Humidity tends to drop in the winter; the colder air just isn’t capable of holding as much moisture as warmer air can during the summer months. This lack of humidity in turn can dry out areas of your body that like to stay moist — like, for example, your eyes and your nose.

Of course, your body does have ways to combat the dryness — so if you feel like your eyes are always watery or your nose is always runny in the winter? That’s because they are. The dryness causes them to boost production in tears and mucus to compensate, which can result in both of them running like faucets all winter long. Fun.

You know how your fingers swell when it’s hot out? The reverse happens when it’s cold: They shrink. The cause of both occurrences has to do with how your body deals with blood flow in differing temperatures. When it’s hot out, your blood vessels expend in a process called vasodilation; the increased blood flow helps keep you cool, but it also makes your extremities swell. Conversely, when it’s cold out, your blood vessels narrow in what’s called vasoconstriction. When this occurs, your body is trying to redirect blood to your essential organs and heat to your core — and because blood flow to your extremities is consequently restricted, your fingers shrink a bit.

This, by the way, is also why it’s actually not a good idea to drink alcohol when you’re going to be out in the cold, despite it being commonly believed that booze “warms you up.” Alcohol expands you blood vessels, which means the heat that should be going to your core isn’t going to get where it needs to be.

Officially called cold-induced diuresis, one of the side effects of the majority of your blood getting directed towards your essential organs is feeling like you need to pee a lot. Vasoconstriction causes your blood pressure to rise — and in response, your kidneys start to attempt to alleviate the pressure by filtering excess fluid out of you blood in order to reduce its volume. As Arkansas Urology puts it, “All this fluid has to go somewhere, and that’s where the increased urination begins.”

Your heart has to work a lot harder when it’s extremely cold in order to keep you warm — which, of course, means that your heart rate and blood pressure shoot up. This extra strain can put you more at risk for heart attacks, especially if you already have heart conditions or concerns.

If you feel short of breath in the winter, it’s because your lungs literally aren’t getting enough air. Breathing cold air in can cause something called a bronchospasm — a " tightening of the muscles" that line your lungs’ airways, per Healthline — which limits the amount of air that can go in and out of your lungs with each breath. This, in turn, can cause shortness of breath, particularly in people who have asthma, chronic obstructive pulmonary disease (COPD), or other lung and respiratory conditions.

It’s called a cold stimulus headache, and it occurs when something cold — like, for example, frigid winter air — stimulates part of your head. According to physician Dr. Mark Brown, it’s believed that when the trigeminal nerve, which is responsible for most of the sensory information your face, head, mouth, throat, and neck get, comes in contact with something cold, it makes the blood vessels in your brain constrict. This, in turn, can cause you to experience a brief, migraine-like headache.

If it reminds you of the “brain freeze” variety of headache sometimes prompted by eating something cold, like ice cream, there’s a reason for that: Brain freeze-y, ice cream headaches are also cold stimulus headaches, with the part of the head that’s been stimulated being the upper palate of your mouth. The good news, though, is that cold stimulus headaches tend to end pretty quickly; you likely won’t suffer for much more than around five minutes.

So, uh, things I discovered today that are utterly horrifying include the fact that frostbite is literally your body tissue freezing. I feel like on some level, I already knew this, but somehow, seeing it spelled out like that did nothing so much as cause me to resolve never to venture outdoors in cold weather ever again.

Anyway, if you want the gory details, frostbite can hit you in just a couple of minutes or over the course of an hour or two, depending on the temperature, how many precautions you’ve taken, and so on. Initial symptoms include a pins-and-needles feeling, which later progresses to numbness; your skin might also turn red, white, blue-white, or gray-yellow, according to Live Science, as well as get sort of… waxy in texture.

Because your tissue has actually frozen.

No, thank you.

This, however, is not a good thing — it means that hypothermia has set in. And when that happens, your body starts responding to the cold less and less, as Dr. Jeff Schaider of Chicago’ John H. Stroger, Jr. Hospital told NPR in January: Any shivering you’ve been experience will stop; your heart starts working more and more slowly; your blood pressure drops; your thinking gets sluggish and you start acting irrationally; and then, probably, you’ll die.

Don’t let that happen, if you can help it.

It’s always worth brushing up on your cold weather safety information, but especially when you’re deep in the throes of a polar vortex or other such event. To get you started, the National Weather Service has tips here; Consumer Reports has more here; and the Centers for Disease Control and Prevention (CDC) has a whole guide to staying safe in extreme cold weather situations here. Don’t forget your hat and mittens!