Heat Units (Standard Deviation) - October

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））的概念早在18世纪中期就已被提出，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总受热总量。这一累积热量的度量标准被称为生理时间（physiological time）。一般而言，气温越低，植物与无脊椎动物（invertebrate animal）的生长发育速率越慢，且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示与近似计算，度日亦俗称热单位，指某段时间内平均气温介于特定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，生物体的生长会趋于停滞甚至衰退。度日概念并非没有局限性，具体包括：假设生长速率与气温呈线性关系；阈值温度可能会随作物或有害生物（pest）的生命周期发生变化；当气温超出上限阈值时，反而可能对发育产生有害或负面影响。南非地区的热单位推导工作，通过计算自然环境中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逐时度法（degree hours）、正弦法（sine method）与三角法（triangular method），以及当气温达到上限阈值时所采用的截断法（cut-off method）。为估算南非地区的热单位（heat units，简称HUs），研究团队使用了覆盖该区域的429700个1'×1'经纬度栅格点（raster point）所生成的50年逐日最高、最低气温时间序列数据（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，逐日计算热单位值，计算公式为：当HU≥0.0时，HU = [(Tmxd + Tmnd)/2 - 10]。相较于本图集1997年旧版中通过Tmx与Tmn的回归月均值计算热单位的方法，本次方法实现了显著改进。基于这些逐日热单位值，可进一步计算得到月、季、年尺度的热单位值及其标准差，并完成空间制图。