Standard Deviations Of Daily Maximum Relative Humidity (%) October

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（Relative Humidity, RH）是重要的气象参数。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；其在一日内往往波动显著，原因在于：尽管实际水汽压ed基本保持恒定，但饱和水汽压ea却随温度呈曲线变化。因此，一日中的最低相对湿度会出现在当日最热时段——此时ea数值较高，ed与ea的比值相对偏小；反之，一日的最高相对湿度则出现在日出前后，彼时气温较低，ea数值偏低，ed与ea的比值趋近于1。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。已有研究证实，在南非地区，针对各月构建以地理因子为主要自变量的多元回归方程，可高精度逐月预测任意站点的未校正ed。因此，任意站点在当年任意月份的ed均可视为相对恒定值，仅因气温范围作为回归方程的变量，在该月内随日期小幅波动。然而，饱和水汽压ea与温度直接相关，因此在任意站点，其数值不仅在单日内（从清晨至正午）会发生显著变化，在不同日期间也会出现明显差异。综上，RHmin与RHmax不仅在单日内波动范围较大，在不同日期间也会呈现显著差异。