Answer: Water Vapor
Explanation:
Water Vapor is the third most abundant gas in the atmosphere but shows the greatest variation because in some places it could be so low that it is practically 0% but in other areas it can get up to 4%.
This is because there are areas where it is more conducive for water vapor to exist and places it isn't. For instance, there will be a lot of water vapor in tropical climates where rainfall is abundant as opposed to deserts where temperatures are too high and rainfall too low for rain to come.
