A Sticky Psychrometric Situation: Wet Bulb Temperature

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wet-bulb-temperature-psychrometrics-sweat

Wet bulb temperature tells you how easy it is for water to evaporate. Sounds simple enough, right? Today I'll give you a couple of examples to see how well you really understand that simple statement. First, early last month my friend Mike MacFarland texted me about the day he wore his wet bulb shirt on a mountain bike ride. Here's what he said:

Sweaty backs and wet bulb temperature

"I was on a Mtn bike ride last weekend and wore a cotton shirt and my hydration pack. 6 Miles in I stopped at a picnic table for a couple mins of reflection and when I put my hydration backpack on again, it pressed my wet bulb shirt against my back and I felt like someone had thrown an ice pack in there. Later after the ride I had to do a quick weather condition lookup and Psych calc to see the conditions- it was 84 outside and 24% RH, making for a pretty cold (60F) t-shirt! :-) my family thinks I'm a geek though..."

How did he determine the shirt was 60° F? By looking up the wet bulb temperature, of course. There are a bunch of smartphone apps that do psychrometric calculations for you. All you need are two independent data, like the dry bulb temperature and relative humidity he used, and you can find any of the other psychrometric quantities you're interested in.

Climate change and wet bulb temperature

The New York Times also got into psychrometrics recently with a nice article relating wet bulb temperatures to climate change. They defined wet bulb temperature as “a measure of how well you can cool your skin by sweating.” When it’s hot outside, as it has been during the heat wave in India and Pakistan, that’s one of the primary ways we keep our bodies from overheating.

Normally, when you want to know how well water will evaporate, you need to know the difference between the wet bulb and dry bulb temperatures (a number that determines how well the Mistbox can work). When those two numbers are equal, you'll get no net evaporation because dry bulb and wet bulb are equal only at saturation. In the New York Times article, however, they discussed the dangers of high wet bulb temperatures without reference to a dry bulb temperature. How can that be?

Here's the paragraph I'm referring to:

A person who is physically active at a wet-bulb temperature of 80 degrees will have trouble maintaining a constant core temperature and risks overheating. A sedentary person who is naked and in the shade will run into the same problem at a wet-bulb temperature of 92 degrees. A wet-bulb temperature of 95 degrees is lethal after about six hours.

It doesn't matter what the dry bulb temperature is—or more importantly, the difference between dry bulb and wet bulb. We can talk about levels of wet bulb temperature that are dangerous to humans because the difference between wet bulb temperature and our body temperature becomes the important number in times of high humidity.

The point of the article is that as the Earth warms, it's not just rising dry bulb temperatures and sea levels we should be concerned about. If current trends continue into the next century, the authors say, "summer outdoor conditions could become physiologically intolerable for humans and livestock in the eastern United States — and in regions currently home to more than half the planet’s population."

And speaking of wet bulb temperatures, climate change, and Mike MacFarland, I can't end this article without talking about data loggers. Mike is a data collector extraordinaire. (See, for example, the article I wrote about his heat pump defrost data and the device he developed to make heat pumps smarter.) In addition to all the home performance data logging he does, he monitors carbon dioxide levels at his home in Redding, California.

As I finish up this article here in Tucker, Georgia, the dew point is 70° F, and the wet bulb temperature is 72° F. How sticky is it where you are?

 

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Photo of sweaty back (which is not Mike MacFarland's) by Gregg O'Connell from flickr.com, used under a Creative Commons license.

 

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Comments

Curt Kinder
Jul 7 2015 - 10:07am

In evaluating humidity / discomfort I think in terms of dewpoint since that figure is regularly reported at National Weather Service stations. It runs steadily in the high 60s low 70s all summer in north Florida.

In another (non-HVAC / building science) role I routinely wear body armor while operating a center console boat. A cooling technique I use while underway at 20-25 mph is to alternately allow air to pass under the armor via my collar cooling the wet cotton tee shirt beneath and then let the cooled shirt back in contact with skin.

Repeating this every few seconds by hunching shoulders back and forth really helps stay cool.

Armando
Jul 7 2015 - 10:28am

Plano, TX: 9:25 AM, 81.9°F, Humidity 77%↓, Wetbulb 76.3°F, Dew Point: 73.9°F↑

Dale Sherman
Jul 7 2015 - 12:12pm

Syracuse, NY: 12 noon, 82°F, Humidity 73%↑, Wetbulb 75°F, Dew Point: 72°F.

James Hinkle
Aug 25 2015 - 7:19pm

We have a bldg. in Lockhart Texas Zone two and it is a new bldg. finished in March 2015 and we have wet walls behind vinyl wall paper, it has tyveck commercial wrap on exterior. The walls behind the vinyl has Mold ,but above vinyl paper has not , where is this moisture coming from Thanks

Allison Bailes
Aug 26 2015 - 6:30am

James, that's an easy one. It's coming from outdoors, where the humidity is. The vinyl wallpaper is a vapor barrier so it doesn't let the wall dry to the inside. The water vapor from outdoors gets stuck and causes the problems you're seeing.

This has been a problem with hotels in humid climates especially because they like install bathroom ventilation that depressurizes the walls in the building, helping to suck in even more humid air.

If you want to understand the details of why the water vapor is condensing, see this article:

What Happens When You Put a Plastic Vapor Barrier in Your Wall?
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/75130/W...