Wet Bulb Temperature Explained

The relationship between wet bulb temperature and dry bulb temperature is a key concept in psychrometry, which is the study of atmospheric conditions, especially the properties of moist air. This relationship is critically influenced by the air's moisture content, specifically its relative humidity.

Dry bulb temperature (DBT) is the most commonly known air temperature. It is measured by a standard thermometer freely exposed to the air but shielded from moisture and radiation. It represents the sensible heat of the air, which is the energy associated with the temperature that can be sensed.

Wet bulb temperature (WBT) is measured by a thermometer with its bulb wrapped in a wet cloth (wick) that is exposed to air flowing over it. As water evaporates from the wet wick, it absorbs latent heat from the thermometer bulb, causing the temperature reading to drop. This cooling effect is called evaporative cooling. The rate of evaporation, and thus the amount of cooling, depends on how much moisture the air can still hold, which is directly related to the relative humidity.

Relative Humidity (RH) and its Impact on Temperature

Relative humidity (RH) is a measure of the amount of water vapor currently in the air compared to the maximum amount of water vapor the air could hold at that specific dry bulb temperature. It is usually expressed as a percentage.

• When the air is dry (low RH), it has a large capacity to absorb moisture. This leads to rapid evaporation from the wet wick, causing a significant cooling effect and a large difference between the dry bulb and wet bulb temperatures.
• When the air is moist (high RH), its capacity to absorb more moisture is reduced. This slows down the evaporation rate from the wet wick, leading to less evaporative cooling and a smaller difference between the dry bulb and wet bulb temperatures.

Wet Bulb Temperature at 100% RH

The question specifically asks about the wet bulb temperature at 100% RH. When the relative humidity is 100%, the air is completely saturated with water vapor. This means the air is holding the maximum possible amount of moisture it can at that particular temperature and pressure.

• Since the air is already saturated, there is no driving force for further evaporation of water from the wet wick of the wet bulb thermometer.
• Because no evaporation can occur, there is no evaporative cooling effect acting on the wet bulb.
• Consequently, the wet bulb thermometer will not show any temperature drop due to evaporation.
• Therefore, the wet bulb temperature reading will be exactly the same as the dry bulb temperature reading.

In essence, at 100% RH, the air cannot absorb any more moisture, eliminating the cooling process that typically makes the wet bulb temperature lower than the dry bulb temperature. Thus, the two temperatures become equal.