What Causes Shooting Pain In Knees On Cold Days?

Almost every morning (unless it's very warm) around 5 am, I'm awoken by shooting pain in my knees, sometimes if I wrap a extra blanket around them the pain calms but most of the time I have to get up, it's much worse in Winter.
The pain does decrease to a an occasional pang once I'm up, more frequent if it's a cold day, .