The wet bulb temperature corresponds to the value of the temperature in contact with water in its liquid state.
Depending on the temperature and pressure levels, the air can hold a greater or lesser quantity of water vapour. This is what we mean by air humidity.
At night, when the temperature drops, the air’s capacity to retain water is reduced accordingly. If the air is saturated with water, the dew point is reached and the water vapour becomes liquid (see article “What is the dew point?”).
This liberation of liquid water contributes to further lowering the surrounding temperature. Monitoring the wet bulb temperature is therefore essential, particularly for preventing and guarding against freezing. It means you can anticipate the minimum temperature that might be reached.
When it comes to protecting against freezing by spraying, if the humidity is under 100% the diffused water will initially contribute towards saturating the air with water and lowering the temperature of the plants to the wet bulb temperature. Subsequent spraying with water will allow the plants’ temperature to be maintained above 0°C.
Thus the wet bulb temperature, in combination with the humidity, plays an important role in forecasting weather conditions and allows us to work out when to initiate protective measures against freezing (spraying, heating, air circulation).