Standard Deviations Of Daily Minimum Relative Humidity (%) September

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.

每日最低与最高相对湿度（relative humidity）是气象分析中不可或缺的基础参数。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。由于实际水汽压ed基本保持恒定，而饱和水汽压ea会随温度呈曲线变化，因此相对湿度的日变化幅度往往较大。据此，一日内的最低相对湿度通常出现在最热时段：此时饱和水汽压ea较高，ed与ea的比值偏小；反之，最高相对湿度则出现在日出前后，此时气温偏低，ea较小，ed与ea的比值趋近于1。
在计算每日最低、最高相对湿度时，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，针对南非任意特定地点的未校正ed，若采用针对各月构建的多元回归方程（multiple regression equations），以地理因子为主要自变量，则可按月度实现高精度预测。因此，任意给定月份内，某一地点的ed可视为相对恒定值，仅因回归方程中纳入了温度范围作为变量，在该月内的逐日变化幅度极小。然而，该地点的饱和水汽压ea会因与温度直接相关，在单日内部以及逐日之间均会出现显著波动。由此，最低与最高相对湿度不仅会在单日内部存在较大差异，在逐日之间也会呈现显著变化。