My nose sometimes bleeds when i'm outside in really cold air. what does that come from?

Dry, cold air: Dry, cold winter air dries out the tissue of your nasal cavities and that causes the vessels to break and bleed more easily. Try a saline nasal spay several times a day. This may help. Arm & Hammer makes a good one. Should be able to find it in most pharmacies.

Not uncommon: It is possible when the weather is more extreme, as in very cold, that the humidity may be rather low and dry out your mucous membranes, particularly the ones that line your nose. When they dry out, you may be predisposed to nosebleeds. Consider when it's cold utilizing nasal saline rinses, avoiding blowing your nose too vigorously, and humidification for your home (aim for 35-45%). Best of luck.

Not unusual: When the weather is cold and dry the tissue inside the noses can become fragile. A little olive oil just inside the nostrils can be helpful preventing the bleeding.