there is a lot of info about dew points, here is a link for the curl chemist article on it:
Humidity, Humectants and the Dew Point
the basic thing about dew point is that it is what is important for our hair, not humidity. the dew point is never higher than the temp, so if is snowing outside, it is 100% humidity, but the dew point is lower than 32, can even be negative.
40 - 60 is mid range, good for curly hair. moisture neither leaves our hair nor does it take in extra.
60 and above our hair tends to take in extra moisture. everyone's hair is different, my hair is good in high dews. if you hair cannot tolerate extra moisture, you would 'seal' it.
below 40 is low dew point. moisture leaves our hair. so it may start out great, but as the day goes on, it gets drier. most curlies, myself included, seal their hair to avoid this. you should avoid products that contain a lot of glycerin as the glycerin seeks moisture, so if there is none in the air, it will take it from our hair.