Okay, it's hard because everyone lives in a different places, so in different climates.
The 2 thing I know and which is logical (for me
):
during winter, humidity level in air can be low(depends on where you live) so you have to use an humectant for catch/capture it.
during summer, humidity level is high, so you don't
need a humectant.
but my hair(4type), are dry naturally, so moisture is
never enough for me
i use a humectant whatever the season we are in. it just my hair doesn't like glycerin, so I use another humectant (there are lots of humectants, sodium lactate, sodium pca, honeyquat, honey(powder form/liquid form, sugar(sucrose, fructose lol) agave nectar, some hydrolysed proteins (rice, wheat, silk) use at 0.5 to 5%)all are with different power, I just use the one which works better for me)
As I said, we live in different climates, and above all in the same country you can have different climates. One thing can help : looking at dew point. I think it's NaturallyCurly who did this great article. depending on dew point you can see if you need to use an humectant or not.
But as I said, I personnaly don't relate to this article, my hair and skin is seriously dry naturally due to my ethnicity, ii'm not gonna check weather if I need to bring moisture on my skin/hair lol.
ETA : Okay I've just found the link of the article which may help you in your request :
http://www.naturallycurly.com/curlreading/curl-products/humidity-humectants-and-the-dew-point