an easterly wind is blowing from the east towards the west.Ĭonventional instruments used to measure wind speed and direction are the anemometer and the wind vane. The direction given for the wind refers to the direction from which it blows e.g. A specified height is used because the wind speed decreases towards the ground. Measurements of surface wind strength are made at 10 metres above the ground. Today, humidity is measured with electronic humidity sensors. The wet bulb temperature is always lower than the dry bulb temperature if the air is not saturated with water vapour. The wet bulb temperature is the lowest temperature to which air can be cooled by evaporating water into it. The dry bulb temperature is the actual air temperature. The best known instrument for humidity measurement is the psychrometer, which consists of two different thermometers to measure the dry bulb and wet bulb temperatures. Relative humidity and temperature probe/sensor In Singapore, a normal diurnal range is 95-100% around dawn to about 60-70% in the afternoon. If the amount of water vapour in the air remains constant, the RH decreases (increases) as the temperature rises (falls). The relative humidity (RH, expressed as a percentage) is a measure of the amount of water vapour in the air compared to the maximum that could be contained by the air at the same temperature. There are various ways of measuring moisture in the atmosphere. As such, the temperature sensor picks up the heat from the road surface and the heat absorbed by the metal body of the car, and this difference could be significant if the car has been exposed to direct sunlight for some time. While the outside temperature sensor itself is accurate, it is usually located under the hood near the lower half of the car. A common example is the outside temperature sensor reading in vehicles which usually reads several degrees higher than the air temperature. Such thermometers are likely to register a temperature reading that is higher than the true air temperature. Thermometers that are exposed to direct sunlight absorb considerable radiation from the sun, the ground and other surrounding objects. There are many temperature readings around us which may differ from the true air temperature measured by the meteorological authorities. The temperature sensor at an Automatic Weather Station is also housed within a louvered radiation shield. (*in accordance with World Meteorological Organisation guidelines for siting of meteorological instruments) The screen is placed at a height* above-ground to avoid heat from the ground affecting the readings. The screen shelters the thermometers from direct solar radiation while allowing air to circulate through the louvres. Thermometers are housed in a Stevenson screen to measure the true air temperature. This is to avoid heat from the ground affecting the temperature measurement. The World Meteorological Organization provides guidelines on the siting of Stevenson screens or radiation shields at heights of between 1.25m and 2m above ground. The shield is usually painted white to better reflect the sun’s rays (as opposed to dark colours which absorb heat). An example of such a shield is the conventional Stevenson Screen. This is the temperature measured with a thermometer or temperature sensor enclosed in a white louvered radiation shield that allows air to circulate within and shelters the thermometer or temperature sensor from rain and the direct exposure to solar radiation. Meteorological Services around the world measure the true air temperature.
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