Edit: Thank you all for the detailed answers! You guys are awesome.
In: 2953
Your body cools down by using sweat.
When sweat evaporates it takes with it a surprising amount of energy.
If you are in a hot space and cannot sweat you cannot cool down.
At any given temperature there’s a maximum amount of moisture the air can hold. For a given humidity the temperature at which the air cannot hold any more moisture is the wet bulb temperature.
If you take a thermometer and wrap it in a wet cloth and blow air on it at the “wet bulb” temperature for the humidity in the room it won’t cool down, because the water can’t evaporate, because the humidity/temperature mix is maxed out.
This is important for humans as if the wet bulb temperature goes about about 37 degrees we lose the ability to keep our bodies at normal operating temperature. If the wet bulb temperature makes it to about 40 degrees you basically start to die. A fan won’t cool you down. Taking your shirt off won’t cool you down. Your body temperature will rise to the wet bulb temperature, shedding enormous amounts of sweat.
If you cannot find somewhere cooler then you’re done for – you’ll succumb to either dehydration or heat exhaustion.
This is being talked about a little more lately as parts of the earth, some with substantial populations, are now seeing wet bulb events at the very edge of what humans are capable of surviving.
As moisture-containing, sweat-producing creatures, we have a limited ability to cool our own bodies. This ability is greatly impacted by relative humidity in the air, which is not reflected by a standard thermometer. 30 degrees is 30 degrees, regardless of humidity, even though it might feel significantly different and pose completely different levels of risk. A wet-bulb temperature is more directly accurate to human experience, being essentially a thermometer that can sweat.
[there’s a pretty good explanation on wikipedia](https://en.wikipedia.org/wiki/Wet-bulb_temperature#Intuition)
If a thermometer is wrapped in a water-moistened cloth, it will behave differently. The drier and less humid the air is, the faster the water will evaporate. The faster water evaporates, the lower the thermometer’s temperature will be relative to air temperature.
Water can evaporate only if the air around it can absorb more water. This is measured by comparing how much water is in the air to the maximum which could be in the air—the relative humidity. 0% means the air is completely dry, and 100% means the air contains all the water it can hold in the present circumstances and it cannot absorb any more water (from any source).
This is part of the cause of apparent temperature in humans. The drier the air, the more moisture it can hold beyond what is already in it, and the easier it is for extra water to evaporate. The result is that sweat evaporates more quickly in drier air, cooling down the skin faster. If the relative humidity is 100%, no water can evaporate, and cooling by sweating or evaporation is not possible.
When relative humidity is 100%, a wet-bulb thermometer can also no longer be cooled by evaporation, so it will read the same as an unwrapped thermometer.
Earth science teacher here. Wet bulb temperature kinda represents how thirsty the air is. If the WBT is close to the air temperature, the air isn’t thirsty, meaning it’s already got a lot of water molecules in it…so your sweat will stay on you, not evaporating. Now, mind you, this isn’t really a problem if the air temperature is reasonable. It only becomes a problem if you NEED sweat to evaporate to cool you.
To understand this, it’s important to remember that for water to evaporate, it needs to take a little heat from somewhere in order to make the jump from a liquid to a gas. In the case of sweating, the sweat takes the heat from YOU, cooling you down.
It’s called wet bulb temperature because it literally comes from a wet bulb. If you wrap the end of a classic glass thermometer in a wet cloth, then let it evaporate, the evaporation cools the thermometer by taking some energy from it (like sweat would cool you.) A bigger drop in temperature means there was more evaporation, which means the air was thirstier.
If air temp is near WBT, the air is wet, so sweating doesn’t help.
If air temp very different from WBT = the air is thirsty, so sweating cools you off.
Wet-bulb temperature is the temperature that an old-fashioned liquid bulb thermometer will read if you wrap it in cotton gauze, wet it, and swing it around in the air. (The “bulb” is the little reservoir at the end of the thermometer that holds the mercury or alcohol.)
Wet-bulb temperature by itself doesn’t tell you much. Wet-bulb temperature compared to dry-bulb temperature is useful. It’s a measure of how much moisture is already in the air, and how easy it is for moisture to evaporate into the air.
So if it’s 95 degrees F out and the wet-bulb thermometer reads 94 degrees F, that means very little evaporation is occurring on the little bulb of the thermometer. So it’s very humid (actually, almost raining), and evaporation will not occur easily.
If it’s 95 degrees F out and the wet-bulb thermometer reads 70 degrees F, that means a lot of evaporation is occurring on the little bulb of the thermometer…..because it’s the water evaporating on the literal wet-bulb that is lowering the temperature of the fluid in the thermometer below the actual air temperature. It’s very dry, and evaporation will occur easily.
When evaporation doesn’t occur easily, all sorts of processes are affected: sweating is less effective – so your body overheats more easily, cooling towers don’t work as well – so you spend more energy to cool your building or power plant, manufacturing processes that require things to evaporate (films, adhesives, etc.) may not work well.
When it evaporation does occur easily, processes that rely on evaporation occur easily. Your sweat evaporates – so you feel cool even in 105 F heat….until you realize that you’re sunburned like a lobster.
The wet bulb temperature is how cold an object coated in water can get by evaporating the water. If it’s above a safe body temperature, you can’t keep yourself from overheating by sweating and letting the sweat dry.
Humidity is measured in 2 ways, either a hygrometer or a psychrometer. The wet bulb refers to the psychrometer. A psychrometer has a wet bulb and a dry bulb, the bulbs being thermometers. The dry bulb is just any old thermometer, measuring the air temperature, but the wet bulb is going to have water evaporate away from it, thus cooling the wet bulb. This cooling continues until no more water can carry away heat faster than the environment heats the bulb back up.and you reach equilibrium. The wet bulb then reads the temperature at which the current amount of moisture in the air would be 100% humidity, this temperature is the dew point. The closer the dew point is to the current temperature, the higher the humidity.
People run a 100 mile unltra-marathon in Death Valley because of the enormous amount of heat dissipated when water CHANGES PHASE FROM LIQUID TO VAPOR.. it pulls energy from the skin (stored as skin temperature).
The short answer is it is the measurement we use to determine if human life is sustainable outdoors.
The way humans keep cool is by sweating, the sweat evaporates into the air and cools our skin. If there is too much humidity in the air the sweat doesn’t evaporate and we can’t cool ourselves and we could overheat and die.
Wet bulb temps measure the temperature and humidity on a scale that we can accurately assess the point at which we will die outside if left without a way to cool down.
Wet bulb temperature is the coldest your body can get via sweating. If your body can’t cool off, Ie wet bulb temperature is above body temperature, you can overheat and die.
Latest Answers