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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?

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  • $\begingroup$ 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). $\endgroup$ – lemon May 24 '15 at 11:02
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Air conditioners and dehumidifiers are the same machine -- just different exhaust mechanisms. They both remove water from the air, thus reducing actual and relative humidity. Heaters raise the air temperature, thus reducing the relative humidity (but not, in general, the actual humidity).

Edit to clarify: "relative humidity" is the amount of water in the air as a percentage of the maximum possible capacity. "humidity," or "absolute humidity," is the molar (or other absolute unit) content of water per unit volume of air. The dew point is the temperature at which a given absolute humidity will start to condense out of the air -- thus an indication of the temperature at which the relative humidity equals or exceeds 100%. The dew point is the same everywhere for a given humidity.

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  • $\begingroup$ I think I don't understand humidity, relative humidity, and dew point fully. So humidity is how much water is in the air and relative humidity is how easy it is for water to escape (from the skin) to the air? So what causes different dew points? Say if it is 20°C and humidity 60%, what causes the dew point to be different in California and in Asia and thus altering the RH? So most hygrometers, either one with a physical pointer and the digital ones, in general measure RH instead of humidity? $\endgroup$ – 太極者無極而生 May 24 '15 at 14:04
  • $\begingroup$ @太極者無極而生 see edits $\endgroup$ – Carl Witthoft May 24 '15 at 15:10
  • $\begingroup$ In most cases, humidity refers to relative humidity. If I have a container of air with 50% relative humidity at 10°C and I heat it up to 20°C, the same air will have about 25% relative humidity. It will have the same absolute humidity since the water is still in the container. To see how much water can be in the air, see: en.wikipedia.org/wiki/File:Relative_Humidity.png From the graph, if you have 50% RH at 40°C, the temperature needs to drop to below 28°C (dew point) for water to condense (fog or dew). $\endgroup$ – LDC3 May 24 '15 at 15:26

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