It is essentially a mercury-in-glass thermometer having a round bulb at one end and the stem is graduated in degree Celsius. The reading of this thermometer gives the free air ambient temperature at a place when exposed at a height of 1m to 1.5m inside a louvered thermometer screen installed in the open. While taking a reading, it should be made sure that no parallax error occurs.
A wet bulb thermometer is precisely similar to dry bulb thermometer except that the bulb of the wet bulb thermometer is always kept wet by means of a muslin sheath fed by distilled water from a bottle through a wick. The bottle must be placed a little on one side of the bulb to prevent the thermometer from reading too high. The difference between the DB and WB thermometer readings is a measure of moisture content in the atmosphere.