What is “wet bulb temperature” and why does it matter?

Question

What is “wet bulb temperature” and why does it matter?

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Answer 1

HVAC engineer here. I’d like to throw in a more general answer that doesn’t talk about hygrometers. As others have pointed out, wetbulb temperature is a number that accounts for both air temperature and moisture level.

Starting with relative humidity: 100% relative humidity is not just the maximum amount of water that air can hold. It’s the point at which water and air are in equilibrium. When air is not at 100% relative humidity, water will naturally evaporate until equilibrium is reached at 100% relative humidity. Hotter air can hold more water. A cube of 100 Deg air at 100% relative humidity has a lot more water in it than a cube of 50 Deg air at 100% relative humidity.

Steam (gaseous water) has more energy than liquid water. It takes a lot of energy to evaporate water from its liquid phase to a vapor.

So here’s what wetbulb temperature is: imagine a cube of air in front of you. Imagine evaporating water into that cube of air to bring the relative humidity up to 100%. Imagine that all energy used to evaporate the water came from the cube of air. So, as the water evaporated into the air, the temperature of the air decreased. The temperature of the air after losing enough energy to evaporate water to achieve 100% relative humidity is known as the wet bulb temperature.

When air is at 100% relative humidity, the air temperature (or dry bulb temperature) is the same as the wet bulb temperature.

As to why it matters, it’s an important concept for cooling. Humans naturally evolved to take advantage of evaporative cooling using sweat. Sweat evaporates into the air. The energy to evaporate the sweat came from the air. The air drops in temperature as it evaporates the sweat. That cooler air moves across our body and cools us.

Cooling towers on buildings work mostly the same. They maximize how much water is evaporated using all sorts of creative ideas. The air that evaporates the water is cooler. That cooler air then cools the rest of the water in the system.