Heat Waves Per Year Maximum Temperature > 30C On 3 Or More Consecutive Days

A heat wave is an extended period of excessively hot weather on consecutive days on which temperatures are well above normal, and which may be accompanied by high humidities, in which case the temperatures `feel` even higher. Within a South African context the following three definitions have been proposed for purposes of mapping: Heat waves are occurrences (per year) with Tmxd > 30 degree Celsius on 3 on more consecutive days, Extreme heat waves are either occurrences (per year) with Tmxd ≥ 35 degree Celsius on 3 or more consecutive days, i.e. a higher temperature threshold, or occurrences (per year) with Tmxd ≥ 30 degree Celsius on 5 or more consecutive days, i.e. a longer duration threshold. Heat waves are usually associated with synoptic scale anticyclonic, i.e. high pressure, circulation systems covering thousands of 2km in size and lasting for several days to weeks.

In mapping heat waves over South Africa, the temperature database created by Schulze and Maharaj (2004) was used to compute frequencies of heat waves. In this temperature database daily maximum and minimum temperatures were generated for the 50 year period 1950 - 1999 at 429 700 one arc minute grid points covering the area, by using observed daily temperatures of varying durations from 973 temperature stations with quality controlled data which were infilled and extended to the common 50 year period by techniques outlined in Schulze and Maharaj (2004) and interpolated/extrapolated using regional and monthly temperature lapse rates.

热浪是指连续多日出现显著高于常态的极端高温天气，且可能伴随高湿度环境，此时体感温度会进一步升高。在南非的研究语境下，学界提出了三类用于热浪制图的定义：普通热浪指年度内出现连续3日及以上日最高温（Tmxd）超过30摄氏度的事件；极端热浪则分为两类，一类为年度内连续3日及以上日最高温≥35摄氏度的事件（即采用更高温度阈值），另一类为年度内连续5日及以上日最高温≥30摄氏度的事件（即采用更长持续时间阈值）。热浪通常与天气尺度的反气旋（即高压）环流系统相关联，这类系统覆盖范围可达数千平方公里，持续时间从数日至数周不等。
在开展南非地区热浪制图研究时，本研究采用了Schulze与Maharaj（2004）构建的气温数据库以计算热浪发生频次。该数据库通过整合973个经过质量控制的气象站点的逐日观测气温数据，采用Schulze和Maharaj（2004）所述的插补与延拓技术，将原始观测时长各异的气温数据补全并延拓至1950—1999年共50年的统一时段，随后结合区域及逐月气温递减率进行插值与外推，最终生成覆盖研究区域内429700个1弧分网格点的逐日最高、最低气温数据集。