Median First and Last Dates of Heavy Frost their Variability, and the Duration of the Frost Period

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.

在强霜日，经认证的气象站会在距地面1.5米高的斯蒂芬森百叶箱（Stevenson screen）中测量气温。尤其在辐射霜天气中，地面结霜的情况并不总能通过斯蒂芬森百叶箱内的气温≤0℃得到反映。因此，当斯蒂芬森百叶箱内测得气温≤0℃时，即被定义为‘强霜’，其前提假设为地面实际温度会更低。在南非霜冻制图统计中，该气温数据库包含来自970余个合格气象站的经过质量控制的日最高、最低气温时间序列；依据Schulze与Maharaj（2004）所述方法，这些台站的观测记录已被补全并（在必要时）延长至统一的1950-1999年50年时长。随后，利用这些逐日台站观测记录，结合经区域与季节分异确定的气温直减率，为覆盖南非（RSA）、莱索托与斯威士兰的429700个1弧分（1.7×1.7千米）栅格点生成了1950-1999年共50年的日最高、最低气温序列。在每个栅格点处，若其50年逐日气温序列中出现日最低气温≤0℃的情况，则该日被定义为‘强霜日’。若某栅格点在50年序列中，出现强霜日的年份不足5年，则该点被判定为‘无霜区’。因此，霜冻分析仅针对那些在50年序列中，出现强霜日的年份≥5年的栅格点的逐日气温数据开展。