what does 100% humidity mean?

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What’s the ratio of air to water at that percent? Is there a cap on the amount of moisture in the air? Is it the same for different elevations?

In: Earth Science

Humidity is the amount of water vapor being carried in the air at a given area. How much water air can carry depends a lot on factors like temperature and how much water is actually evaporating into the air. Hotter air can hold more water vapor *but* if you’re at a desert where there’s no water, the humidity is going to be lower. Colder air can’t hold as much water vapor.

The capacity is “100% humidity” – once the air hits that limit, anything else either… stays where it is (like sweat on your skin, it can’t evaporate), or, if the air was rising upward and hits a temperature where it can’t keep the water vapor any more, then the water condensates into clouds and that can result in rain or snow or hail depending on other circumstances. If the temperature differences are very low elevation you end up with fog, which is just a cloud of condensed water vapor at or near the surface level.

There’s (almost) always some amount of water vapour (steam) just floating about in the air. 100% humidity means that the air can not accept any more water vapour, meaning any additional water that enters the air condenses as liquid, this is what fog and clouds are.

The amount of water air can hold depends on temperature, the hotter the air the more water it can hold. This is also why cold cans of drink will form a layer of water on them. The air around the can is cooled so much that the amount of water the air can hold drops below the amount of water already in the air, so that water has to condense as a liquid.

Same effect happens when you can see your breath on a cold day

It means the air is fully saturated with water. This is very similar to when you have a glass of tea and you’re dissolving sugar in it, there is a point where if you add any more sugar, the sugar will just sit on the bottom and not dissolve into the tea. This is the saturation point and it occurs when water vapor is dissolving in air as well. once the air reaches 100% humidity, it has reached it saturation point.

The air can hold a certain amount of water. That amount changes based on the air temperature. The percent humidity is the percent of water the air has already absorbed compared to what it can totally absorb. That percent can change as the temperature changes, even if the amount of water in the air stays the same.

When people say the humidity is 50%, they’re really talking about the *relative* humidity. That’s a comparison between how much water vapor is in the air and how much the air can possibly hold at that temperature. The actual amount of water the air can hold varies with temperature and pressure, but generally hot air can hold a lot more than cold air.

When the number gets to 100%, that means the air can’t take any more water. At that point, your sweat doesn’t really evaporate, so you tend to be covered in sweat as your body tries to cool itself. Anything colder than the air, like a soda can, will be covered in water, because the air next to it gets cooled down, can’t hold the water it already has in it, and dumps that water onto the can.

You’ll also see water dripping out of air conditioners, because they use cold coils to cool the air, and that also makes water come out of the air. That’s also why your sweat dries up so fast when you walk into an air conditioned building.

It means it feels like hot as hell and you sweat like crazy. It means that when you run you feel like it’s 150 outside. And to those people who state Arizona and places like that are different because of low humidity, let me tell you hot is hot. So don’t let anyone talk you into moving to a place because it’s dry heat. Hot is freaking hot, ok.

> What’s the ratio of air to water at that percent?

That depends on the temperature. At the freezing point of water (0°C), air at 100% humidity has about half a percent of water by weight, while on a hot day (30°C), air at 100% humidity has about 3% water.

>What’s the ratio of air to water at that percent?

Sorry not ELI5, but here is the equation that answers this question:

h = 1e-2 * RH * p_vp / p

where:

h = fraction of air molecules that are H2O

RH = relative humidity, expressed as percentage

p_vp = vapor pressure of water (increases with ambient temperature)

p = ambient pressure

Bonus round: supersaturation (RH>100%) is possible as well, though it is short lived and usually happens in clouds..at best you briefly add a percent or two on top of the 100.

“What’s the ratio of air to water at that percent?”

It depends on the temperature. There’s more water at 80 degrees and 100% relative humidity than at 60 degrees and 100% rh.

At 100% rh and 60 degrees there’s 0.0107 pounds of water for every pound of air. At 80 there’s 0.02244 pounds of water per pound of air.

The air becomes solid water!

But seriously, it is just the max amount of water that the air can “absorb”. It is a relative number, and the physical amount will vary based on air temperature. So 10% humidity in cold air is not the same amount as 10% in hot air.

Basically if 100% humidity happens, than it is likely to rain. Keeping in mind that humidity also varies based on altitude as well, so if it is X amount humid at the surface, it means up high where it is colder, the % humidity is likely to be more, causing high potential for rain.

If you live in Florida like I do – it means you are going to be sweating in places that you don’t want to sweat in.