Monthly Means Of Daily Maximum Relative Humidity (%) August

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参数。相对湿度（relative humidity, RH）在实际应用中定义为实际水汽压与饱和水汽压的百分比比值，其日内变化幅度往往较大：尽管实际水汽压（actual vapour pressure, e_d）基本保持稳定，但饱和水汽压（saturated vapour pressure, e_a）会随温度呈曲线形式变化。因此，一日中的最低相对湿度（RH_min）通常出现在当日最热时段，此时e_a较高，e_d与e_a的比值相对偏小；反之，一日中的最高相对湿度（RH_max）则出现在日出前后，此时气温较低，e_a偏小，e_d与e_a的比值趋近于1。
在计算RH_min与RH_max时，通常假设未校正的实际水汽压e_d在日内无显著变化。已有研究证实，针对南非任意特定地点的未校正e_d，通过采用基于各月地理因子构建的多元回归方程，可实现按月度尺度的高精度预测。因此，任意地点的e_d在对应月份内可视为相对恒定值，仅因气温范围作为回归方程中的自变量，在月度内逐日存在小幅波动。
不过，该特定地点的饱和水汽压e_a因与温度密切相关，在逐日之间以及单日的清晨至正午时段均存在显著波动。因此，RH_min与RH_max不仅在单日之内存在较大变化范围，在逐日之间也会呈现显著差异。