Why is hot air dry?

Question

Everyone knows that hot air from a furnace is dry - that's why people buy humidifiers. But why? Shouldn't hot air retain more water vapor?

Answer 1

It depends on the existing water content in the air. When a furnace heats the interior of most structures, the heat evaporates the water in the air, unlike in my house. Due to the high humidity in my home, I have water condensation on the windows, much like the effect of heat on water laden clouds. In warmer air, there is more energy for more water molecules to hold themselves in the air (and overcome hydrogen bonds which seek to pull water molecules together).

Answer 2

Hot air retains more water vapor like you said. However when you heat up air your furnace does not add water to it. The air is now able to accept more moisture than before and it does this by taking it from people and furniture, drying them out.

Answer 3

Good question. The higher the temperature the more moisture the air CAN hold and higher the dewpoint temperature the more moisture the air DOES hold. If I take air that has a high humidy (temperature and dewpoint are close together) and I heat the air I therefore reduce the humidity because now the air can hold more water vapor.

Answer 4

Dry air is a term describing the relative humidity of air. If you heat air without adding water to it you will decrease the relative humidity of the air. As a result, it will feel dryer even though the amount of water has not been changed. To see the math of this, please review the wikipedia entry:

http://en.wikipedia.org/wiki/Humidity

Answer 5

I think the heat separates the moisture

Answer 6

It isn't. The Rainforests are extremely hot and humid.

Answer 7

i'm confused. in Indonesia,the air is hot and moisture.