Wet bulb temperature takes into account how effectively sweat could cool a person by evaporating.
Basically, imagine taking a thermometer and covering it with a damp cloth. That thermometer would show a lower temperature than a thermometer without the cloth, assuming it is hot enough for some of the water to evaporate, and not so humid that the water cannot evaporate. This evaporation cools the thermometer just like your sweat evaporating cools you on a hot day.
The more humid the air is, the less water can evaporate. That is why dry heat (like in a desert) doesn’t feel as hot as humid heat (such as in the Gulf of Mexico region).
Too high of a wet bulb temperature means people cannot cool off sufficiently by sweating, and will die of overheating without artificial air conditioning or similar. Many people in areas like India and Africa do not have air conditioning available to them, and thus will not survive if the wet bulb temperature is too high for a sustained period of time.
I'm surprised no one has mentioned this. You test it with a sling psychrometer. It's a thermometer with a wet fabric over the bulb, that you actually spin through the air Crocodile Dundee style. Then you can measure the difference between a regular thermometer and the wet bulb of the psychrometer.
Or you can get a little handheld device with a fan that blows air over two thermometers, one of which has a wet cloth cover.
I prefer the Dundee style variant
Hmmm. Like some sort of wet bulb almost. It is called a sling psychrometer
I think his point was that you don’t have to “actually spin through the air Crocodile Dundee style” to make it work. You can also blow a fan across it.
Exactly. Makes it a lot easier to make measurements at sea.
Well I have a digital one that I don't have to get wet or spin about.
The sling psychrometer existed... people didn't just drag a fan about for WB readings
You get that we’re all saying the same thing, right?
Do you get we are saying the same thing?
My point is 25 years in the trade taking WB readings daily and never have even seen a psychrometer with a fan in the field, slings were common until digitals existed.
Thanks for making that point finally! lol. Yeah, I mean it’s pretty old fashioned to thing we’d actually need to get something wet or swing it around. I imagine people had assumed we figured that one out.
Interesting. I used both, but preferably the handheld with the fan, 35 years back. But then I was on ships at the time.
25 years in the trade, first time in a while I was just not old enough... lol.
The fan style seemed more like lab tools than ones for in the field
Remember what sub you are in, please.
I am familiar with the basics of wet bulb temps, but never heard of a psychicmeter until today, and will forget about it tomorrow.
"Regular temp is in the shade and with no wind, wet bulb is with a damp cloth over it and blowing air over that" is enough of an explanation without the pedantry of when people use slings vs fans to blow air over the thermometer.
Have you ever even spoken to a 5 year old? The bulk of the top replies sub wide are over a 5 year olds head.
Have you ever read the sidebar? The 5y old thing isn't literal, it's meant to indicate you should write layman-friendly comments.
You’re referring to relative humidity
Wet bulb is the lowest temparature air can cool to due to evaporating water… aka the lowest temp you can get while sweating. The hotter it is, the less effective you sweating will be. Coincidentally it can also be used to determine what temperature it will cool to if it starts raining when the dew point and temperature aren’t equal. It’s very useful in the winter to know if there will be winter precipitation.
Source: meteorologist
Is wet bulb or WBGT more common? I see WBGT almost exclusively in NWP models.
Depends on what you are doing. WBGT takes into account sun and wind. It’s used heavily for heat safety for outdoor activity. Wet bulb is used more for cold weather, as when precipitation happens the dew point increases and the temperature decreases through evaporation. This will end up at the wet bulb, so knowing the wet bulb, you know if it will be freezing at the surface after precipitation starts.
aka the lowest temp you can get while sweating.
The lowest temperature for something that does not generate body heat, I assume. I've seen 95F wet bulb cited as lethal within hours no matter what, which is slightly less than body temperature let alone the overheating point.
Yes. Without account for body heat. Basically once it’s 95 degrees your body can’t cool itself anymore
The wet bulb temperature is basically a way to combine temperature and relative humidity in a way that represents the body's ability to remove heat.
When you're hot, you sweat. Sweat evaporating is what actually provides cooling, there's a property called "enthalpy of evaporation" which is a fancy way to say "amount of energy to turn liquid to gas". Since water has a pretty high enthalpy of evaporation, it's very good at providing cooling through evaporation. Your body releases water and that water uses the heat your body is producing to turn in to gas, removing that heat from you. When humidity is high, the air is already full of water, so sweat doesn't work as well cause there's no where for the water to go.
The way wet bulb temperature is measured is you literally take a wet rag and wrap it around the thermometer bulb. This measures the current temperature and determines how well water can evaporate to lower the temperature, eventually it'll settle on some number which is called the "wet bulb temperature".
Theoretically, there's a value where your body will no longer be able to cool itself, which means you can quickly overheat and die (I wanna say it's at a wet bulb temp of like 94F or so but I can't remember).
Air absorbs water (we call it humidity) and it's measured by taking in two temperature values: wet and dry bulb temperatures.
Dry bulb temperature is the actual air temperature if you stick out a (dry) thermometer out in the air. Measuring wet bulb temperature is a bit complex: You need to put a moist wick around the thermometer bulb, attach a string on the other end and sling it around. You'll observe that the temperature in the thermometer drops.
When it stops dropping, that's your wet bulb temperature. It should be clear by now why it's called "dry" and "wet" bulb. It's because the thermometer bulb has to be dry (or wet) when taking in air temperature measurement.
When dry bulb > wet bulb, it means the surrounding air is capable to absorb moisture. When air has no moisture in it (called bone dry air), it has the maximum potential to absorb water and consequently the difference between dry and wet air is at maximum.
As air absorbs water, the difference between dry and wet bulb temperature becomes smaller, until such time the air has reached its limit in absorbing water (we call it humid air = saturated air = air with 100% relative humidity) and at this point, dry bulb = wet bulb.
It goes without saying that dry bulb < wet bulb is impossible.
It's important in a number of applications. One practical one is when you cool yourself by sweating. When sweat evaporates, it needs to absorb heat from the surroundings, since evaporation requires energy to happen. When your sweat dries out, it will try to absorb heat from your skin and the surrounding air. Removing heat results in lowering the temperature.
Effectively your skin temperature drops to the wet bulb temperature of the surrounding air.
However, there's a limit. Once you saturate the air with your sweat, your sweat will not dry up. This will give you the "humid' feels, which is uncomfortable. Air can still cool you down by direct absorption of heat from your skin, as long as the dry bulb temperature < 37.8°C. If it goes higher, it will effectively heating you up and no amount of fanning air around you will make you cooler.
Cooling towers (the huge, fat "chimneys" with plume of cloud on top) that you see in industrial complexes utilizes evaporative cooling to cool water in a more efficient manner.
Crime scene cleaner here! We use a wet bulb and have to take pictures of its reading every hour. Wearing a full hazmat suit with full face respirator and hood heats your body 20 degrees. So even in a frozen room you're constantly sweating your tits off. Unfortunately for us the nastiest jobs are always in summer in homes often without electricity so if its 90 degrees out youre actually 110 or more in the suit. The wet bulb beeps above a certain level and then instead of breaks every hour now its every 20 minutes or so. When you take the mask off you spray sweat everywhere lol
On the flip side, in the winter it is used for snow making considerations. You don’t just start at 32F as the typical freezing point. It will be an icy mess. OK snow making starts at 27F wet bulb temp where the water vapor gets room enough to crystallize in the drier air. It doesn’t get good dry snow until 20F WB or below.
Cuz it’s not the heat that gets you, it’s the humidity.
A regular thermometer is the stick, with a glass bulb, and as the temperature changes, a tiny tube gets more or less liquid pushed up from the bulb.
A wet bulb thermometer puts the bulb inside a wet rag. As water evaporates from that rag, it produces a cooling effect, like a swamp cooler, or like a sweating human.
This wet bulb temperature provides a more accurate temperature "feel" for a human being. It also helps to calculate relative humidity.
Wet bulb is literally a thermometer with a damp sock on it. It simulates you, a human, with damp skin due to sweating.
Depending on the wind and humidity a thermometer with a wet sock will read cooler than a dry thermometer. Just like you will feel a bit better after sweating if there’s a breeze and it’s not too humid out. The reading is a clue to predicting your comfort should you go outside and get sweaty.
Also you must know the difference between wet bulb temperature and wet bulb globe temperature: https://zelusports.com/blog/difference-between-wet-bulb-temperature-and-wet-bulb-globe-temperature-wbgt/
When i was rowing competitively, the wet bulb temperature was used as the measure for when regattas would be called off, rather than just the forecast temperature.
It is the theoretical lowest temperature achievable through evaporative cooling (such as sweating). It does not account for the generation of body heat, hence why wet bulb temperatures slightly below body temperature are lethal within hours, even if lying down in front of a fan in a shaded area with an unlimited supply of Gatorade.
Old fashioned mercury/liquid thermometers have a bulb at one end, and to measure a web bulb temperature the bulb was wrapped in a damp cloth. This would tell how much evaporative cooling lowers the temperature. High humidity reduces the amount of evaporation, and causes the wet bulb temperature to be higher than in dry conditions.
Since humans rely heavily on sweating, and the evaporative cooling it provides, to stay cool in high temperatures, the wet bulb temperature can be very important to knowing how well sweating will allow a person to cool down. High heat combined with high humidity makes it much harder to stay cool than just high heat alone.
There is a method of cooking meat called “sous vide”. It uses low temperatures and long times to cook meat gradually and evenly. If the wet bulb temperature exceeds a certain threshold, your body loses the ability to regulate its temperature. You’ll start cooking like someone is sous vide’ing you.
Your body regulates its temperature by sweating. When your body gets hot, it produces sweat which evaporates into the air, removing heat from your body and cooling you off.
When the humidity in the ambient air is past a certain point, your sweat will not evaporate and your body cannot cool itself off. This increases the risk for heat related issues like heatstroke and dehydration if someone is outside for too long or engaged in some strenuous activity like manual labor or sports.
So just knowing the temperature of an area isn’t enough to determine if it’s safe to be outside for an extended amount of time. You need to pair the humidity with that temperature to get a fair reading of the bodies ability to cool itself off. The “wet bulb” is exactly that. It’s a thermometer that also reads the humidity and gives a reading of the ability of water to evaporate into the air and its cooling effects.
You’ve probably often heard in humid areas “the temp is 95°, but because of the humidity it feels like 105°” The wet bulb is helping give a measure to that but measuring how well evaporation can cool something from the ambient temperature given the current humidity.
If the temp is too high and it’s too humid, your body can’t regulate its heat.
Source: am a college sports official in a very humid area and we have to use wet bulb readings before games to determine if we have water breaks during the game and how often.
Good explanation, except probably worth noting that unlike the "feels like" temp they like to report, wet bulb is below the actual temperature.
Right, I kinda always get that backwards since it’s measuring the ability of water to evaporate at a given temp so the reading is the “cooled” reading. Edited some words for clarity.
Why isn't the wet bulb temperature widely reported during heat waves? Seems like that'd be incredibly useful data.
Because it's lower than the actual temperature, it'd have the possibility of confusing people into thinking that the temperature outside isn't as bad as it really is, and perhaps some of them would go out and die of heat stroke.
Instead we use the heat index, which on a hot humid day is going to be higher than the actual temperature, so that's more useful for judging what kind of activities you might do in the heat, and if you get confused by it and think it's the actual temperature, that's going to lead you to be more careful, not less careful.
Because most people don't really know what to do with that information. The concept only really makes it to the public during excessive heat waves, and since the ambient temperature is already going to be so comparatively high for that, people generally still get the message of "oh it's gonna be really hot".
It's like opening task manager and looking at all the extra tabs to see random performance numbers. To some experts, it might be useful information to have in extreme circumstances. But for the average person, it's not going to tell you anything you didn't already know, or change your plans/behavior in any meaningful way.
So how do you know what the limit is? At what point do you stop evaporating sweat?
Your average person won't really know, but they won't really care either. The weather programs already report heat and humidity pretty often, and that's enough. People know high temps = hot, high humidity percentage = muggy and gross, and both being high is especially bad.
Anyone who does care about heat safety, is already going to plan to stay cool based on those two factors, or maybe even just on the heat alone. And anyone who doesn't already pay attention to those two, isn't going to pay attention to yet another number.
*Adding: it might also be a safety measure. If you're reporting that it's 98 degrees outside, but the 'wet bulb' temperature is only 87, then people might think it's not actually that bad out because "Oh, 87 isn't too bad, I'll just stay in the shade", even though a wet bulb temp of 87 is still quite hazardous. Going by "feels like" temperature is probably a better way to get people to take dangerous conditions more seriously.
The wet bulb temperature is regularly reported. However, instead of calling it wet bulb, it's called the dew point. If the dew point is 80F and it's 90F temp, it's super uncomfortable (think Miami in summer) If it's 90F temp and 55F dew point (Think Denver in summer), it's pretty comfortable.
No,
No, wet bulb temperature and dew point are not the same; they measure different aspects of moisture in the air.
Definitions
In addition to what u/ReelyAndrard said, the wet bulb temperature will usually be close to 2/3 of the way between the dew point and the dry bulb temperature.
This really pinpoints my age range and where I grew up, but we used to call that the "humiture"
Isn't it still referred to as such? I left Canada seven years ago and am not familiar with today's practices it seems.
It was coined by the meteorologist in Jacksonville Florida back in the '70s. But then became more standardized as the heat index soon after.
They were still calling it the humature until at least the early '90s but then that dropped off in was commonly called the heat index after that around there.
I didn't realize they still called it that up in Canada
British here. Never heard that.
In Canada the name for the heat index/humidity/“feels like” is “Humidex”
Is the wet bulb exposed to wind?
The wet bulb globe temperature is directly affected by the wind and sun intensity. Maybe web bulb (no globe) is not.
https://www.weather.gov/tsa/wbgt
No. For correct temperature measurements the thermometers need to be in the shade, sheltered from the wind and shielded from being affected by other heat sources.
I might be confusing wet bulb and wet bulb globe but wbgt by definition has to be in the sun.
Never understood that. I'm really hot, not thirsty. No amount of water is going to cool me off.
Because you are sweating more and need to replenish that water. The sweat cools you, the water just makes sure you can keep doing that.
Yeah but I'm inside a chemical suit.
Sir, this is a kindergarten.
Air can be hot.
Water in air can make it feel hotter.
If there’s too much water in the air you can’t cool off.
Mommy and daddy aren’t yelling because they’re mad, they’re just hot and sweaty.
Go get a popsicle from the freezer.
This is absolutely the best ELI5 that I’ve ever read.