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A few questions about relative humidity

  1. Oct 1, 2015 #1
    I have a couple of wonderings about relative humidity under different circumstances,
    • During hot weather periods RH gets high. Is this because water evaporates from oceans and such? Shouldn't how weather result in low RH since hot air can hold more moisture?
    • During the winter RH usually gets low. Is that because the air simply isn't as moist during the winter? I know cold air can't hold as much water, but then, where did it go?
    • When it rains or snows, does that necessarily mean that RH is around 80-90, or can it still be around 60 even with such weather?
    • Say you have a basement with concrete walls that are quite cool. If there is an issue with mildew, is the best remedy to heat the basement? Won't the hot air just condensate when it hits the walls like on a cold glass of beer in the summer?
  2. jcsd
  3. Oct 1, 2015 #2


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    Well now in parts of Arizona it can be 100 degrees F with a rather dry 10 percent relative humidity. In Florida, might be 100 degrees F and 60 percent relative humidity. On the east coast during a winter snowstorm you can have 32 degrees and near 100 percent relative humidity. It's all relative. Dew points are a better indicator of air moisture rather than relative humidity. In fog, you have near 100 percent independent of air temp. The moisture appears as precipitation when the air can't hold it any more.
  4. Oct 1, 2015 #3


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    Warm air holds more moisture than cool air, but warmer temperatures also mean that more evaporation is occurring from nearby bodies of water.

    In the northern hemisphere, cooler air usually moves south from the northern latitudes around the pole. Not only is this air cold, but it is also dry, since much of its moisture has precipitated in the form of rain or snow.
  5. Oct 1, 2015 #4


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    Generally, but in dry climates, a light rain might not be enough to make the air much more humid. Sometimes rain even evaporates before reaching the ground.
    If you heat the air, it may help, but what matters (temperaturewise) is the temperature of the walls, not the temperature of the air. So it depends on how much heating the air heats the walls. Generally, it wouldn't be the best option. Dehumidification is the best option.
  6. Oct 1, 2015 #5


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    That's contradictory. Hot air hold more water vapour = higher humidity
    The cooler temperature of fall and winter means that less water evaporated from lakes and oceans. And what water does evaporate gets rained out with each cooler day.

    Rain can raise humidity somewhat, but it doesn't stay long. 100% humidity is fog.

    Yes, the vapour will condense on the walls if the walls are cold.
    If you run an heater AND you have adequate insulation, then the walls will warm up.
    But ultimately, as Russ says. dehumidification is the better option.[/QUOTE][/QUOTE]
  7. Oct 2, 2015 #6


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    0. Surprized you didn't ask the obvious: WHY isn't humidity at near 100% - always? Its not like 70% of the Earth's surface has recently acquired a layer of water... I don't know "the" answer to this, but I expect it says something about temperature variations and the speed at which water evaporates.
    1. "During hot weather periods RH gets high..." You ALWAYS need to be careful when you make broad generalizations. I can't think of any which are correct, generally (ha-ha). Journalists used to be taught (before truth took a back seat to click-bait) to ask (and answer) the questions: Who?, What? Where? When? How? and Why? In this case, I draw attention to the 3rd, Where? During hot weather, NEAR THE SURFACE (very very very near). And that pretty much answers your question. The hot air blowing across the moist soil and all the plants and bodies of water evaporate more water faster ..I guess it doesn't have to "blow" now that I think about it... Another related question is Why (is the air hot)? Well, turns out that its not just air temperature of the body of air moving into a region, but also SUNLIGHT heating the surface...and guess what surface heating does for evaporation?
    2. During winter RH gets low. This is just factually wrong.* Very wrong. INDOOR RH gets low. Outdoor evaporation is limited because soil is frozen (water is much less mobile) and plants are in a dormant state. (Etc.) *Although at lower temps, the energy available to evaporate will also be less...And I'm hoping your question about where the water "goes" was hasty...I'd be astounded if you really didn't know where the water "goes". (hint: precipitation)
    3. Snow or rain occurs because A. The clouds are over-saturated and B. The air between the clouds and ground isn't so dry as to evap. (or sublime) the droplets (or crystals). Again, humidity is measured very very very near the ground. (I leave it to you to find out how RH differs with height above ground level). Clearly, someone on the roof can pour water down on you and you'll get wet regardless of your RH. So, the conclusion should be that ground level RH has only a small impact on whether you need an umbrella. The factors at work in the clouds may NOT be predicted by your RH (the question is how much mixing is going on of the air between the different heights).
    4. What do you mean by "best" ? Most economical? quickest? cheapest to install? A little thought would indicate that if the air's RH is zero, then you're not going to have to worry about it. Of course, there's dry-rot which is caused by wood getting wet for long periods... But in modern USA home building (various climates and countries have different solutions - it is a matter mostly of economics), you design a home to have a certain amount of air turn over (the volume of air in the home being replaced per hour or per day). You don't design homes to be 100% air tight, and modern practice is to use a fan to suck in air at the lowest level of the home (basement, say). Its clear that this air is likely to be hotter (and therefore carry more water) than the ground temperatures. So what do you do? Two complimentary approaches: A. Insulate the walls so that the interior surfaces are hotter than the ground temperature and B. condition the air (dehumidify). Warming the wall is a possibility, but guess how expensive that would be. Remember the ground will be "sucking" the heat out of the wall continuously. (When we leave our doors and windows open, we don't expect the outside air temperature to increase, similarily if we heated the walls, would we expect the outside ground temperature to increase? We could do it only at tremendous cost...Oh, I should add a C. Make sure there are no SOURCES of water (leaks, etc.) in the basement.
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