The wet bulb temperature corresponds to the value of the temperature in contact with water in its liquid state. In other words, it is the temperature that takes account of suspended moisture (air humidity).
Depending on the temperature and pressure levels, the air can hold a greater or lesser quantity of water vapor. 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 vapor becomes liquid.
Why is the wet bulb temperature lower than the dry bulb temperature?
Its name comes from the fact that the traditional device used to measure it is a mercury thermometer, the bulb of which is covered with a water-soaked cloth through which a current of air can pass. The air current causes the water in the cloth to evaporate, cooling the ambient air.
More technically, if the air is saturated with water, the dew point is equalized and the water vapor becomes liquid, causing the temperature to fall.
This is why the wet bulb temperature is always lower than the dry bulb temperature.
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.
In summer, for both plants and humans, our ability to lower our body temperature depends on the humidity of the air. When the air reaches a relative humidity (RH) of 100%, it no longer accepts water vapor. If this happens, we may sweat, but our sweat doesn't cool us down, simply because the ambient air is so saturated with water that it no longer evaporates.
That's why humid heat is less bearable than dry heat.
In winter, when the ambient humidity is low, the plant continues to transpire and the water evaporates. This causes the plant to cool down, as the evaporation process requires energy from the air.
A loss of energy is equivalent to a loss of heat and therefore to cooling.
The wet bulb temperature to prevent the risk of frost
Monitoring the wet bulb temperature is therefore essential for preventing and defending against frost. It allows you to anticipate the minimum temperature that may be reached, so you can activate your anti-freeze systems at the right time.
With sprinkler frost protection, if humidity levels are below 100%, the water sprayed will initially help to saturate the air with water. Once the environment is saturated, the humid temperature will have been reached, which corresponds to the minimum temperature that the plant can reach when the air is saturated with water. Water spraying keeps the temperature of the plants above 0°C.
In this way, the wet temperature, in conjunction with the humidity, plays an important role in forecasting weather conditions and helps to determine when frost protection should be triggered (spraying, heating, air circulation).