Dew Point, Humectants, Humidity
Disclaimer: While guidelines are helpful to many these posts are only meant to be used as a reference point. This is just information that you can tailor to your own individual needs. As always, you will need to do some experimentation with your hair to find the combination of products that give the results you prefer.
Weather plays a big role in how our hair behaves. What a lot of us may not know is that humidity has little to do with how our hair acts and responds to the climate. In order for our hair to behave at its best (at least for a lot of us), the dew point is the information we need to focus on. Dew points can be a deciding factor in figuring out when to use humectants and when to avoid them (see photo slide above).
Humectants are a very broad category including glycerin, salts, plant gels like aloe vera, algae extracts, hyaluronic acid, hydrolyzed proteins, sodium PCA, lactic acid, witch hazel (without alcohol), and other ingredients that attract water.
With that being said not all humectants are created equal and different humectants behave differently in hair. Most of the problems people have with humectants are with the simple ones like glycerin, propylene glycol, sorbitol, aloe vera, maybe honey, agave nectar and panthenol.
For those who don't know what humectants do, they attract water to themselves and the humectant, glycerin is great at grabbing water vapour out of the air. For example, when you have a hair gel with glycerin in it, when there is ample water in the air, the air is going to be hydrating the glycerin in the product, which will help your hair stay hydrated. Another benefit of glycerin in products (when there is ample water vapor in the air) is that glycerin keeps hold providing ingredients that would otherwise create a brittle like, dried candy like finish from feeling brittle.
Glycerin, sorbitol and propylene glycol take water vapor from the air to hydrate the dry gel in your hair and keeps it more flexible. What happens when there's not enough water vapor in the air (low humidity and/or low dew points), the gel loses that benefit from the glycerin and the gel becomes more brittle, creates friction which means hair will feel dry and look dull.
According to Wendy (hair scientist) from her Science-y blog, whether humectants actually dehydrate the hair or pull water from the hair is not well-studied. And it's an "it all depends" sort of question. It's based on a reasonable hypothesis; that if glycerin attracts water from the air, when the air is drier than your hair, water will move from your hair to the glycerin. For that matter, when the air is less humid than inside of your hair, the water will tend to move from your hair to the air around it, glycerin or not.
So does glycerin create a stronger "pull" than dry air alone? It probably does exert a stronger pull on the water in your hair than dry air alone, but not all humectants do that. Here's a great example, think of glycerin like brown sugar. In humid air, a bag of brown sugar absorbs moisture and forms clumps. But it dries out quickly too and the clumps become hard as a result.
Why must glycerin be so difficult? Glycerin can be a hit or miss for some, especially during the winter months. It can be tricky to get great results with glycerin even with an ideal humidity/dew point. To get to the heart of why glycerin can be a problem ingredient for some people and some weather, one big issue is the size of the molecule. The smaller the molecule, the less water it can bind and hang on to when exposed to very dry air. Glycerin, sorbitol and propylene glycol are "sugar alcohols" which are small molecules. There are not a lot of places on the molecule to bind water. Think of glycerin as a "simple sugar" like candy. Glycerin, sorbitol and propylene glycol aggressively pull water to themselves, but they also lose it easily. Emollients (oils and conditioners) can slow that down, but not stop it. So when the air is dry, glycerin is a much less effective ingredient.
Formulation Can Be a Problem
Often, a problem with a glycerin containing product is that it uses only glycerin for a humectant and "flexibilizer" and doesn't use any emollients or film forming humectants at all. Well balanced products avoid this pitfall. Different humectants have different actions and a combination of different size and molecular weight humectants might be ok for a person who finds that just glycerin and no emollients or film formers is a mess.
There are people who live in climates that are dry and can use glycerin with no problems at all. And there are people who can only use glycerin when the humidity/dew point is "just right". There is no simple rule to determine how your hair will respond because it's not just a porosity issue. It's an issue of climate and weather, what products you use in your hair, how sensitive your hair is to increased friction, how often you go outdoors. It's trial and error as usual. (I know, my eyes were rolling too ).
So let me try to sum this up a bit. In essence, when the air is dry, humectants like glycerin, propylene and sorbitol are small molecules and they hold water when they air isn't dry. But when the air is dry, they have the ability to pull water from your hair. A lot of how strongly and ingredient can pull water from your hair or skin has to do with how much of it is there, how strongly it attracts water to itself and whether or not the formula includes ingredients which slow water loss like oils or conditioners. It's the same principle as salting vegetables before making pickles for example or cooking them otherwise to remove excess water--The salt pulls water to itself and out of the vegetable. Only you use so much salt, you overwhelm the vegetables ability to hold water. Similar in principle, but not entirely similar to glycerin.
It is possible that glycerin, propylene glycol, and sorbitol could pull water from your hair when the indoor/outdoor humidity/dew point drops. That can have a lot of side-effects from causing a styling product to take on a dried candy, tacky feel, causing frizz, making the hair "flash-dry" meaning some locks dry almost immediately or it make your hair feel rough.
Some of this is an interaction of the product with the dry air. Long-term roughness or a dullness or stiffness is probably your hair being dried out by the product contain glycerin, etc. Some people can work around this by using conditioning glycerin containing (those with oil or conditioners included) products or using leave in conditioners when using glycerin containing products or sealing in moisture with oils on damp hair under glycerin containing products. Others just need to avoid glycerin and dry weather if they find it gives their hair problems. Some people's problem with glycerin in dry weather is purely cosmetic and some people's problem with glycerin is dehydration.
The Glycerin Debate
The glycerin debate: there's an ongoing debate about using products that contains glycerin in the winter. Some say it dries out the hair because the air doesn't have enough moisture to pull from.
If you're one of these people, after using a product with glycerin in it, steam is humid air and will act to help glycerin attach water without necessarily wetting your hair. You can take a warm shower without wearing a shower cap for a few minutes before getting out. The moisture from the steam will bind to the glycerin and make your hair strands super moisturized or you can use a handheld steamer for a couple of minutes, but hold it at a distance and very lightly steam your hair, then finish by adding a tiny amount of sealing oil. This will help to act as a barrier to keep water on the surface of your hair from evaporating. Another thing you can do is to cover your hair with a hat, hood or scarf while out in the cold so that it will act as a barrier to the air outside as well.
If you live in a dry climate, glycerin will take moisture from your hair and transfer it into the air instead of taking moisture from the air and transferring it to your hair, that's why it's important to steam your hair before you get out the shower if you live in a dry climate area.
Sources: Dictionary, National Weather Service, CurlyNikki, NaturallyCurly, The Natural Haven Bloom, Science-y blog