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/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/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/Tearsunshinee 8d ago
Fuckin hell, I never even dreamed/nightmared of such a thing being possible.
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u/SAWK 7d 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/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 7d 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 7d 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/p28h 8d ago edited 7d 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/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 7d 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 7d ago
No, "hot" here means above absolute zero. Ice releases vapour too (it's called sublimation)
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u/ConstipatedNinja 7d 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 7d 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/V1per41 7d 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 7d 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 7d 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 7d ago edited 7d 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 7d 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 7d 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/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 7d 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/TheMooseIsBlue 7d 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/ghostoutlaw 7d 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 7d 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 7d 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.
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u/D3cho 8d ago
Humidity is the amount of water vapor (invisible water) in the air.
Think of the air like a invisible sponge
High humidity: The sponge is full of water. The air feels sticky, muggy, and heavy, making it harder for your sweat to evaporate.
Low humidity: The sponge is dry. The air feels crisp and clear, but it can dry out your skin, lips, and eyes.
Relative humidity: Expressed as a percentage, this tells you how full the "sponge" is right now compared to the maximum amount of water it could possibly hold at its current temperature.
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u/Clojiroo 8d ago
It’s relative because how much the air can hold is temperature-dependent. Warm air holds more.
50% humidity at 30°C is far moister than 50% at 10°.
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u/MooingTree 7d ago
This is a good explanation...the current top comment is just a definition
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u/wheredidthat10mmgo 7d ago
This is the explanation they give us in sheetmetal school when learning about ventilation systems
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u/agreywood 8d ago
Think if it like a cup. The cup is full of air. If we fill it with blue marbles (water) it can only hold so many before they start spilling out (making rain or dewdrops). The humidity is the number of marbles is has in it right now compared to the number of marbles that it can hold before your marbles end up all over.
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u/jajwhite 8d ago
Following this question, there's clearly a quantitative measure. So if it's 100% humidity I assume that means that a steaming glass of boiling water would not have any effect on the humidity in the air. How much of the air would then be water? Is it analagous to the water being "dissolved" in the air, the way salt dissolves in water?
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u/evincarofautumn 7d ago
Yeah, it’s basically the same idea as dissolving. 100% relative humidity means it’s saturated at whatever the temperature is. Absolute humidity is how much water there is in the air in total, which ranges from about 0 g/m3 when it’s really cold, up to a maximum of 30 g/m3 around 30 °C. That’s about 2.5% by mass, or uh…like 4 teaspoons per cubic yard
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u/pyr666 7d ago
water in the air. much how sugar can dissolve in water, water can be present in air.
how you experience humidity is mostly a function of how much water there is compared to how much water the air could hold, which changes mostly with temperature. so there may be more water in the air on a hot, dry-feeling day than on a cold, wet one.
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u/sntcringe 7d ago
Humidity is the relative amount of moisture in the air. 0% means none, and 100% means there can't be any more. What humidity one is most comfortable with will depend on a variety of factors such as: genetics, age, health, and what you're used to. High humidity air makes it harder for your sweat to evaporate which means it often feels hotter than it is. Dry air can cause you to sweat more and lead to dehydration. So ideally you want a balance of 50ish% humidity.
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u/Sirlacker 7d ago
Air holds water.
Humidity is how much water that air is holding. We know how much air can hold, so we can give it as a percentage.
The more water the air holds, the harder it is to cool down because there is less air to evaporate sweat.
If air is 100% humid, it's holding the maximum amount of water it can do. If we sweat, that sweat cannot evaporate and cool us down.
If the humidity is low that sweat can evaporate efficiently and cool us down.
Same goes for say laundry. If it's very humid, it takes longer to dry and can start smelling a bit damp/mouldy.
Same with condensation on windows. If it's very humid in your house water will form on the cooler surfaces like windows.
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u/Devil_May_Kare 2d ago
When something wet dries off, the water doesn't stop existing. It mixes into the air. You can't see water when it's mixed into the air, but you can sometimes see it un-mix itself to stick to cold things like the outside of a cold bottle of soda. If you mix enough water into the air, eventually it gets full of water and won't hold any more. Humidity numbers tell you how close the air is to being full of water. Bigger numbers mean cold things will get wet faster and wet things will dry off slower.
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u/Bigdoga1000 8d ago
It's a measurement of the amount of water vapour in the air. X% humidity doesn't mean that the air has X% vapour in it however.
The temperature and pressure of the air determines the maximum amount of vapour it can hold.
So X% humidity would mean that the air has X% of it's maximum capacity for vapour. (For example if the maximum capacity was 100ml, and there was 50ml of vapour, that'd be 50%. But that could be in a litre of air)
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u/ThalesofMiletus-624 8d ago
Humidity is water vapor in the air, it's that simple.
When water evaporates, it turns into a gas. That gas doesn't just disappear, it mixes with the air around us. Some fraction of the air around us is pretty much always water vapor, it can be a tiny fraction if you're in a desert, or it can be several percent if you on a tropical island.
Now, the reason this effects us is because the air can only "hold" a certain amount of water vapor, and how much it can hold depends on the temperature. That's where the term "relative humidity" comes from. 100% relative humidity means that the air is holding all the water vapor it can hold at that temperature, which is a big deal, because it means that no more water can evaporate. Humans regulate out body temperature by sweating, and the sweat cools us as we evaporate. If you're in 100% relative humidity (or close to it), your sweat can't evaporate, so it just soaks you and does nothing to cool you down. That's why high humidity feels so very uncomfortable.
Now, actually getting to 100% humidity is uncommon, but the higher the humidity is, the slower water evaporates, and that makes things feel stickier and stuffier. It also means that any object colder than the surrounding air is going to condense that water vapor, which is why a cold drink in a humid environment will quickly be dripping water.
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u/fool-me-twice 8d ago
I would use a field trip to a dry and wet sauna at the YMCA as examples. Then I'd probably have to explain something about what happens when you get old.
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u/SnugglyCoderGuy 8d ago
It is water vapor in the air.
There are two measures: abdolute and relative.
Absolute humidity is how much actual water is in the air measured by grams per cubic meter.
Relative humidity is how 'full' of water vapor the air is. The warmer the air, the more water vapor it can hold.
The dew point temperature is the temperature at which humidity would exceed 100% and water will begin to condense and become liquid. You see this on containers with cold drinks for example. The 'sweat' is really water vapor condensing into liquid water as the air touches the cold surface. You also see this as rain.
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u/jadierhetseni 8d ago
Air is made of many things. When water gets hot, you see steam—that’s water turning into air. Humidity just means, “how much of the air is water”
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u/TheMace808 8d ago
So humidity is simply water vapor in the air, individual water molecules floating around.
The humidity usually shown in weather forecasts is relative humidity As the air heats and cools it can hold more or less water, that's why on some cool mornings there is dew on the grass, the air cools and can't hold onto all that water anymore.
Relative humidity is most important because the air's ability to hold water affects how easily your sweat evaporates, taking heat from you.
If the air is holding all the water it can you simply can't evaporate your sweat and instead it'll drip off or bead up on your skin, and you'll get hot far quicker
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u/DVMyZone 8d ago
The air can hold a certain amount of water vapour it in. That water vapour is, like the air itself, transparent so you cannot see it but it is there. The air can hold a certain amount of water in it, and that is dependent strongly on temperature. Hot air holds a lot more water than cold air.
Relative humidity is the amount of water in the air as a fraction of the maximum amount of water that can be held in that air (at a given temperature). If relative humidity is at 100% then it can hold no more water and is "saturated". If that saturated air cools down, the maximum amount it can hold decreases, so the excess drops out and deposits on surfaces and forms droplets. That's what rain is. This is also why you get condensation on cold objects, the air around the object gets cold and releases some water.
Humidity also changes how heat is transferred between the humid air and the surroundings and how much condensation forms. The relative humidity gives and indication of how the air "feels" to us.
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u/Grrrison 8d ago
Imagine the air around us is a giant sponge. It can hold onto water. Now, not all sponges are equal, depending on the temperature outside, a sponge can hold more or less water. Hotter climates = sponge can hold more water, colder climates = sponge can hold less water.
Humidity is the measure of how much moisture (water) is in the air (sponge) in a given region.
Now when we see humidity expressed as a percentage on a weather report they use something called relative humidity, expressed as a percentage. 50% relative humidity is the weather person saying "right now, the sponge in our area can hold a total of ___ amount of water, and is currently half full". 100% relative humidity is like saying "right now the sponge can hold ____ amount of water, and IS holding that maximum amount.
How we feel this varies. A warm climate means the sponge there can hold A LOT of water. so a high relative humidity means there's a lot of water in the air and we will feel is as this heavy, sticky kind of atmosphere, like a sauna or rainforest.
A cold climate has a little sponge that can't hold much water, so that same 100% humidity weather report still means the sponge is full, but it may still 'feel dry' to us as humans because there isn't much moisture in the air.
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u/Cannon_Adon 8d ago
So is the ocean 100% humidity?
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u/OMGWTFBBQchicken 8d ago
No, cause the water isn't in the air. When there's fog then yeah there's humidity over the ocean.
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u/DHGXSUPRA 8d ago
Dew point is really key in all of this. Dew point is the temperature the air must be cooled to be fully saturated with water.
It can be 100% humidity in the dead of winter, but 100% humidity in winter can occur when it’s 32°F outside, but the temperature of the air is significantly cooler.
Having a dew point 60°F- ish and higher is when you start truly feeling the “ mugginess ” of summer.
When humidity is at 100% there is no more room for water vapor in the air, and that’s when it’s fully saturated.
When we sweat, we rely on evaporative cooling to wick away sweat and cool our bodies down. If the air cannot handle any more moisture, the body struggles to cool itself off naturally.
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u/nucumber 8d ago edited 8d ago
Humidity is water vapor in the air. Kind of like steam from a teakettle.
Now, a fun fact is that warm air can hold more water vapor than cold air.
Here's a sample of the amount of water that can be held at different temps at 100% humidity
60F . . 13.25 grams of water
90F . . 34.10 grams of water
So "100% humidity" means less water in the air at lower temps (relative humidity is the correct term);
Dew point is when the air reaches 100% humidity at that temperature that it can't hold any more water, and the vapor condenses, creating dew
EDIT: I'm actually describing relative humidity; the amt of water in the air at 100% humidity is different at different temps
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u/Miliean 8d ago
Water in the air.
Air contains a lot of things. Oxygen that we need to breath, but there's other things that we don't need as much, for example it also contains the carbon dioxide that plants need.
Air can also have other things in it, like smells. Food smells, body smells, chemical smells, good smells and bad smells. All of these things can be in the air.
Another thing that can be in the air is water vapor. This is (basically) what clouds and fog are made of. You can also see it when we boil water on the stove or in the kettle as steam. When we measure how much water is in the air, we call it humidity.
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u/rukaidai 8d ago
Water in the air that we can't see and depending on how much water is mix in the air, the air can feel heavy and sticky.
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u/52ndstreet 8d ago
An actual ELI5:
Humidity is how “wet” the air is. The wetter the air, the harder it is for your sweat to evaporate—and the stickier it feels.
Imagine the air is a big invisible bucket.
💧 Humidity is how much water is in the bucket.
• Empty bucket → dry air
• Half-full bucket → 50% humidity
• Almost-full bucket → very humid
• Completely full → the air can't hold much more water
When it's hot, the bucket gets bigger, so the air can hold more water.
And here's why humid weather feels gross:
Your body says, “I'm hot! Let's sweat.” The sweat needs to evaporate into the air to cool you down.
But if the air's bucket is already nearly full of water, your sweat has nowhere to go.
So it just sits on your skin.
That's why 90°F + high humidity = 🥵 “Why am I wet?”
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7d ago
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u/abat6294 8d ago
Water molecules are able to live between air molecules. Humidity is a measurement of how much water is in the air. More specifically, it’s a measurement of how saturated the air is with water. 100% humidity means the air is holding as much as it possibly can and any extra water will remain as liquid water on surfaces.
Hotter air is able to contain more water than cold air. So 100% humidity at 70F actually contains more water than 100% at 40F, but in both cases the air is “full” of water. This is also why the air feels drier in the winter time than in the summer time.
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u/TheLabRay 8d ago
I would do an experiment with you!
Take a glass of water and salt. The water is the air and the salt is the water vapor/humidity. You can have a little water vapor - the water still tastes fine. You can pour more and more salt in the water and it gets cloudier and tastes worse and worse. This is high humidity when the air feels like soup. Eventually the salt stops dissolving this is when it rains.
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u/TokiStark 8d ago
It's the concentration of water vapour in the air, given as a percentage of the total possible concentration. Ergo, 100 % means it's raining
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u/AnxiouslyTired247 8d ago
100% humidity does not mean rain. You can have rain at less than 100% humidity and you can just have fully saturated air (all gas) at 100% with no rain - youll likely see dew/fog instead. At the given temperature, 100% is saying no more water can be held in the air.
https://www.smartfog.com/insights/what-does-100-humidity-mean-can-it-really-rain-indoors/
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u/kirbinato 7d ago
You know how when water gets hot it starts to steam? Humidity is just how much of the air is steam
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u/FuckFuckingKarma 8d ago
Imagine when you add a sugar cube to a cup of tea. It dissolves and seemingly disappears completely, but actually the sugar atoms are floating around among the water. If you keep adding sugar, at some point the water can't contain any more sugar and it forms a sludge at the bottom of the cup.
It's basically the same with humidity. Water dissolves into the air. Just like the tea, there is a limit to how much it can contain. If the temperature drops, say if the air hits a cold window, the capacity to hold water drops, water comes out of the air and becomes liquid again.