Monthly Means Of Daily Average Relative Humidity (%) June

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.

日最低和最高相对湿度（Relative Humidity, RH）是不可或缺的气象参数。出于实用目的定义的相对湿度，实为实际水汽压（actual vapour pressure, ed）与饱和水汽压（saturated vapour pressure, ea）的百分比比值，其日变化幅度往往较大。尽管实际水汽压（ed）基本保持恒定，但饱和水汽压（ea）会随温度呈曲线变化。因此，在一日中最热的时段，饱和水汽压较高，实际水汽压与饱和水汽压的比值（ed:ea）偏小，此时对应当日的最低相对湿度。反之，日出前后气温较低，饱和水汽压亦偏低，ed:ea比值趋近于1，此时则为当日的最高相对湿度。

在计算日最低相对湿度（RHmin）与日最高相对湿度（RHmax）时，通常假设未校正的实际水汽压（ed）在一日内无显著变化。研究表明，针对南非境内任意特定地点的未校正ed，若为每个月份构建以地理因子为主要自变量的多元回归方程（multiple regression equations），即可凭借较高置信度逐月预测其数值。因此，任意地点的ed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变量，在该月内逐日存在小幅波动。然而，该地点的饱和水汽压（ea）与温度相关，不仅逐日变化幅度显著，在单日的晨间至午间时段亦会产生较大波动。由此可见，日最低与最高相对湿度不仅单日变化范围较大，不同日期间的差异亦十分显著。