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Maximum temperature The maximum thermometer indicates the highest temperature reached since the thermometer was last reset. Owing to the constriction in the mercury capillary tube, the maximum thermometer is not affected when the temperature drops as this constriction prevents the mercury from returning to the bulb when it is getting colder.
The maximum thermometer is only read at the time of the 08:00 SAST observation, and two readings are taken, namely the maximum temperature and the maximum reset value (which should be within 1,5 ºC of the 08:00 observed dry-bulb temperature). The maximum temperature as read between 08:00 SAST yesterday and 08:00 SAST today is recorded against yesterday’s date on the climate database. Minimum temperature The minimum thermometer indicates the lowest temperature reached since the thermometer was last reset. Factors influencing horizontal variations in temperature include distance from the sea, ocean currents and aspect. The difference between wet-bulb temperature and dew-point temperature and relative humidity.
The wet-bulb temperature, dew-point temperature and relative humidity all relate to the amount of moisture in the atmosphere. The wet-bulb temperature of air is measured by a thermometer whose bulb is wrapped in muslin and which is kept moist with distilled water. The evaporation of water from the muslin around the thermometer bulb has a cooling effect and thus the temperature indicated by the wet-bulb thermometer is less than the temperature indicated by a dry-bulb thermometer.
The rate of evaporation from the wet-bulb thermometer depends on the humidity of the air. Evaporation is slower when the air is already full of water vapour. The difference in the temperatures indicated by the wet-bulb and dry-bulb thermometers gives a measure of atmospheric humidity. The dew point is the temperature to which the air must be cooled to reach saturation. The difference between the air temperature and the dew-point temperature is proportional to the relative humidity.
The closer the two temperatures are, the higher the relative humidity. When the air temperature is the same as the dew-point temperature, the relative humidity is 100%. Condensation will then take place resulting in visible water in the form of either clouds or fog or dew.