# Why air conditioner, dehumidifier, and heater all can lower the relative humidity?

I think so far I understand it this way: why is humidity in Asia 75%, and humidity in California 82%, but California feels dry, and Asia feels humid. It is because there is something called Dew Point. So in Asia, Dew Point might be 17°C, and the current temperature of 22°C will only let a little bit of water escape from the skin. Versus in California, the Dew Point may be 10°C only, so a temperature of 20°C already allows a lot of water escape from the skin.

This also explains why in some ski resort during the winter, the humidity is 85%, but inside the hotel room, our lips can dry up: it might be due to a Dew Point of 12°C, so in the hotel room, the temperature is 25°C or 28°C, so a lot of water escape from our skin, making us feel very dry. (25°C or 28°C due to the heater, and it does not matter whether it is an oil-filled heater or a heater which you can see the glowing metal wire inside)

This also explain what somebody told me: inside the house, since the garage is usually at the lowest level of the house, it is usually colder there, and therefore it is more humid inside the garage than the second or third level of the house.

(that is, whenever the current temperature is the same as the Dew Point, it feels very humid, because this is what 100% relative humidity is defined: whenever the current temperature is the same as the Dew Point. The current temperature needs to be higher than the Dew Point for our body moisture to have a net effect of evaporating into the air).

So the question is: why the opposite is true: when we turn on the air conditioner or dehumidifier, the room temperature is lowered, but why do we also feel dryer?

• I don't see any contradiction between the two. A/C units maintain a constant relative humidity in the range 40-60% which is comfortable for us. (Note also that at a constant relativity humidity, the dew point varies linearly with air temperature). – lemon May 24 '15 at 11:02