Standard Deviations Of Daily Maximum Relative Humidity (%) November

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的观测与计算具有重要实用价值。相对湿度（Relative Humidity，RH）在实际应用中被定义为实际水汽压与饱和水汽压的比值，以百分比形式表征；其在一日内通常会发生显著波动：尽管实际水汽压（记为ed）基本保持恒定，但饱和水汽压（记为ea）会随温度呈曲线变化。因此，一日中的最低相对湿度会出现在一天中最热的时段——此时ea较高，ed与ea的比值较小。相反，一日的最高相对湿度则出现在日出前后：此时气温较低，ea较小，ed与ea的比值趋近于1。

在计算日最低相对湿度（RH_min）与日最高相对湿度（RH_max）时，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，通过为每月构建以地理因子为主的多元回归方程，即可以较高置信度逐月完成预测。因此，任意地点的ed在一年中的任意月份均可视为相对恒定值，仅在当月内随日期存在小幅波动——这是因为回归方程中纳入了气温变幅作为变量。不过，该特定地点的饱和水汽压ea则会因与温度相关，而在单日的早间与午间之间发生显著变化，且不同日期之间的差异也十分明显。因此，RH_min与RH_max不仅会在单日内部存在较大波动，不同日期之间的差异也会十分显著。