there are articles here on nc, and many websites. do a search here and then a google search.
in brief, curly hair is more dry -- CURLY HAIR NEEDS MOISTURE and is always seeking it.
humectants, most particularly glycerin, also seeks moisture. in low dewpoints it cannot get moisture from the air, so it will take it from our hair, causing it to be dry, lose curl. in high dew points glycerin can take in too much moisture causing our hair to 'swell.'
40 to 60 range is considered the best for curly hair. everyone's hair is different, tho. my hair loves glycerin in mid and high dew points, seems my hair cannot get enough moisture! lol!! i even consider the places i will be -- ie, will i be spending a good part of the day in an overheated, dry bldg which has low dew points? watch my curl disappear, my hair look like a dry nest!! unless i seal in my moisture that day.
i always check the dew point before i wash my hair, makes it easier to determine which products to use. you can use the frizz factor on the home page here, just put in your zip code. i go to weatherunderground.com for my zip code.
remember it is the dew point, not the humidity that is important. ie, it can be 20 degrees out, snowing so it would be 100% humidity, but the dew point is never above the actual temp, so dew point would be 20 or most likely lower, so a dry day for hair, no moisture in that cold air.