulp of the thermometer just in contact with the tips of the blades of grass. It is also described as the temperature at 5cm (2in) above ground.
The grass minimum temperature often varies substantially from air temperature, which is measured at 2m above ground. In deserts or on the beach during a hot summer day the "grass temperature" might be much higher than the air temperature and much, much lower during clear spells under a starry winter sky. In fact, the grass minimum temperature might be 0C (32F) or less, while the air temperature recorded simultaneously at the same site is 4C to 5C or higher. This is known as ground frost and is an important information for farmers, gardeners and drivers.
The grass minimum temperature is measured by a minimum thermometer . As with all minimum thermometers the actual thermometer is held inside a glass jacket and the contraction of the alcohol column overnight causes a small indicator within the stem of the thermometer to descend and record the lowest temperature. The grass minimum temperature is usually read after night at 9 am local time, or0900 GMT in Britain.
After snowfall, the instrument is positioned above the snow surface and a note made to the register. Some stations also record the concrete minimum thermometer , above a slab of concrete, where the snow is swept away and the thermometer reset.
Have a look at the UK's daily grass minimum temperatures here