Further data on the climate of the Alpine Vegetation Belt of eastern Lesotho

Rainfall data from 31 stations in the Alpine Vegetation Belt of eastern Lesotho, together with temperature, relative humidity and evaporation data from Letseng-la-Draai are presented and discussed, and comparisons, where possible, are made with data from Mokhotlong in the Subalpine Belt.

Carter (1967) demonstrates convincingly in his Fig. 3a that rainfall in high mountains varies inversely with altitude.From Table 1 it can be seen that the higher-based stations such as PI, P2, P3, P5, P7 and P8 have the lowest mean annual rainfall.This is a well-known phenomenon o f mountain climates.
Another observation made by Carter and reflected in Fig. 3b is that in the Oxbow area mean annual rainfall is high close to the north-western escarpment (e.g.gauges B, I, R, T and U), but decreases towards the east (e.g.gauges PI, P2, P5 and X4).A similar situation exists from west to east 90 km south o f Oxbow (Fig. 3c).However, C arter states that along the Drakensberg escarpment the rainfall is high (e.g.Organ Pipes Pass summit, M A R 1 609 mm), but that this high rainfall effect does not extend more than 1 , 6 -3 , 2 km into Lesotho.
It should be mentioned that Carter (Fig. 3d) has produced a isohyetal map o f the Upper Orange River ca cim ent, which should prove most useful to plant ecologists.A further look at Table 3 reveals that the monthly mean relative humidity varies considerably from year to year, the percentage difference sometimes being as much as 339% (August).Mean annual range o f relative humidity is 33% .

Van
Unfortunately it is not possible to compare the relative humidity data for LetSeng-la-Draai with M okhotlong, because the times o f observation are different, viz.14h00 and 08h00 respectively.However, there is a marked difference in the seasonal march o f relative humidity at the two stations (Fig. 4).A t LetSeng-la-Draai relative humidity is at a minimum during winter and early spring and at a maximum during summer, the wet season.This would be described by Schulze (1965) as a " m onsoonal" type o f variation.A t M okhotlong, on the other hand, the opposite situation prevails: relative humidity is at a minimum during spring and summer and at a maxi mum during winter, the dry season.This is apparently a "continental" type o f variation.The reason fo r the seasonal difference in relative humidity is not clear and deserves study by climatologists.

TEMPERATUREA
ir temperature data for Let3eng-la-Draai have been used to produce the Deasy temperature graph in Fig. 2. The curves for mean daily maximum (B) and mean daily minimum (C) with the hatched area between representing diurnal temperature range, show that air temperature is cool to mild in summer, but cold in winter.The absolute temperature curves A and D reach degrees o f hot and frigid.that maximum temperatures can cer tainly exceed 16 °C: the mean daily maximum tem perature for January is 1 6 ,6 °C and the absolute maximum temperatures exceed 16 °C from September to April reaching 31 °C in January 1972.U nfort unately there are no data for temperature at soil level.However, if a screen minimum o f 3 °C is employed as a criterion o f light ground frost (as used by Schulze, 1965), it will be clear from Fig. 2, curve C, that only from March to November are ground level tem peratures below freezing point likely to be a daily feature o f the alpine climate-certainly not throughout the year as claimed by Van Zinderen Bakker & Werger.The present author has often been on the summit o f the Drakensberg during summer and has only occasionally seen evidence o f frost heaving and frozen bog pools even at dawn and at altitudes o f above 3 000 m.In plant ecological studies considerable use is made o f " klimadiagramme" (W alter & Leith, 1960).W ith air temperature and rainfall data available for Letsengla-D raai, it is possible for the first time to produce such a diagram (Fig. 3 .1 ) for the Alpine Belt.The chief conclusions to be drawn from this diagram are as follow s: precipitation always exceeds temperature, therefore the climate is humid; and ecologically, winter is a time o f stress, because o f low rainfall coupled with low temperature.The climatic diagram' for LetSeng-la-Draai shows affinities with three o f W alter & Leith's clim atic classes, namely VI (cold, humid), VIII (Boreal) and X (mountain areas in other regions).It differs from VI in that the monthly summer rainfall exceeds 100 mm by a considerable amount (reaching 1 2 6 ,6 m m ); the onset o f the rainy season in October and its decline in A pril is much sharper; and the mean m onthly summer temperatures are usually lower and persist for a longer time.It differs from VIII in the higher summer-rainfall, the sharper onset and decline o f summer rain, the usually higher mean annual temperature and the lower mean m onthly summer temperatures (in VIII these temperatures are rarely lower than 20 °C).The diagram approxim ates closest to some o f the examples o f X, the mountain clim atic class, which is an extremely variable class.area showing the variation o f rain fall with altitude.This profile shows clearly the effec tiveness o f the scarp in creating a " rain-shadow" on the leeward side o f the Drakensberg with resultant low rainfall at M okhotlong.M okhotlong is also warm er than Letseng-la-Draai with a mean annual temperature o f 1 1 ,5 °C and with the mean monthly temperature never dropping below 0 °C.M oreover, the absolute minimum temperatures do not fail below 0 °C between December and March.The average number o f frost days at M okhotlong is 100.

F
given in Table 3. W hile not as significant ecologically as data for absolute relative humidity, these data do illustrate the variation in humidity from month to month, season to season and year to year.Tyson, Preston-W hyte & Schulze (1976) regard rela tive humidity as the least satisfactory way o f measur ing humidity, because o f its high degree o f dependence on temperature.Relative humidities at Letseng-la-Draai are high during summer, but low during winter and in early spring.The mean relative humidity fo r January is 54% with the highest m onthly mean being 72 % in January 1974.The mean relative humidity for August is 37 % the lowest mean being 18% in August 1969; in September the corresponding figures are 34 % and 21 % in September 1969.Tyson, Preston-W hyte and Schulze (I.e.) explain why there is low vapour pressure over the Drakens berg area in winter." ..........the winter intensification o f the high pressure all over South A frica is accom panied by increased upper air subsidence with as sociated atmosphere drying.Under these conditions the moisture content o f the air over the Drakensberg, which is already low due to elevation is further reduced.Onshore north-east winds are weak in this season and with the Drakensberg generally elevated to the level o f the subsidence inversion, little moisture is advected over the area.This is reflected by low pressure over the Drakensberg area" .