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Contrasts between air and grass minimum temperatures

I have for some time been puzzled by the variations between the lowest (air) minimum temperature and the number of ground/grass frosts noted in the Climatic Data pages of Weather log.

For example in the log for September 2010, Lowestoft had a lowest air minimum of 5.4°C and one ground frost, Valley had a lowest temperature of 5.1°C and two ground frosts – and Aldergrove a minimum of 2.9°C but no ground frosts.

I would appreciate an explanation of these variations.

Reply by Roger Brugge, editor of the Climatological Observers Link

On most nights the lowest temperature occurs close to the ground, and to measure it a thermometer freely exposed to the sky is placed at grass-tip level and read at 0900 utc every day: the grass it is placed on should be short, akin to a freshly-mown lawn. This gives us the grass minimum temperature, as opposed to the air minimum temperature which is recorded inside a Stevenson screen. A ground frost is recorded for every morning when the grass minimum temperature reading is below 0.0°C.

At night, in the absence of cloud and much wind, it is the ground/grass surface that cools most quickly, and this cooling is transferred to the overlying air. Thus, the grass minimum temperature is almost always lower than the air minimum – on a clear, calm, night over a snow surface by as much as 10 degC. Grass is a relatively poor conductor of heat and so a thermometer in contact with the grass tips will usually also read lower than one in contact with bare soil or concrete.

Grass minimum temperatures can vary widely in any given locality. Hollows and sandy lowlands will experience lower readings than level marshy areas and anyone with a garden will have noticed how the frostiest part of the lawn on a winter's morning is near its centre – away from radiating objects. Sandy soils have a low thermal capacity and a low thermal conductivity because of their air content, but clay soils (usually containing more moisture) have a higher thermal capacity and higher thermal conductivity. As a result dry soils topped by grass will have a tendency towards a greater fall in grass temperature (they contain less heat and conduct it less well towards the soil/grass surface) – and thus a larger difference between the grass minimum and air minimum temperatures.