Wet Bulb Temperature (WBT) and Dew Point (DP) are distinct, though closely related, measurements of atmospheric moisture. Both are temperature-based metrics used to understand humidity, but they quantify different phenomena. Dew Point measures the absolute amount of water vapor present in the air (moisture content). Wet Bulb Temperature measures the air’s capacity for evaporative cooling (cooling potential). These separate purposes make each measurement useful for different applications, such as predicting fog or assessing human heat safety.
Defining Dew Point
The Dew Point is the temperature to which air must be cooled, at a constant barometric pressure, for the water vapor within it to begin condensing into liquid water. This temperature marks the point of saturation, where the air can no longer hold all the moisture it contains. The higher the Dew Point, the greater the mass of water vapor present in the atmosphere.
A simple way to visualize the Dew Point is by observing condensation on a cold beverage glass. The air immediately surrounding the cold glass is cooled to its Dew Point, causing water vapor to condense onto the surface as liquid water. When the air temperature and the Dew Point are nearly identical, the air is highly saturated, holding close to the maximum amount of moisture possible.
Defining Wet Bulb Temperature
Wet Bulb Temperature represents the lowest temperature achievable by the evaporation of water into the air. It is determined using a thermometer whose bulb is covered in a wet cloth and exposed to airflow. As water evaporates from the cloth, it draws heat away from the bulb, causing the temperature reading to drop.
Cooling continues until the rate of heat loss from evaporation balances the heat gained from the surrounding air, yielding the Wet Bulb Temperature. This measurement reflects the air’s ability to absorb moisture. Since dry air allows for more rapid evaporation and cooling, the difference between the standard air temperature and the Wet Bulb Temperature is greater in dry conditions.
The Relationship Between the Two
The Dew Point and the Wet Bulb Temperature are interconnected through the concept of saturation, which is also expressed by relative humidity (RH). Relative humidity describes the amount of water vapor in the air relative to the maximum amount the air can hold. When the air reaches 100% relative humidity, it is fully saturated, and the Dew Point, the Wet Bulb Temperature, and the standard air temperature are all equal.
When the air is not saturated, the Wet Bulb Temperature will be lower than the standard air temperature due to evaporative cooling, and the Dew Point will be lower still. The difference between the standard air temperature and the Wet Bulb Temperature indicates how dry the air is and how much cooling potential exists. Dew Point measures the quantity of moisture, while Wet Bulb Temperature measures the effect that moisture has on the environment’s cooling capacity.
WBT as a Measure of Heat Stress
The Wet Bulb Temperature is an indicator for assessing human heat stress because the human body relies on evaporative cooling through sweating to regulate its core temperature. When the air’s Wet Bulb Temperature is high, the air is saturated with moisture that sweat cannot evaporate effectively from the skin. This prevents the body from shedding excess heat, leading to a dangerous rise in internal temperature.
The theoretical limit for human survival is often cited as a Wet Bulb Temperature of 35°C (95°F), where the body can no longer cool itself even with shade and water. Recent research suggests that the critical limit where a healthy person’s core temperature begins to rise uncontrollably may be closer to 30.6°C (87°F). Monitoring this measurement is a public health tool for predicting conditions that can lead to heatstroke and organ failure.