What is “wet bulb temperature” and why does it matter?
In: 3256
Dry bulb is just the temperature taken with a dry thermometer. Wet bulb is when they soak some cloth in water and wrap that around the temperature-sensing bulb of the thermometer, so the effect of evaporation cooling the thing is taken into account – wet bulb temperature will always be lower than dry bulb because of this.
It’s important because when we sweat, usually it cools our bodies when it evaporates. When a dry thermometer gives you a very high temperature but it’s not very humid out, you might be ok because when your body sweats it will keep you cool enough. But if someone talks about wet bulb temperature instead of dry bulb, and it’s alarmingly high compared to what your body can handle, you should probably stay indoors in the AC, because staying hydrated or in the shade won’t help you stay cool. On a very humid day, there’s a lot of water suspended in the air that’s already evaporated, and more water and sweat won’t evaporate very fast, so the cooling effects of evaporation won’t help your body or a wet thermometer cool down.
How many cooling tower manufacturers made it into this thread?
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.
A lot of good explanations about the definition already, I just want to add another reason it’s important.
I work on data centers. We care a lot about the wet bulb temperature because it’s a good measure of how effective evaporative cooling will be. You can look into how data center cooling works online, I can’t really explain anything in detail.
Ah… the good ol’ WBGT…. Think of most temperatures as a piece of data without context…. Having a car with a lot of horsepower doesn’t do much good if the engine will destroy the rest of the car, right? Ok, we’ll, a wet bulb temperature is supposed to help simulate “perceived” temperature to the body.. your body does an amazing job cooling itself and you don’t notice but shade, humidity, etc make a lot of cooling harder for your body… the wet bulb simulates a “sweating” person cooling off (so a dry 113 in West Texas may still suck, but it’s a lot easier for your body (with breaks and hydration) than 95 with 100% humidity in Ohio… the primary purpose is to regulate work rest cycles for people doing harder work outside. I could go on for hours about the details but I think that basically covers it.
Latest Answers