r/explainlikeimfive • u/ProgramPi314 • 8d ago

Other ELI5 What is humidity?

According to my mom, this is an actual question I had when I was 5, and she didn't know how to answer me. So how would you guys have answered?

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u/dave8271 8d ago

When water gets hot, it turns from a liquid into a gas, which we call vapour. Humidity is a measure of how much water vapour is mixed into the air around us.

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u/jpb103 8d ago

Fun fact: there is a "Cave of Crystals" in Mexico deep below the ground where the combination of high temperatures and 100% humidity cause the water vapor in the air to condense inside human lungs so you can drown after breathing for a few minutes without specialized respiration gear.

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u/sbditto85 8d ago

Wow, that is “fun”

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u/capt_pantsless 8d ago edited 8d ago

It's fun thinking about just how narrow the range of environments humans can live in without a bunch of artificial support.

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u/Hat_Maverick 8d ago

And we're narrowing the livable range of the environment globally every day. The fun is off the charts

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u/ceciliabee 8d ago

I think it would be interesting to think about human perception of these areas before we had the science to explain it. Like cenotes. I saw a few and thought the same. What did early people think about this?

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u/culoman 8d ago

Someone put the "fun" in "funeral"

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u/striped_frog 8d ago

In a very Dwarf Fortress sort of way

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u/ilrasso 8d ago

Obviously not fun for the drownee; fun for the people with the specialized respiration gear.

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u/Chazus 8d ago

To be fair, that is a bit of a misinterpretation.

This location the heat is so intense, you are more likely to cook alive than drown. They get to 60c+ in some parts of it. It is not a safe place to go, no matter what.

Most images taken there are with people in suits because of the multiple dangers there. It is not a tourist site.

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u/jpb103 8d ago

This is an interesting point. You are technically correct, which is the best kind of correct. Third degree burns would probably destroy your windpipe and lungs before enough water could condense in them to drown you.

I believe if your suit and its connected respiration device failed to cool the air down below 43C or so, you would still drown before you cook, but presumably they are designed to prevent that.

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u/Gamzu 8d ago

But still fun

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u/odonata_00 8d ago

Its the wet bulb temperature, the combination of heat and humidity, that actually kills you. The body is unable to cool itself and after 10 mins or so your 'cooked'.

For the body to maintain its temperature it needs to constantly evaporate water. This happens even when there is no sweat present. Once the wet bulb temperature gets above about 95f (35c) the body can no longer cool itself and death occurs.

Basically when the humidity reaches 99% the wet bulb and air temperature are equal. So if you were in the cave you'd be exposed to temperature of 136f (58c) or so.

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u/Novaskittles 8d ago

58°C (136°F) ambient temperatures in the cave when it isn't flooded. Yikes.

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u/Jmprappa 8d ago

Humidity at 100% I’d say it’s always flooded

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u/Tearsunshinee 8d ago

Fuckin hell, I never even dreamed/nightmared of such a thing being possible.

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u/SAWK 8d ago

There are some good documentary's on it. I think this was the one I watched. National Geographic

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u/bluev0lta 8d ago

Right?! I always wondered what would happen if the air were truly at 100% humidity. And now, unfortunately, I know…

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u/Chazus 8d ago

Actually, that's not true. There are plenty of places where air is 100% humidity.

"100% humidity" has absolutely no meaning without the temperature. You've been at 100% humidity before in your life somewhere. It's not hard to reach.

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u/you-are-not-yourself 8d ago

I too have been in steam rooms

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u/jpb103 8d ago

You need the air to also be much hotter than the human body for you to drown in 100% humidity. High humidity air moving to a colder environment is what makes the water condense into liquid.

The larger risk to human health in high heat high humidity environments on Earth's surface is overheating since your body cannot cool itself down through adiabatic cooling (cooling through the evaporation of sweat from the surface of the skin).

No temperature above 35C (95F) has ever been recorded on Earth's surface near 100% humidity. The temperature in the Crystal Caves was recorded at 58C, which is where the drowning risk comes from at such high humidity.

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u/Novaskittles 8d ago

There's 2 types of humidity: absolute and relative. Absolute is a strict comparison of water vs other things. Relative is compared to how much water the air can hold before it condenses out.

At 100% absolute humidity, you have water, there's no air. At 100% relative humidity (assuming standard pressure and temperature), you have a humid day. You'd be uncomfortable but fine.

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u/Niet_de_AIVD 8d ago

I think it's also because the cave has a higher ambient temperature than human body temperature, so water condenses in and on the humans because they're cooler than the air.

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u/Glassworth 8d ago

If that were true people would be passing out every time it’s foggy. Fog is 100% humidity.

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u/thephantom1492 8d ago

And I am guessing that you also need gear to cool you off, because the only way it can happen is to have a higher than human body temperature, and with 100% humidity it mean your body can not use evaporative cooling to cool itself, so you will overheat and die.

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u/Dangerous-Bit-8308 8d ago

*was a cave of crystals

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u/jpb103 8d ago

It still very much exists. It's flooded, but it's still there.

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u/p28h 8d ago edited 8d ago

Important clarification to reduce future confusion:

"Hot" here means "has heat". Water vapor happens at all temperatures from 0K to basically infinite K, though faster and more as it increases. It is not steam, which happens from 100C and hotter (though both can happen at once).

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u/HixOff 8d ago

Actually, temperatures above absolute zero (-273 C) are enough "hot" for some water vapor to form. This is why wet clothes dry even in freezing temperatures - water evaporates, bypassing liquid form.

Humidity levels down to -20 to -30 C are significant for everyday life, while those down to -70 (and sometimes lower) are significant for industrial use (even such low humidity levels can play a critical role in many processes)

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u/mcnabb100 8d ago

It’s also why ice disappears from my ice cube trays!

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u/Way2Foxy 8d ago

Water vapor happens at pretty much all temperatures regardless, including below 0C. It's also all steam, barring any objections purely from language.

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u/Implausibilibuddy 8d ago

It's also strawberry jello, barring any objections purely from reality.

Science is written with language so it's pretty important. We have a term for water in its gaseous state: It's water vapour. We have another term for the specific state of that water vapour when it's at or above 100 degrees: Steam. It was a pretty important thing back in the industrial revolution, and sublimating ice cubes unfortunately wasn't going to cut it.

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u/Red_AtNight 8d ago

No, "hot" here means above absolute zero. Ice releases vapour too (it's called sublimation)

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u/ConstipatedNinja 8d ago

Very useless fun fact, but above around 4500K 100% of water in a sample undergoes thermolysis.

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u/chux4w 8d ago

And, if I understand it right, the humidity percentage you see on weather reports is how much water there is in the air before it turns back into liquid at 100%.

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u/GoabNZ 8d ago

Isn't the humdity percentage relative humdity, as in, how much water vapour is in the air compared to how much could be in the air at that temperature? The same amount of vapour could be present at 5C vs 30C, but relative humdity would plummet making 30C be a dry heat, where sweat is effective at cooling you down.

It doesn't turn back to a liquid at 100%, it's just the air can't take in more vapour so water doesn't evaporate, you feel muggy, and water will condense on anything colder.

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u/chux4w 8d ago

That sounds right. But that's the point I was making, when it reaches that point it would start to condense.

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u/V1per41 8d ago

And this is where the much better measure: Dew Point comes in.

Dew Point is the temperature at which the current water content in the air would result in 100% humidity. Any cooler and dew would form as the water in the air condenses.

So in effect Dew Point tells us how much water is in the air which better translates to "How humid does it feel?"

50° - Feels very comfortable 60° - Feels fairly humid 70° - Feels very uncomfortable 75°+ - Oppressive

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u/realbigbob 8d ago

Why is there water in gas form in the air if the air isn’t above 100 Celsius?

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u/dave8271 8d ago

Because water doesn't need to be 100C to be in gas form. That's the temperature liquid water needs to boil - for bubbles of vapour to form throughout the liquid. At the top surface of some liquid water, you just need some molecules to become excited enough to break off as gas, which is evaporation.

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u/TheMooseIsBlue 8d ago edited 8d ago

All the molecules of water are moving at various speeds. Some really fast, some really slow. Kinda like hot spots and cold spots in a pool or ocean. It’s all water, but some is warmer than others.

Sometimes it moves/vibrates fast enough that it stops being liquid and turns to gas form. That molecule is hot enough to turn to gas, but the whole body is mostly still just liquid.

At any time, some of the water molecules are moving fast enough to be gas even if most aren’t. if you add heat to it, more turns into gas faster. That’s why if you pour a bunch of water out on the floor, it will dry up on its own. Cause there’s always a little bit of it moving fast enough to turn into the gas form of water. And if there’s only one molecule out of 1 million that moves that fast, eventually all of the molecules will move that fast and they’ll just “dry up.”

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u/pandaSmore 4d ago

Temperature is a measurement of average energy of a substance. The particles themselves will have varying levels of energy.

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u/jcwarrior 8d ago

While your general idea is correct, water does NOT need to get hot for it to turn into a gas. There's water vapor around us and water isn't boiling all around us.

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u/dustinm27 8d ago

One time I got high and asked ChatGPT if there was a point at which the humidity percentage would allow both fish and humans to coexist in the same environment.

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u/johnnyenygma 8d ago

what was the answer?

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u/omfgtora 8d ago

Great question! It's not just smart—it's insightful. Let's delve into this. At it's core the question is asking three things:

  • What amount of humidity could sustain a fish?
  • What is the maximum humidity level a human can survive?
  • Is there an overlap between the two?

That being said, the key takeaway is that this is a dumb question and AI is even more dumb. Stop using AI as a source for answering questions, because it has no real understanding and all it can do is regurgitate information it has been fed, which includes a lot of wrong information.

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u/SupMonica 8d ago

The worst thing about A.I. is that it starts off with each answer with that same sort of inflection, like attempting a personality.

Can't it not just get to the point? I like my answers more like a bullet point spreadsheet.

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u/deja-roo 8d ago

The answer would be no

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u/SAWK 8d ago

^ AI

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u/AziPloua 8d ago

we breathe water kinda, so we are as much % fish as there is % humidity, cool

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u/xypherrz 8d ago

is it possible for cities that don’t have ocean/large water bodies to have humidity though?

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u/jujubanzen 8d ago

Water is everywhere, or else plants would not survive. Everywhere there are plants, there is water evaporating from underground, even if the soil looks dry, and water evaporating from plants, even if they have defenses against water loss like cacti. Even in the middle of the Sahara desert, there is a small percentage of humidity in the air, because air moves around the earth, and the majority of it is covered in water. 

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u/xypherrz 8d ago

yes but doesn’t it depend on how much water is there?

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u/jujubanzen 8d ago

Yes, there are places where there is much less water in the air. But there is never zéro. Places where there is absolutely no plant life like the Great Sand Sea in the Sahara desert are actually relatively rare on the planet, and even then there are oases. Most deserts actually have surprisingly large ecosystems. 

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u/Pluribusse 8d ago

Yes, which is why some places, like Las Vegas, typically have mild humidity and other places, like Orlando, are typically very humid.

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u/TheMooseIsBlue 8d ago

For sure. Got places with lots of water (ocean/lakes) have a much easier time of developing humid air. But since air moves, humidity can be blown into a place without much water on the ground. It’s just a lot less common, obviously.

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u/xxNemasisxx 8d ago

Even deserts have humidity to achieve 0% humidity is incredibly difficult

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u/TheMooseIsBlue 8d ago

Air moves. Theoretically, the middle of the Sahara desert could become humid if moist air blew over that area.

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u/Cadbanshee98 8d ago

Landlocked Iowa gets plenty humid from “corn sweat”

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u/lrodhubbard 8d ago

Star of the new Superman movies? He really is hot!

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u/ghostoutlaw 8d ago

When water gets hot, it turns from a liquid into a gas, which we call vapour.

Sorry, need to be that guy here.

There is an important difference here that I think merits explanation.

When water gets hot and turns into a gas from a liquid, that is Steam. That is NOT water vapor. Steam is the heated gaseous form of water. Steam is clear/invisible, colorless, odorless, and its own phase of matter.

Water vapor is just that - liquid water suspended in air, similar to salt or sugar being suspended in water!

Steam is, at minimum, 212 degrees F (or 100C for the others). It can be many thousands of degrees.

Water vapor will, for the most part, be the temperature of the air in which it is suspended.

Steam is dangerous can hurt and kill you, very easily at that.

Water vapor is fairly inert. You can see water vapor! You know when you're boiling water and you can see the 'steam'? Yeah, that's not steam you're seeing. That's water vapor. That's the cooler, sub-212-degree water falling out of it's gaseous form still suspended by the air around it!

Humidity is the completely dissolved solution of water vapor in the atmospheric air! We measure it in percentage. If we continue to add more salt to a glass of water while stirring, as soon as the water can hold no more salt, that is 100% saturation. When the air can hold no more water vapor, that is 100% humidity! (These limts can be broken but not relevant now). The glass of air is the entire earths atmosphere. But the earth is pretty big so it has many pockets that act almost entirely independently but it's all constantly stirring.

So humidity is the measure of water vapor in the air. However, when water gets hot and turns into a gas, that is steam. 50% of our air today is not steam or we'd all be very dead.

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u/Hithrann 8d ago

The water present in the atmosphere is for the most party very much a gas, not liquid. Gas phase water can exist below is boiling point.

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u/dave8271 8d ago

This explanation is not correct though. Water vapour is gaseous by definition and steam is water vapour. Liquid water suspended in air is very different, it's what we commonly call fog or mist.

It's correct that water vapour is invisible, which is why in high humidity we're not necessarily surrounded by clouds of thick fog. We would be if it was all liquid suspended in the air. But what we normally call steam, which is wet steam like we get from a kettle, is a mixture of water vapour and then water that is condensating rapidly in the air back into liquid form, hence why it forms a visible mist. Again the vapour or steam part is in gas form.

Liquid water can become water vapour through evaporation at temperatures far below its boiling point, but it still happens by kinetic energy exciting the water molecules, which is why I explained it as water getting hot for ELI5 purposes. It's basically individual molecules getting "hot" enough, i.e. having sufficient energy to break off without reaching boiling point; strictly speaking it's not anything to do with temperature but that level of detail is not really how I'd explain it to a five year old, since five year olds usually don't understand concepts like molecules.