If the humidity level reaches 100%, does that mean we're underwater?

September 30, 2005 5:35 PM

I know they say that there are no stupid questions, just stupid people. Therefore,I ask this question with the full disclosure that, on the subject of humidity at least, I am a stupid person. So, my question then is; if the humidity level reaches 100%, does that mean we're underwater?
posted by Effigy2000 to Science & Nature (20 answers total)

No. It means that the air is holding 100% of the amount of water vapor it can hold. The capacity of air to hold water vapor varies based on temperature and barometric pressure.
posted by kindall at 5:36 PM on September 30, 2005

If by "underwater" you mean under falling rain drops, then yes, at 100% humidity it will rain.
posted by darkness at 5:38 PM on September 30, 2005

Obligiatory Wikipedia link
posted by cillit bang at 5:45 PM on September 30, 2005

The air can actually hold more than 100% if certain aerosols are present (supersaturation) like dust and pollen. The 100% assumes pure air and water.
posted by rolypolyman at 5:51 PM on September 30, 2005

Sorry, I gaffed, it's the lack of a nuclei that allows for supersaturation.
posted by rolypolyman at 5:53 PM on September 30, 2005

So it's not really more than 100%. As humidity is relative (to temperature), so the percentage itself is relative (to contaminants). It's like hockey--those guys are not really giving 110%.
posted by weapons-grade pandemonium at 6:32 PM on September 30, 2005

In other words, at 100% the water starts to precipitate out of the air.

(Oh, and by the way: there are three kinds of people in the world. Ones who can do math, and ones who can't.)
posted by alms at 6:51 PM on September 30, 2005

darkness, you can have 100% humidity and no rain if you have a sufficiently dense fog; also, related to what rolypoyman said: check out the next-to-the-last comment here.
posted by kimota at 7:02 PM on September 30, 2005

This is the difference between "Relative Humidity" and "Absolute Humidity". Weathermen never give the latter value, so you often hear "Humidity" as shorthand for the former. Occasionally you need to convert between the two (doing air conditioning calculations for example), and for that you need a psychometric chart.
posted by Popular Ethics at 7:24 PM on September 30, 2005

What is "pure" air, rolypolyman?
posted by madman at 9:44 PM on September 30, 2005

well, from his statement I would assume it was air that contained no dust and/or pollen.
posted by Iax at 11:11 PM on September 30, 2005

100% Humidity>> dew point and temperature are equal
posted by raildr at 2:25 AM on October 1, 2005

Best answer: Effigy2000,

Stir some salt into water. It dissolves right? But there's a point where you can't add any more salt and it forms a sediment on the bottom.

Well, in air, you can add water. And you can keep additing it...until droplets start to form (at which point the local air is holding 100% of the water it can - 100% humidity.)
posted by filmgeek at 4:36 AM on October 1, 2005

I know they say that there are no stupid questions, just stupid people.

This is actually not true. There are plenty of stupid questions. My belief is that it's fine to ask them. The problem arises when the same person keeps asking the same stupid questions.

Your question, by the way, is not stupid. It neatly gets right to the heart of day-to-day physics.
posted by Mo Nickels at 6:24 AM on October 1, 2005

Heh. Somewhere, some scientist has a site that angrily critiques people who say that the air holds water, or that it has anything to do with temperature. Over on MoFi I got yelled at with it once, but I can't seem to find the site.
posted by klangklangston at 8:05 AM on October 1, 2005

Humidity.
posted by furtive at 9:10 AM on October 1, 2005

As to wether you can have 100%+ humidity, it depends on what you mean by the air 'holding'. A fog would be a heterogeneous suspension (right?) rather then water vapor being dissolved in air you'd have tiny droplets of water that are to light to fall to the ground as rain. So the air is 'holding' the little droplets, but the humidity of the air around the droplets is only at 100%.

Technically you are also 'under' water, because there are droplets of water above your head...
posted by delmoi at 1:36 PM on October 1, 2005

Oh, by the way there is something rather intresting about Gatorade I discovered after trying to freeze it before going to the gym.

You can cool it to below freezing, and it won't freeze right away. But if you try to take a drink of this super-cooled solution, you get icy slush in your mouth, rather then the liquid. then a 'shockwave' of ice crystals will travel through the bottle in a couple seconds, freezing the entire thing.

Very cool to watch, IMO.
posted by delmoi at 1:38 PM on October 1, 2005

I recall a part of Heinlein's Red Planet (the book, not the movie) where the kids are trying to explain to the Martians that people need to breathe a little water along with their air.

Eventually, the Martian goes out, turns up the humidifier in his cave, and then comes back and asks "whether the water that flies with the air is now sufficient."

Also, the original poster is ignorant, not stupid; stupid would be not wanting to ask the question in the first place. And the instrument that measures humidity is a psychRometer, not a psychometer.
posted by ikkyu2 at 3:00 PM on October 1, 2005

the original poster is ignorant, not stupid

Was ignorant, by the time s/he reads this comment. Problem solved.
posted by squidlarkin at 3:17 PM on October 1, 2005