What's the difference between a ground frost and an air frost?
In day-to-day meteorology, the temperature of the lowest layer of the atmosphere is measured at a height of 1.25 m (about 4 feet) above local ground level. Usually, though not always, this is achieved by placing thermometers in a double-louvered screen with the bulbs of the thermometers, or the sensor heads (for distant reading thermometers), placed so that they cluster around the 1.25 m standard. The temperature so read is usually called 'the air temperature' and it is these values that appear, for example, in the World Cities reports in newspapers/teletext, or plotted on standard synoptic charts, and also it is at this level that the forecast temperatures seen on tv weather maps are based.
When the temperature as measured in this way falls below 0.0 deg C, then an AIR FROST is recorded. For other purposes though, e.g. horticulture, road gritting operations etc., we need to know what the temperature is at the surface of the ground, and most weather stations set at least two thermometers to record these values: a grass minimum thermometer, set just above/in contact with short grass, and a concrete minimum thermometer, set so that its sensor/bulb is in contact with a concrete slab of standard dimensions/composition.
When the temperature as measured by the thermometer set over grass falls below 0.0 deg C, then a GROUND FROST is recorded. (In spring and early summer, when the temperature is expected to produce a frost using the grass minimum thermometer, but not over other surfaces (due to thermal inertia of surfaces such as concrete, tarmac etc.), then the unofficial term 'grass-frost' may be heard in weather forecasts - this is to try and avoid panic by road, railway and airport operators as soon as they hear the word 'frost' but alert gardeners, growers etc., to the risk of damage).
The difference between the two levels can be considerable: On still, clear nights, with air of a low humidity content, 5 degC or more is not uncommon.