What is “wet bulb temperature” and why does it matter?
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A lot of these answers relate directly to what it means for humans. While this is important, there is a slightly different explanation, so here is what means to those of in the Wildland fire community:
We have a tool called a sling psychrometer. It has two thermometers on it. One is the dry bulb, you hang it somewhere (preferably in the shade) and wait for it to stop moving. That’s the ambient air temperature. The second thermometer is the wet bulb. It has a small cotton wick on the bulb. We dip that in distilled water. The whole thing is on a little chain so you can spin it. The air movement dries the wet bulbs wick and the temp drops. When it stops dropping, that’s your wet bulb temperature.
We then have charts, based on elevation, that allow us to cross reference the wet and dry bulb to get the relative humidity (%) and the dew point (°). RH is very important for determining how a fire is going to behave. We track all of this, and more, consistently on a wildfire and the trends/observations let us make more informed tactical decisions on how to fight the fire.
Wet bulb temp is the temp that a wet temperature bulb would read in free air. Assuming dry air, the wet bulb temp will be lower than the actual temp. If humidity is at maximum, that means the wet bulb temp will be equal to the actual temp. Let’s say the wet bulb temp is above 100f, then this means that sweating will not cool you down and you will die.
Wet bulb temperature is the theoretical coolest temperature that evaporative cooling will get you to.
(Evaporative cooling is the process of wet things evaporating off their moisture and some of their heat along with it. That’s why sweat keeps you cool.)
At extreme heat and humidity, sweating physically cannot keep you cool enough to survive longer than a few hours. These are called “wet bulb events.”
The maximum survivable wet bulb temperature is actually cooler than body temperature (somewhere between 88F and 95F), because wet bulb thermometers do not generate body heat, thus can get cooler than a human body.
Lots of good answers already, but I thought I’d share some info on “wet bulb”
~30 years ago, it was common for classrooms to have one of these on the wall:
https://www.instrumentchoice.com.au/Labelled%20Sling%20Psychrometer.jpg
The wick was kept supplied with water. Students would record both values, and you could look up an XY table, and the crossing point shows the relative humidity of those two temperatures.
In general, the further apart those two temps (dry vs wet), the lower the humidity.
Basically a wet thermometer that measures the temp with 100% humidity.
Once your sweat, measured by the thermometer with a wet “bulb” around it, it means your body can’t actively cool itself (sweat evaporates from your skin cooling you) it means no matter what, since you can’t cool yourself, your pretty screwed and things like heatstroke etc can happen.
In Texas they stopped waterbreaks by law and it’s been crazy hot there. Funny thing is, in the military (at least in my experience) they had this board and temp measuring stuff theyd put outside. There were levels of heat and certain levels required certain precautions to be put in place during training, outside labor, etc.
When it got *real hot*, we’d have everyone roll their sleeves down (keeping sweat from evaporating quickly) thus keeping you cooler (and yes it works even tho you’d think you’d be hotter) and other stuff like water breaks at set intervals, etc.
So even basic stuff the military has had around forever, Texas outlawed by nixing water breaks.
Absolutely batshit insane.
Ok I don’t understand. I lived in NC when I was a kid. It gets hot and humid there. Like 100f and 100% humidity. A lot of the south is like that. 100%humidity happens a lot. Why hasn’t this been a big deal before? I never heard of this in the south.
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The wet bulb temperature is one of two thermometers on pyschrometer, an instrument to determine humidity and/or dewpoint.
The air temperature is referred to as the dry – bulb. The wet bulb has a wet cloth on it. When it it very dry (low humidity and low dewpoint) the cloth evaporates causing cooling. There will be a larger difference In wet bulb and dry bulb.
It matters because it can tell a meteorologist if it will be muggy and if there is a chance of rain.
– an earth Sci teacher
Our bodies generate heat constantly, requiring cooling.
Our cooling system consists of sweat evaporating from our skin.
Wet bulb temperature tells us how much we can cool ourselves off given a temperature and humidity.
If it rises above 72°, you are officially in the danger zone for heat illness. And if it rises above 85° you are now in the seriously fucked part of the danger zone.
There is an important element to recognise here, this is a term that has only recently become common among the general population. Previous generations generally had no real need to understand this term as it really became a concern.
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