Frost Duration

During a heavy frost day, temperature at recognised meteorological stations is measured in a Stevenson screen at a height of 1.5 m above the ground. Particularly with radiation frosts, a frost occurrence on the ground is therefore not always recorded as a temperature at or below 0 degree Celsius in a Stevenson screen. A temperature of ≤ 0 degree Celsius recorded in a Stevenson Screen is therefore termed a `heavy` frost, on the assumption that the temperature on the ground would be even lower. In the mapping statistics on frost over South Africa, the temperature database contains quality controlled time series of daily maximum and minimum temperatures from > 970 qualifying temperature stations, the records of which had been infilled and extended (where necessary) to a common 50 year time period 1950 - 1999 by techniques described in Schulze and Maharaj (2004). These daily station records were then used with regionally and seasonally determined temperature lapse rates to generate 50 years of daily maximum and minimum temperatures at each of 429 700 one arc minute (1.7 x 1.7 km) raster points covering the RSA, Lesotho and Swaziland. At each raster point a minimum temperature of ≤ 0 degree Celsius in the 50 year daily time series was designated a `heavy frost day. Any raster point where < 5 years out of the 50 experienced at least one heavy frost day, that point was deemed to be `frost free`. Frost analyses were therefore only performed on daily values at those raster points with 5 or more years recording at least one heavy frost day.