Okay, I didn't explain this very well before and since I've got NOTHING else for today I'm going to try again. Does anybody remember yesterday my writing about a little rain when I left for my walk? Did anybody even read the thing? Is anybody out there right now? 'Doesn't matter I'm going to do this anyway. Everybody knows this stuff because they've experienced it but if your trying to explain it I think the best way is to use numbers.
First from Wikipedia so I don't have to try and explain this all by myself, I can cut corners and get this over with. I took all the footnote links out because they're annoying here.
The relative humidity of air depends not only on temperature but also on the pressure of the system of interest.
Humans are sensitive to humid air because the human body uses evaporative cooling (perspiration) as the primary mechanism to rid itself of waste heat. The rate at which perspiration evaporates on the skin under humid conditions is lower than under arid conditions. Because humans perceive a low rate of heat transfer from the body the same as a higher air temperature the body experiences greater distress of waste heat burden at a lower temperature with high RH than at a higher temperature at lower RH.
If, for example, the air temperature is 24 °C (75 °F) and the relative humidity is zero percent then the air temperature feels like 21 °C (69 °F). If the relative humidity is 100 percent at the same air temperature then it feels like 27 °C (80 °F). If the air is 24 °C (75 °F) and contains saturated water vapor, in other words, then the human body cools itself at the same rate as it would if it were 27 °C (80 °F) and dry. The heat index and the humidex are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body.
Humans can be comfortable within a wide range of humidities depending on the temperature — from thirty to seventy percent - but ideally between fifty and sixty percent.
When I left here it was 80º and the humidity was 60%. To figure out the humidex, short for "humidity index" you have to consider both numbers and use this simple formula...
Or you cheat and use this handy little calculator I found online. Punching in my numbers indicates: