I know I’m gonna feel silly when I hear the answer to this, but here goes: Radiators tend to dry out the air in a room. As I understand it, heating humid air makes it rise, since it is less dense than dry air. Ok, but where does the dry air come from to fill my room? If the ambient humidity is higher than the humidity in my radiated apartment, where is the water vapor going? Is the air near my ceiling becoming saturated with water vapor and that’s where it all goes?
Corollary question: if I want to prevent the dehumidifying effect with a humidifier, where is the optimal place to put it in the room? Close to the ground? Far from the radiator?
The humidity in the air doesn’t go anywhere, the relative humidity decreases when the temperature of the air is increased. Air at 0 degrees C and 70% relative humidity holds 3.4 grams of water per cubic meter.
When that same air is heated to 20 degrees C the relative humidity drops to 20%. That is because the amount of water air can hold is temperature dependant. Cold outside air infiltrates the house, is heated to ambient temperature, and becomes relatively drier, or more capable of absorbing moisture.
If you want to increase the humidity in a room, you need to put the water where it can evaporate. On top of the radiator is the traditional spot.
I’m still not fully grokking this. So, I have bunch of cold air in a room. I heat up the air, so now the air only holds 2.4 grams of water per cubic meter (or whatever). Where does that other gram of water go?
Also, the second question was about a commercial humidifier. It’ll evaporate wherever I tell it to. But is there a most efficient place to make it do so?
No, as the temperture rises it can hold more moisture. You feel the wetness more when it’s near the limits of what can be absorbed. When it heats up the amount of water in the air is the same as the cold air had, but the warm air can absorb more, so water evaperates into the air with greater speed, or in other words stuff dries out faster.
Ok, I’m starting to understand. So the hot air is able to suck up more moisture from, say, my nose. But doesn’t that mean there is more absolute water vapor in the air?
It doesn’t go anywhere. The 20° air still has the same 3.4 grams of water as it did at 0° but the capacity of the has increased. Relative humidity is the comparison of how much water the air is holding compared to the amount it can.
Analogy: I have a one-liter bottle and put a half-liter of water in it. The bottle is 50% filled. I then pour the water into a two-liter bottle. Now the bottle is 25% filled. Same amount of water (500ml) – bigger bottle.
On review: Beaten to the punch, but I hope my explanation is clear.
Which it does at a greater rate in the warmer air so your nose dries out and the plants dry out faster. Your nose is adding very little moisture to the volume of air in the house. To reduce the rate of evaporation you have to add more water to the hotter air. This is why people in cold areas run humidifiers in winter.
The way moisture evaporates is analogous to the way sugar dissolves in water. Here’s an experiment. Add a cup of cold water to a cup of sugar and stir. Continue adding sugar until no more will dissolve, then add another quarter cup. You have a sugar solution that is saturated at it’s current temperature. That’s like cold outdoor air with a low absolute humidity, but a high relative one.
If you heat the sugar solution, the undissolved sugar goes into solution. It doesn’t go anywhere, but the water is capable of holding more sugar at a higher temperature. That’s like the outside air when it gets warmed. It becomes capable of holding more water.
Now the most important part. You’ve got simple syrup, so make yourself an Old Fashioned, and kick back.
>Relative humidity is the comparison of how much water the air is holding compared to the amount it can.
This isn’t correct but it’s commonly stated. More correctly, relative humidity is the comparison of how much water the air is holding compared to the amount it would if it were equilibrated with a flat surface of pure water. With flat water nearby, if you put more humidity into the air it would condense into the water. If the water surface were convex, as it is in a cloud where the air is equilibrated with zillions of tiny droplets, the relative humidity would be over 100% (in some situations with fine uniform droplets you can get relative humidities up to around 300% or 400% - this is how a “cloud chamber” works in particle physics). The rise in relative humidity associated with droplets is called the Kelvin effect. No doubt you have heard of “supersaturated air”; you couldn’t define that as “air holding more than it possibly can”. If the water surface were concave, as it is near a solid whose fine pores are partly full of water by the capillary effect, it would equilibrate at less than 100% RH. This is how physisorbants like silica gel lower humidity. The same thing happens with other vapors in solids having an affinity for them, for example organic vapors around activated carbon.