100% humidity?

My flat is quite damp; the humidity was at 80% before I got a dehumidifier. It’s now down to between 60 and 70%.

I got thinking last night (dangerous sport :eek: ) what is 100% humidity? Is it a cup of water or is it that the air is so saturated that it can’t take up more water without “raining”? :slight_smile:


Last year on a geography camp, us learning-type people were being taught how to use various environmental data-gathering instruments. There was this thingy that you took out into an open area and spun around for a minute or so, and then it gave you a humidity reading. It was raining when we did this, though, and the teacher guy said “well, we’ll do it anyway, even though it’s just going to say the humidity’s 100%.” I took this to mean that 100% humidity = precipitation. So there you go.

At 100% relative humidity, the air is saturated. If you then increase the relative humidity (for example by cooling) some of the water will condense out in the form of fog. It will also appear as “dew” where it comes in contact with solid surfaces.

100% RH does not equal precipitation. Precipitation does not require 100% RH at ground level; rain can fall through dry air.

Air does not “hold” water, it does not become “saturated”.
From Bad Meteorology page

Each gas in the atmosphere has its own dew point. It just that water’s is in the range of climatic norms, so we notice its phase changes more often.

OK so if I have 100% humidity the vapour pressure equals the equilibrium vapour pressure (giving a value of 1, 1 x 100 = 100 so 100%)?

Does it mean that if the humidity is less than 100% then water vapour is being “sucked” in? Also why do you get condensation occurring when the humidity is less than 100% Is it that the equilibrium vapour pressure changes with temperature?
: desperately trying to get my head round that quote:

AWB, you are correct, and I should have thought more deeply before posting. “Saturation” is such a useful and intuitive concept that it is hard to give it up, especially when trying to provide a clear and concise explanation.

If the RH is less than 100%, liquid water will evaporate (clothes on the clothes line, or dishes on the rack, will dry). AWB would make the point that the net evaporation is the combined effect of more water making the phase transition from liquid to vapour than the other way.

Condensation occurs on cold surfaces where the water vapour in the air in contact with the cold surface is cooled below its dew point. The RH is locally 100%.

For more about humidity, dew points, and other weather terms, see the Bad Meteorology page.

Thanks everyone!

Of course, if you want the real experience of 100% humidity, come down here to Louisiana this summer. It’s not a rare occurence, either. I’d estimate that it gets that humid probably 25-50% of the days of any given year. Step out your door when it’s 80 degrees out and 100% humidity and you’ll be sweating in seconds. Feckin’ miserable.

For the engineer’s approach to relative humidity (as opposed to the meteorologist’s), try


By the way, Erroneous, the thingy you used was probably a sling psychrometer. (So called because you have a dry bulb thermometer and a wet bulb thermometer mounted in the same frame, and you sling it around in the air to give the dry bulb and wet bulb temperatures ---- which you then look up on your psychrometric chart to determine what the relative humidity is.)

Note that the “engineer’s approach” refers twice to the “moisture-holding capacity of air,” verbiage which Bad Meteorology condemned.

That’s the point I was trying to make. I don’t understand Bad Meteorology’s condemnation of the term. Engineers have been using the psychrometric chart quite successfully for some time for design purposes. The line at the top of the chart (the 100% relative humidity line) is referred to as the saturation curve. If a single iota of additional water is introduced to a particular volume of mixed air at a particular dry bulb temperature at 100% relative humidity, water will condense out. Hence, no moisture can be added, and it is saturated. I think that the argument that the term saturation is used by incompetent authors is just playing with semantics.

Bad Meteorology also argues that

"some incompetent authors present it to their readers as a percentage of the air’s holding capacity. "

Check out the psychrometric chart located at


Pick any dry bulb temperature (measured along the axis along the bottom of the page. Travel upward along the constant drybulb temperature line until it hits the saturation curve (RH=100%). Then travel in a line straight to the axis on the right hand side of the curve to read the humidity ratio, which measures the moisture in mixed air (in pounds of moisture per pound of dry air). Now go back again to your dry bulb temperature line. Follow the line upward until you reach the RH=50% curve. Now follow the line to the right axis to read the new humidity ratio. Divide the humidity ratio at 50%RH by the humidity ratio at 100%, and PRESTO! You have a ratio of 50%. The relative humidity curves DO show that, at a given dry bulb temperature, the mixed air at that given relative humidity does contain that percentage of moisture in relation to what how much moisture the air mixture would contain at saturation.

Where does the incompetence come in?

The “incompetence” comes from people who are incredibly anal about arguing over terms that are and have been in common use for decades or longer. “Saturation” is the common engineering term in every country I have been to. For this site to use the term “incompetence” is really pretty damn incompetent itself, and violates Una’s Rule of Public Speaking Number 4:

  1. Do not insult or mock your informed, well-intentioned, yet incorrect audience.