Every time the cold weather comes my knee swells up with fluid. why? i have a meniscus tear but in summer is doesnt swell

Internal derangement: will cause periodic effusion ( fluid accumulation in the joint space ). I suggest repeat MRI and have your ortho draw off the fluid and send it for analysis.

Vaso-constriction: When it's cold, the veins and surrounding nerves constrict blood vessels in the limbs, to minimize heat loss and help keep warm the core of the body, where vital organs are. But the increased activation of these nerves around joints in response to cold weather might also lead to an increase in the pain a person feels, and cause less fluid/blood flow out of the knee-thus the increased swelling.