Monthly Means Of Daily Minimum Relative Humidity (%) December

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.

日最低与最高相对湿度是不可或缺的气象观测要素。实用中，相对湿度（Relative Humidity, RH）被定义为实际水汽压（actual vapour pressure, e_d）与饱和水汽压（saturated vapour pressure, e_a）的比值，以百分比形式表示。由于尽管实际水汽压基本保持恒定，但饱和水汽压e_a会随温度呈曲线变化，因此相对湿度的日变化幅度通常较为显著。由此，一日内的最低相对湿度会出现在当日最热时段：此时e_a较高，e_d与e_a的比值e_d:e_a偏小；反之，一日内的最高相对湿度则出现在日出前后：此时气温较低，e_a亦偏低，e_d与e_a的比值e_d:e_a趋近于1。
在计算日最低相对湿度（RH_min）与日最高相对湿度（RH_max）时，我们假设未校正的实际水汽压e_d在一日内无显著变化。已有研究证实，针对南非境内任意特定地点的未校正e_d，若采用逐月构建的多元回归方程并以地理因子为主要自变量，即可实现高精度预测。因此，任意地点的e_d在当年对应月份内可视为相对恒定值，仅在月内随每日气温范围的变化产生小幅波动——气温范围是该回归方程中的自变量之一。不过，该地点的饱和水汽压e_a会随单日时段（如清晨至正午）以及不同日期间产生显著波动，因其与温度直接相关。由此，RH_min与RH_max不仅在单日内变化幅度较大，且不同日期间也存在显著差异。