system
Given the following post, respond as if you're commenting on Reddit:
human
Why do we use Relative Humidity in all the weather reports? Surely how dry it feels is related to the Absolute Humidity? Or if how dry we feel is because of Relative Humidity, why?
gpt
Relative Humidity is a handy because it measures the ability of air to hold more water. Humidity (the moisture content of air) effects us because of its influence on the rate of evaporation. Absolute humidity measures the same thing but it is in practice a less useful scale because the saturation value changes so dramatically with temperature. The weather report could tell us the specific humidity is 1.7 grams water per kg of air, but unless they also tell us the saturation value for air at that temperature, it really isn't usually as informative as saying 25%, or 50% of 98% relative humidity.  If the weather reports did routinely report absolute specific humidity and saturation values, the first step you would want to take to make it into a useful number is calculate the ratio, e.g. relative humidity.

If air is 100% relative humidity it is incapable of holding any more moisture and thus is lousy for providing evaporative cooling. If you sweat in 100% relative humidity air, it won't cool you down, regardless of the temperature. But if the air were relatively dry (25% humidity) than the air is capable of holding much more water and will more quickly evaporate puddles or the sweat on our skin.