The last post was all about the dewpoint. On a normal day, that's the measurement that tells you how much water you might expect to be in the air - so you know whether your skin and hair will be dehydrating or not.©Science-y Hair Blog 2013
Relative humidity is how much moisture the air is actually holding right now, vs. (relative to) how much it can possibly hold (the dewpoint). I am looking at a temperature of 67°F (19°C) with a dewpoint of 60°F and a relative humidity of around 80%. This is very juicy air! We are expecting strong storms from an air mass like this. It is usually foggy and very dewy in the morning when the air temperature is this close to the dewpoint and it's not very cold.
When the dewpoint is close to the temperature, the air is wet. I think this is some of the worst "weather" for maintaining bouncy waves and curls because the moisture is being taken on by the hair and the wave and curl is expanding in some places, tightening in others and just not cooperating in general! Skin, however, is very happy because it's not losing water to the air.
I am looking at a couple different sources which put the cutoff point at which hair and skin begin to stop losing water to the air around it and begin taking water from the air at between 60% and 70% relative humidity. ©Science-y Hair Blog 2013
That is a good figure to keep in mind because there are those days when you have a high dewpoint and high relative humidity - especially when it stays that way all day long - which make it difficult to keep your waves in shape. Take the same dewpoint, but with a lower relative humidity - either because the day got very warm, or because a different air mass moved in, and the air will not feel as wet.©Science-y Hair Blog 2013
If your hair is losing definition, getting bigger or more limp and sprouting frizz, you might be a victim of high relative humidity plus high dewpoint. I think of it as the "limp and frizzy" point.
I also like to think of air with a low dewpoint like an air-dried, stiff washcloth. Air with a high dewpoint, then, is a wetted-and-wrung-out washcloth. Air with a high dewpoint and high relative humidity (above 70%) is like a washcloth you've dipped in water and pulled out, still dripping wet and heavy.©Science-y Hair Blog 2013