Question 1

What is the difference between wet-bulb and dew-point temperature?

Accepted Answer

Both fall below the dry-bulb temperature, but they measure different things. Dew point is the temperature at which air becomes saturated if you cool it without adding or removing moisture — it depends only on how much water the air holds, so it is the number that predicts condensation on a cold surface. Wet bulb is what a thermometer with a wet wick reads in moving air; evaporation cools it, so it sits between the dew point and the dry bulb. The two are equal only at 100% relative humidity.

Question 2

Why does heating air lower its relative humidity?

Accepted Answer

Heating adds no water, so the humidity ratio and the dew point do not move at all. But warmer air can hold much more moisture, so the same water becomes a smaller fraction of what is possible and relative humidity drops. This is exactly why heated buildings feel dry in winter — nothing removed the moisture, the air just gained capacity.

Question 3

What is humidity ratio, and why use it instead of relative humidity?

Accepted Answer

Humidity ratio (W) is the mass of water vapour per kilogram of dry air. Unlike relative humidity it does not change when you simply heat or cool the air, which makes it the right variable for tracking moisture through a system. Relative humidity is what people feel; humidity ratio is what you do the arithmetic with.

Psychrometrics