When I was 9 I sprained my ankle, and ever since depending on the weather (i think) it cracks. Is this something I should be concerned about?

No. If you develop pain, i would be concerned.
Depends. If you ankle cracks, it just means the fluid is moving due to the laxity of your ankle joint. It can be concerning if you have pain in the ankle. You can wear a ankle brace to reduce play in the ankle. If you have pain then you should get an x-ray to see if you are getting arthritis in the joint.
Quite common. You as well as many other patients will sometimes be able to tell its gonna rain before the news tell you. I'm one of those too.