Standard Deviations Of Daily Maximum Relative Humidity (%) February

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.

日常最低与最高相对湿度的计算具备重要实用意义。相对湿度在实际应用中被定义为实际水汽压（actual vapour pressure, ed）与饱和水汽压（saturated vapour pressure, ea）的百分比比值，其日内变化幅度通常较大：尽管实际水汽压ed基本保持恒定，但饱和水汽压ea随温度呈曲线性变化。因此，一日内的最低相对湿度（RHmin）通常出现在当日最热时段，此时饱和水汽压较高，实际水汽压与饱和水汽压的比值偏小；反之，一日内的最高相对湿度（RHmax）则出现在日出前后，此时气温较低，饱和水汽压偏低，实际水汽压与饱和水汽压的比值趋近于1。在计算每日RHmin与RHmax时，通常假定未经校正的实际水汽压ed在日内无显著变化。研究证实，针对南非任意特定地点的未经校正ed，若采用以地理因子为主的逐月多元回归方程，可实现高精度逐月预测。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，仅在当月内随日变化小幅波动——这是因为回归方程中将气温极差作为变量纳入考量。然而，对于该特定地点而言，饱和水汽压ea与温度密切相关，因此其不仅在逐日之间存在显著差异，在单日的晨间至午间时段内也会发生明显变化。因此，每日的RHmin与RHmax不仅在单日内部存在较大波动，在逐日之间也会呈现显著差异。