Because cold temperatures cause the blood vessels to constrict, especially in the extremities. This causes poor circulation as the blood becomes ';sluggish'; and ';builds up'; in the hand.
Hope this helps,
Have a Merry Christmas!
To all the non-physicians down there...
Raynaud's Syndrome is extremely rare. Also, the asker never mentioned pain, which is a symptom of Raynaud's Syndrome. So before you just google, answer...think. This is someone's health. Don't scare people to death by suggesting the most horrific answer possible. Leave it to professionals.
To the asker:
If you believe this to be a serious condition see your physician immediately.Why do my hands turn dark and nails blue in cold?
That sounds like the beginning of frost bite to me.
When you're in a cold environment, the blood vessels in your extremities (fingers, toes, hands, etc.) constrict to direct the blood to your core. A cold core body temperature means death pretty quickly, this is a natural survival technique. The first thing you might notice is ';blanching'; where your fingers and toes, starting from the tips, begin to turn white. This is because you don't have the red blood there anymore. Then they turn blue, this is because the little bit of blood that was trapped is deoxygenated. Then black as necrosis sets in.
To warm your hands, run COLD water from the tap, it should feel pretty warm, then gradually increase the temperature until they are back to normal.Why do my hands turn dark and nails blue in cold?
Consult your physician, you may have something called Raynauds Phenomenon. Check out this website:
There is something called Raynauds Syndrome.