Why Did the Car Roof Freeze and the Thermometer Say Four Degrees? Height Matters

Official air temperature is measured in a ventilated screen at a standard height of roughly one and a quarter metres, which is a convention adopted so that readings from different places can be compared. The ground itself is not at that temperature. On a clear calm night the surface radiates heat directly to the sky and cools rapidly, and it cools the thin layer of air immediately touching it, while the air a metre above is barely affected. Differences of four or five degrees between the surface and the screen are ordinary, so the screen can read comfortably above freezing while the ground is well below it.

The conditions that produce a big difference are specific and predictable:

• •A clear sky, so heat radiates away rather than being returned by cloud
• •Still air, since wind mixes the layers and removes the difference
• •Dry air, which radiates heat away more effectively
• •Long nights, giving more time to cool
• •A surface that radiates well and conducts heat poorly, such as grass
• •Low ground, where the cold dense air collects

Cold air is denser than warm air, so air chilled on a slope drains downhill exactly as water does and pools in the lowest ground available, producing what gardeners call a frost pocket. A valley floor, a hollow in a field or the uphill side of a solid wall, hedge or embankment can therefore be several degrees colder than ground a short distance away and higher up. That is why orchards are planted on slopes rather than in valley bottoms, why a gap left in a hedge at the bottom of a sloping garden lets the cold air drain away, and why the coldest recorded temperatures in Britain come from a small number of known hollows.

Weather services publish the distinction explicitly and the terms have defined meanings that are worth knowing. A forecast of frost without qualification generally refers to the ground. An air frost means the screen temperature itself falls below freezing, which is a colder night and affects plants well above the ground. Grass minimum temperature is measured by a thermometer resting on the tips of short grass and is the standard measure of surface cooling, and it is the figure that most nearly predicts whether a lawn will be white in the morning. Road surface temperature is forecast separately again, since the thermal behaviour of tarmac differs from that of grass.

The distinction matters most to anybody deciding when to plant out, and the rule follows from the mechanism. A forecast of two or three degrees on a clear still night means frost on the ground, so tender plants at ground level are at risk while the forecast sounds safe. Plants in pots are more exposed still, since the root ball can freeze from all sides. Covering plants works by blocking the radiation to the sky rather than by insulating them, which is why even a light fleece or a sheet of newspaper is effective and why the cover must be in place before dusk. Watering the soil beforehand helps, since wet soil holds and releases more heat overnight.

Air temperature is measured in a screen over a metre up, while the surface radiates heat to a clear sky and cools several degrees further, so a forecast of three degrees means frost at ground level. Still, clear, dry nights produce the largest gap. Cold air drains downhill and pools in hollows and behind hedges. Covering plants works by blocking radiation, so it must be done before dusk.