Heat Units (April)

The concept of the heat unit (or degree day), known since the mid-18th century already, revolves around the development of a plants or organism`s being dependent upon the accumulated heat to which it was subjected during its lifetime, or else during a certain developmental stage. This measure of accumulated heat is known as physiological time. In general it holds that the lower the temperature, the slower the rate of growth and development of plants and invertebrate animals and the amount of heat required to complete a given organism's development does not vary. Physiological time is usually expressed and approximated in units of degree days, also popularly called heat units, where these are an accumulation of mean temperatures above a certain lower threshold value and below an upper developmental threshold (above which growth is considered to remain static or even decline), over a period of time. The degree day concept is not without its limitations. These include that, a linear relationship is assumed between growth and temperature, threshold temperatures may change during the life cycle of a crop or Pest, temperatures exceeding the upper threshold may, in fact, have a detrimental/negative effect on development. Derivation over South Africa of heat units were done by calculating degree days for the daily temperature fluctuations that occur in nature. These range from degree hours, sine and triangular methods based on hourly temperatures, as well as so-called cut-off methods when the upper threshold of temperature is reached. To estimate heat units (HUs) over South Africa, the 50 year time series of daily maximum and minimum temperatures (Tmxd, Tmnd), generated at each of the 429 700 raster points covering the area at 1` x 1` latitude/longitude, was used (Schulze and Maharaj, 2004). The generation of this time series is described in Section 2a. HUs was computed for each day for a threshold temperature of 10 degree Celsius, i.e. HU = [(Tmxd + Tmnd) / 2 - 10] for HU ≥ 0.0 This method is a considerable advance on the computation of HUs in the previous version of this Atlas (Schulze, 1997), in which HUs were calculated from regression derived monthly means of Tmx and Tmn. From these daily values, monthly as well as seasonal and annual HUs and their standard deviations, could be calculated and mapped.

热单位（heat unit，又称度日（degree day））的概念自18世纪中期便已为人所知，其核心逻辑为：植物或生物体的生长发育依赖于其生命周期内或特定发育阶段所累积的受热总量。这种累积热量的度量方式被称为生理时间（physiological time）。一般而言，温度越低，植物与无脊椎动物（invertebrate animals）的生长发育速率越慢，而完成某一特定生物体发育所需的总热量保持恒定。生理时间通常以度日为单位进行表达与近似计算，度日也常被称为热单位，其定义为一段时间内，平均气温介于某一下限阈值与上限发育阈值之间的累积值——当气温高于上限阈值时，生物体生长将趋于停滞甚至衰退。度日概念并非毫无局限：其一，该方法假设生长与温度呈线性关系；其二，作物或害虫的生命周期中，阈值温度可能发生变化；其三，超过上限阈值的气温实际上可能对发育产生有害或负面影响。针对南非地区的热单位推导工作，通过计算自然环境下的每日气温波动对应的度日值完成，计算方法涵盖基于逐时气温的度日小时（degree hours）法、正弦法与三角法，以及当气温达到上限阈值时的截断法等。为估算南非地区的热单位（heat units, HUs），研究使用了覆盖该区域的429700个1'×1'经纬度栅格点的50年逐日最高、最低气温（Tmxd、Tmnd）时间序列数据（Schulze与Maharaj，2004）。该时间序列的生成方法详见第2a节。本次研究以10摄氏度为阈值温度逐日计算热单位，即当HU≥0.0时，HU = [(Tmxd + Tmnd)/2 - 10]。相较于该图集1997年旧版中通过最高、最低气温的月度回归均值计算热单位的方法，本方法实现了显著改进。基于上述逐日热单位数值，可进一步计算月度、季节与年度热单位及其标准差，并完成可视化制图。