Monthly Means Of Daily Minimum Relative Humidity (%) March

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, RH）是不可或缺的气象参数。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）的比值，以百分比形式表示；其在一日内的波动通常较为显著，这是因为尽管实际水汽压（ed）基本保持稳定，但饱和水汽压（ea）会随温度呈曲线变化。因此，一日内的最低相对湿度出现在当日最热时段，此时饱和水汽压（ea）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出现在日出前后，此时气温较低，饱和水汽压（ea）偏低，实际水汽压与饱和水汽压的比值趋近于1。在计算每日最低相对湿度（RHmin）与最高相对湿度（RHmax）时，我们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实际水汽压（ed），若在每月的多元回归方程中主要纳入地理因子，则可按月度实现高置信度预测。因此，任意地点的未校正实际水汽压（ed）在一年中的任意月份均可视为相对恒定值，仅在该月内随日期小幅波动——这是因为回归方程中将气温极差（temperature range）作为变量纳入。然而，在该特定地点，饱和水汽压（ea）因与温度相关，不仅会在单日的清晨至正午间发生显著变化，且不同日期之间的差异同样显著。因此，RHmin与RHmax不仅在单日之内的波动范围较大，且不同日期之间的差异也十分显著。