What does it mean when your knees ache when its cold or when its raining?

Knee ache: We hear this all the time... Typically it means you have arthritis in your joints. Many people believe their joints can better predict a cold front or changes in barometric pressure than the weather person. An x-ray of the joint is usually used to diagnose osteoarthritis.

Very real: Slowly falling barometric pressure seems to be the major association. You will notice that in a storm system coming in fast you are fine. Its that front that takes days to come in. There is also fair weather pain from a high pressure area moving away from you. The mechanism is completely unknown, but is a measurable phenomenon.