Monthly Means Of Daily Average Relative Humidity (%) November

Daily minimum and maximum relative humidity is necessary as relative humidity, defined for practical purposes as the ratio of actual to saturated vapour pressure, expressed as a percentage generally varies considerably in the course of a day because, despite actual vapour pressure, ed, remaining relatively constant, saturated vapour pressure, ea, varies curvilinearly with temperature. The minimum RH of a day is thus experienced during the hottest part of the day when ea is high and the ratio of ed : ea is therefore small. Conversely, the maximum RH of a day is experienced around sunrise when temperatures are low, thus ea is low and the ratio of ed : ea therefore approaches unity.

In computations of daily RHmin and RHmax, the uncorrected actual vapour pressure, ed , is assumed not to change significantly in the course of a day. Because uncorrected ed at any specific location has been shown to be predictable over South Africa on a month-by-month basis with a high degree of confidence when using predominantly geographical factors in multiple regression equations for each month the ed at any location is therefore held at a relatively constant value for any given month of the year, varying only slightly on a day-by-day basis within that month because temperature range is a variable in the regression equation. However, at that specific location the saturated vapour pressure, ea , can vary considerably from one day to the next, as well as within a given day between morning and midday, because it is related to temperature. Therefore, RHmin and RHmax can range considerably within a given day and vary markedly from one day to the next.

日最小与最大相对湿度（relative humidity）是必需的，因为相对湿度（实际应用中定义为实际蒸气压（actual vapour pressure）与饱和蒸气压（saturated vapour pressure）的比值，以百分比表示）在一天中通常会发生显著变化——尽管实际蒸气压ed保持相对恒定，但饱和蒸气压ea随温度呈曲线变化。因此，一天中的最小相对湿度（RHmin）出现在日间最热时段，此时ea较高，故ed与ea的比值较小。相反，一天中的最大相对湿度（RHmax）出现在日出前后，此时温度较低，ea随之降低，因此ed与ea的比值趋近于1。

在计算日最小相对湿度（RHmin）和日最大相对湿度（RHmax）时，未校正的实际蒸气压ed被假设为在一天内无显著变化。由于在南非地区，当针对每个月使用以地理因素为主的多元回归方程时，任意特定地点的未校正ed已被证明具有高度可靠的月度可预测性，因此该地点的ed在一年中的任意给定月份均保持相对恒定的值，仅在当月内的每日之间因温度范围作为回归方程的变量而略有变化。然而，在该特定地点，饱和蒸气压ea可能在每日之间以及一天内的早晨与中午之间发生显著变化，因为它与温度相关。因此，RHmin和RHmax不仅在一天内的波动范围较大，且每日之间也存在显著差异。