n. an instrument for measuring the relative humidity of the atmosphere, i.e. the ratio of the moisture in the air to the moisture it would contain if it were saturated at the same temperature and pressure. In the dew-point hygrometer a polished surface is reduced in temperature until the water vapour from the atmosphere forms on it. The temperature of this dew point enables the relative humidity of the atmosphere to be calculated. In the wet-and-dry bulb hygrometer, there are two thermometers mounted side by side, the bulb of one being surrounded by moistened muslin. The thermometer with the wet bulb will register a lower temperature than that with the dry bulb owing to the cooling effect of the evaporating water. The temperature difference enables the relative humidity to be calculated.
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