At its absolute most basic explanation, gravitiy causes wind. Air is, as we all know, not very dense but its density isn't zero. Earth's gravity keeps it from drifting off into space. Uneven temperature sets up a complicated but easily measured circulation patterns affecting the entire Earth, wherein heated air at the equator rises and cooled air at the poles descends, again caused by gravity. Simply stated, winds are the result of the actions of gravity and temperature differentials on the envelope of air surrounding our planet...