What is Meant by Wet Bulb Temperature?

Wet bulb temperature is the temperature a parcel of air would have if cooled to saturation (100% relative humidity) by the evaporation of water into it, with the latent heat being supplied by the parcel.

Understanding Wet Bulb Temperature

Wet bulb temperature (Tw) is a thermodynamic property of air that provides a measure of both temperature and humidity. It is always between the dry-bulb temperature (actual air temperature) and the dew point (temperature at which condensation begins).

• Measurement: It's essentially the temperature read by a thermometer covered in a water-soaked cloth (usually muslin) over which air is passed. As water evaporates from the cloth, it cools the thermometer.
• Evaporative Cooling: The amount of cooling, and thus the difference between the dry-bulb and wet-bulb temperatures, depends on the humidity of the air. Drier air allows for more evaporation and greater cooling, resulting in a lower wet-bulb temperature. In 100% humid air, no evaporation occurs, and the wet-bulb and dry-bulb temperatures are equal.
• Adiabatic Saturation: The wet-bulb temperature is closely related to, and under certain conditions equal to, the temperature of adiabatic saturation. Adiabatic saturation is the process where air is saturated by evaporating water into it adiabatically (no heat exchange with the surroundings).

Factors Affecting Wet Bulb Temperature

• Dry-bulb temperature: Higher dry-bulb temperatures generally lead to higher wet-bulb temperatures.
• Humidity: Higher humidity results in a wet-bulb temperature closer to the dry-bulb temperature.
• Airflow: Sufficient airflow over the wet bulb is crucial for accurate measurement, as it ensures continuous evaporation.

Applications of Wet Bulb Temperature

Wet bulb temperature is a useful parameter in a variety of applications, including:

• Meteorology: It's used to assess atmospheric conditions and predict the potential for heat stress.
• HVAC: In heating, ventilation, and air conditioning, it helps in designing efficient cooling systems and understanding the cooling load.
• Agriculture: It can be used to estimate evapotranspiration rates and irrigation needs.
• Human Comfort: The wet-bulb globe temperature (WBGT), which incorporates wet-bulb temperature, dry-bulb temperature, and radiant heat, is a widely used index for assessing heat stress in occupational and athletic settings. A high wet-bulb temperature indicates a lower capacity for the human body to cool itself through sweating, leading to a greater risk of heatstroke.

Wet Bulb Temperature vs. Other Temperatures

In summary, wet bulb temperature is a valuable measure combining temperature and humidity, reflecting the cooling effect of evaporation and serving as a key indicator in various fields ranging from meteorology to human comfort.