Humectants and Dew Points

I am not a scientist.  I'll add links to blog posts of people who thoroughly understand the science behind this stuff.


Humectants
What is a humectant?  A humectant is an ingredient that draws water to itself.  In normal dew points (see below), humectants draw moisture (water) to your wavy/curly hair from the air and make your waves/curls bounce and behave.  In dry weather (think cold winter or desert), there is no moisture in the air.  In dry weather (low dews) humectants cannot draw moisture from the air, so humectants pull moisture from the only place they can find it, your hair.  In humid weather (sticky air, think summer in the south east), humectants draw too much moisture to hair and cause frizz.

Which ingredients are humectants?  The link I've posted has a list.  The main humectant that seems to cause problems is glycerin.  If you only avoid glycerin in high or low dews, you may be fine.  Every one's hair is different.  Some people can use humectants in low dews without any problems.  I'm not as familiar with high dews (I see few of them), but I would guess there are people who can use humectants in high dews without issues as well.
The Curl Chemist has a list of humectants at the end of the article.   Honey isn't listed, but it is also a humectant.

Dew Points
Relative Humidity is the percentage of water in the air.  Dew points are the temperature at which it will rain or snow.  If your temperature is 70 and your dew point is 50, when the temperature drops to 50, it rains.  Again, the links explain this stuff much better.   Dew points are what you want to pay attention to for your hair.  You can find what your dew point is on Weather Underground  or go to Naturally Curly's home page and click on the frizz forecast.

Science-y Hair Blog explains humidity and dew points more scientifically.
Seasons are Changing- Dew Point Visual Aid

Low Dew Points
Low dew points may start at under 45 to under 30, depending on your hair.  For my hair, a low dew point is under 30.  Avoid humectants, especially glycerin.  Use as moisturizing products as your hair can handle without being over conditioned.  Use more moisturizing rinse outs, more DTs.  You may need less protein.  Even if you don't normally use a leave in, you may want one in low dews.  You many want to seal your hair with a couple drops of oil will seal the moisture in.  After your leave in conditioner, apply a small amount of oil and style as usual. Buy a humidifier for you house.  Even if you only have a small one for your bedroom, your hair spends 1/3 of its life there. When I have low dew points, I avoid glycerin.  I find I only have to avoid glycerin in things that are left on my hair (leave in conditioner and styling products).  You might need to cowash more and low poo less often.


Science-y Hair Blog on dealing with low dew points.
Autumn to Winter Hair and Humidity

Normal Dew Points
Between 30-45 and 60-70.  Use humectants.  Humectants are your friend in normal dews and will make your hair bouncy and lovely.  My hair's ideal dew point is 65.

High Dew Points
Above 60-70.  Avoid humectants, especially glycerin.  Seal your hair with oil to seal the excess moisture out.  You should seal the same way as I described for low dew points.  You will probably need less moisturizing products.  You may not need a leave in conditioner, even if you normally use one.  You may find you need to add more protein into your routine.  You may want to low poo more often and cowash less.

Curl Talk High Dew Point Support Thread
High Dew Point Support- Wavy Edition

This link has the dew points broken down into more categories and more information on what to do at each dew point.  You can find this information at the end of the post.  If you only read one link on this page, read this one.
Pittsburgh Curly- Dew Points
This link has info on anti-humectants (A.K.A stuff for sealing hair)
Curl Chemist- Humidity, Humectants and the Dew Point.