Normally people think Long Hair for the Winter months and Short Hair for the Summer months.
Here’s why I think they should be reversed. In winter I am wearing a woollen jumper plus a scarf wrapped high up on my neck. Then I squash a beanie on my head. The beanie and the scarf overlap and my hair gets completely caught up under the scarf-beanie combo and rubs on my jumper and by the end of the day I have hair that is so matted it is impossible to comb out the knots. Short hair in winter easily tucks in under beanies and scarves and sits close on the head for added warmth, plus it is quick to dry with the hair-dryer so you don’t get frozen-icicle-hair when you go outside in sub-zero temps.
I think summer is the best time for long hair. It easily ties up away from the neck if you want to cool down, you can wet it throughout the day which is very cooling, it dries easily in the warmth and there is no need for styling (especially if you like the beachy look), it provides sun protection on back of neck when worn loose, and it doesn’t get matted because there’s nothing to rub against.