During frost, when the muslin is thinly coated with ice, the readings are still valid because
evaporation takes place from a surface of ice as freely as from one of water. If the muslin is
dry it must be given an ice coating by wetting it slightly with ice-cold water, using a camel-hair
brush or by other means. The water will usually take 10 to 15 minutes to freeze. Excess of
water must not be used as it takes much longer to freeze and will also not give accurate
readings. After the wetting of the muslin, the temperature generally remains steady at 0 °C
until all the water has been converted to ice. It then begins to fall gradually to the true ice-bulb
reading. No reading must be recorded until the temperature of the ice-bulb has fallen below of
the dry-bulb and remains steady. Dry, windy weather may cause the ice to evaporate
completely before the time of the next reading, in which case the procedure of wetting the bulb
must be gone through again. The original coating of ice will give satisfactory results as long it
lasts. It must be pointed out that supercooled water may exist on the wet-bulb at temperatures
well below freezing point and that, if this is not noticed by the observer, serious errors will
occur. The freezing can be started by touching the wet-bulb with a snow crystal, a pencil, or
other object. (source: The Marine Observer's Handbook; Met Office UK)
NOTE
- For convenience the default
'state of wet-bulb' is 'wet-bulb not frozen' on this temperatures
input page. If the wet-bulb is frozen please check the 'frozen wet-bulb' button.