Why is my hair sometimes quite thick, and at other times seems thin?

Not a phenomenon at all, perfectly natural for the treatment and maintenance of your hair. For instance more than likely it’s currently dry or it’s naturally frizzy but thin when wet. This is especially true for straightened hair.
Now if it’s naturally straight and becomes frizzy without your interference, or vice versa, then I too would like an explanation (unless it’s simply static friction)

Not to quibble overmuch with @HolographicUniverse‘s response, any “occurrence of a thing” is a phenomenon. So the fact that you even have hair is a phenomenon, as well as every incident that involves your hair. They are each a phenomenon, collectively phenomena. With that said, I’m sure your hair is phenomenal, @Shippy.

Hair, like every other natural organic fiber, is susceptible to changes due to humidity, temperature and moisture content. So when your hair is clean and uncoated by hairsprays (or weatherproof hats and other coverings) it can absorb moisture in the air and expand, to a point. When it starts to absorb more moisture than that optimal amount, then it begins to get limp – more limp than it was while dry – and the air spaces between individual hairs collapse, and it feels thin again.

An accumulation of natural and artificial hair oils can have the same effect.

When you add hair sprays and conditioners, then you’ve modified the structure of the individual hairs as well as the “structure” of the entire hairdo, which will also have an effect on its feel to your fingers.

@CWOTUS

Fair point well made, I dont intend on letting it happen again :-)

Thanks guys! Although I have fine blond hair with no frizz it is dead straight. Perhaps the moisture in the air is thickening it. Since I wear no hair aids like hairspray! @CWOTUS Yes it is rather phenomenal loll

I have been cutting it myself for a year perhaps that is why. I never had mane hair before.

My first thought was Humidity.