Standard Deviation (C) Of Daily Minimum Temperature July

Minimum (Night-Time) temperatures, there is abundant evidence for the climatic limitation of crop distributions to be described by minimum temperatures, from a perspective of crop survival (on Frost for tolerance levels of crops). Many subtropical crops may be killed at 5 degree Celsius already through chilling injury, even in the absence of frost (Chang, 1968). High minima increase respiration loss, thereby decreasing net photosynthesis.

Daily minimum temperature time series were derived from over 970 qualifying temperature stations` quality controlled daily records for the 50 year period 1950 - 1999. From these time series, 50 years of daily values of minimum temperatures were generated at each of 429 700 one arc minute (i.e. 1.7 km x 1.7 km) raster points covering South Africa, using regional and seasonal lapse rate and infilling/data extension techniques developed by Schulze and Maharaj (2004). Note that in the generation of daily minimum temperatures no account could be taken of cold air drainage into hollows or valleys, which can take place from surrounding higher-lying areas through radiative cooling during calm, stable clear night conditions. Reasons for this are provided in Schulze and Maharaj (2004). From these daily values, monthly means of daily minimum temperatures, as well as standard deviation of daily minimum temperatures for each month of the year, could be computed for purposes of mapping and production of statistics which are tabulated for Swaziland, Lesotho and for each of the nine provinces of the RSA. This approach to mapping minimum temperatures is thus fundamentally different to that used in the previous `South African Atlas of Agrohydrology and Climatology` (Schulze, 1997), in being much more physically based.

已有大量研究证实，作物分布的气候限制可通过最低气温表征，这一结论的依据是作物存活所需的霜冻耐受阈值（Frost Tolerance Levels of Crops）。许多亚热带作物即便未出现霜冻，也可能在5摄氏度时因冷害致死（Chang, 1968）。较高的夜间最低气温会加剧呼吸消耗，进而降低净光合速率。

本数据集的日最低气温时间序列源自1950—1999年这50年间、970余个经质量管控的合格气象站的日气温观测记录。依托Schulze与Maharaj（2004）提出的区域及季节气温递减率、插值/数据补全技术，研究人员在覆盖南非的429700个1弧分（即1.7km×1.7km）栅格点（raster points）上，生成了对应50年的日最低气温序列。需要说明的是，在日最低气温的生成过程中，未考虑冷空气从周边高地经晴朗静稳夜间的辐射冷却向洼地或山谷堆积的过程，相关原因详见Schulze与Maharaj（2004）的研究。基于上述日最低气温数据，可计算得到各月日最低气温的月平均值，以及全年各月日最低气温的标准差，用于制图及统计分析，相关统计结果已针对斯威士兰、莱索托以及南非共和国（RSA）的9个省份分别制表。与此前的《南非农业水文学与气候图集》（South African Atlas of Agrohydrology and Climatology, Schulze, 1997）所采用的方法相比，本数据集的最低气温制图方法更具物理机制支撑，二者存在本质区别。