Monthly Means Of Daily Maximum Relative Humidity (%) January

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是不可或缺的气象参数。相对湿度（Relative Humidity，RH）在实用场景中被定义为实际水汽压与饱和水汽压的比值，以百分比形式表示；其在一日内通常会发生显著波动，这是因为尽管实际水汽压（actual vapour pressure，记为ed）相对保持稳定，但饱和水汽压（saturated vapour pressure，记为ea）会随温度呈曲线变化。因此，一日中的最低相对湿度出现在一天最热的时段：此时饱和水汽压ea较高，ed与ea的比值因此较小。反之，一日中的最高相对湿度则出现在日出前后：此时气温较低，饱和水汽压ea较低，ed与ea的比值因此趋近于1。在计算日最低相对湿度（RHmin）与日最高相对湿度（RHmax）时，假设未经校正的实际水汽压ed在一日内无显著变化。研究表明，在南非任意特定地点，若针对每个月份采用以地理因子为主的多元回归方程，即可高置信度地逐月预测未经校正的ed。因此，一年中任意月份内，任意地点的ed均可视为相对恒定值，仅在该月内随日期存在小幅变化——这是因为回归方程中纳入了气温范围作为变量。然而，对于该特定地点而言，饱和水汽压ea与温度相关，因此在单日的清晨至正午之间，以及不同日期之间，均可发生显著变化。因此，日最低与最高相对湿度不仅会在单日发生显著波动，在不同日期之间也会呈现明显差异。