Naked Science Forum
Life Sciences => The Environment => Topic started by: roberth on 14/02/2005 00:00:18
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Can someone explain the humidity thing, please. I have a thermometer and a hygrometer. During the day the temperature may range between 28 and 35 C, with the humidity around 65%. Most mornings in the summer, the humidity is 100%, with the temperature ranging between 20 and 24 C. It's not raining. Does it need recalibrating or are the readings correct?
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They could be correct. I take it you are on the coast, since in the outback the humidity would be about 10%. You should be able to get approximate corroboration from a local radio or TV station. However, the only way to be sure is to check the hygrometer with a sling psychrometer.
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Your measurements seem to make sense.
If the actual mass of water vapor present in the air remains constant, the relative humidity will change only as a result of the change in temperature; therefore, the relative humidity will be higher in the morning (coolest part of the day) than in the afternoon (warmer part of the day).
If the morning temperature is 20C with a RH of 100% then the same air (i.e. same absolute humidity) will have a RH of 65% when the temperature rises to 27.35C.
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Thanks, gsm and DrPhil. I do live near the coast and the readings during the day are corroborated by the local news. I guess my question is more to do with what relative humidity is. How much moisture is in the air when the RH is 100%? When we breathe this humid air, are we getting usable moisture into our system? Would this alleviate the need for consumption of water? If the air is (about) 70% nitrogen and 30% oxygen, what changes when it's chock a block full of water vapour? What's the difference between relative and absolute humidity?
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What's the difference between relative and absolute humidity?
Absolute humidity is the actual amount of moisture in the air (i.e. g/m^3)
Relative Humidity it is a measure of the amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature. That's why the RH changes when the temperature changes. Warm air can hold more water vapor than cold air, so with the same amount of absolute humidity, air will have a HIGHER relative humidity if the air is cooler, and a LOWER relative humidity if the air is warmer.
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How much moisture is in the air when the RH is 100%?
If the relative humidity is 100% it simply means that the air is saturated. The air contains the maximum amount of moisture it can hold at that particular temperature.
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When we breathe this humid air, are we getting usable moisture into our system?
Unless the air temperature is greater than body temperature I think there will be more evaporation than condensation - your lungs will heat up the air and then it will be able to absorb more water.
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Would this alleviate the need for consumption of water?
I would have thought you loose quite a lot of water by breathing, so it will reduce the amount of water you loose through breathing - although if you are sweating buckets because of the humidity, you may well loose more water than you gain.
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It might interest you to know that many people drown every year on life support systems, that nebulize moisture into the lungs. Do you get a fair number of respiratory problems? More so when you are away from an airconditiong system?
Andrew
"The explanation requiring the fewest assumptions is most likely to be correct."
K.I.S. "Keep it simple!"
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This is a really interesting thread, thanks Rob!
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The traditional dial type of hygrometer that you can buy most easily is quite sluggish to react and gets confused with rapid changes in temperature or direct sunlight, worth bearing in mind. If anyone can explain how these work I'd be interested - I did hear it was with a bimetalic coil.
If you want to experiment, you can make a humidity chamber and check the calibration of your device. The basics law is that in any system at equilibrium, all phases must have the same humidity. Since water has a "relative humidity" of 100% the space above the water in a sealed container must have a humidity of 100%. It turns out that the relative humidity above a saturated solution of table salt is 75% at room temp.
You can easily make a saturated solution of salt by dissolving as much salt as you can in warm water with stirring and then letting it cool, salt crystals should still be present. Don't use any more than half a glassful of water, or get it much over warmness, as you'll find you'll need a lot of salt.
One warning, is that a relative humidity of 100% tends to lead to condensation, so perhaps best not to try that if your hygrometer might be damaged! 75%, of course, doesn't lead to condensation as it defies the key law.
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The most humid place I've ever been to was Hong Kong !!..I think it was May, it completely ruined my camcorder and was unbearingly uncomfortable, within 5 minutes you were soaked !
Sorry, I know this has nothing to do with the science of humidity but the thread bought it all back.
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