Standard Deviations Of Daily Minimum Relative Humidity (%) May

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。出于实用目的定义的相对湿度（relative humidity，RH）为实际水汽压（actual vapour pressure，ed）与饱和水汽压（saturated vapour pressure，ea）的比值，通常以百分比形式表示。由于尽管实际水汽压ed可相对保持稳定，但饱和水汽压ea随温度呈曲线变化，因此相对湿度的日变化幅度往往较大。由此，一日内的最低相对湿度（RHmin）会出现在一天中最热的时段——此时饱和水汽压ea处于较高水平，ed与ea的比值ed:ea较小。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ea亦偏低，ed与ea的比值趋近于1。在计算日最低与最高相对湿度时，我们假设未校正的实际水汽压ed在一日内无显著变化。已有研究表明，在南非地区，针对每个月构建以地理因子为主要自变量的多元回归方程（multiple regression equations）后，即可高置信度地逐月预测任意站点的未校正ed。因此，任意站点在一年中任意给定月份的ed可视为相对恒定值，仅在该月内随日变化存在小幅波动，这是因为气温极差是回归方程中的自变量之一。然而，对于该特定站点而言，饱和水汽压ea因与温度密切相关，会在单日之内以及逐日之间均发生显著变化。因此，日最低与最高相对湿度不仅在单日之内存在较大波动，在逐日之间也会呈现显著差异。