You can buy a space-heater, if they aren't a banned item in your dorms? Also you can buy a window sealing kit for like 3 to 5 dollars at walmart.. It's just like a plastic wrap you tape over the window and you use a heat source like a blow-dryer to shrink it. It's very effective and easy to setup. When it's warm again you can rip it down. If it get cold again, you can always put more back up again, it comes with enough material for multiple windows.
(If you don't want to support Walmart because it is a terribly unethical place destroying individual town culture, identity, and local business competition, you can buy the window-sealing kit at a different store)
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