More information is needed and this is a maths question.
Well I’m no meteorologist, but it seems to me the closer you get to the equator the denser the air mass becomes. The farther away it gets from the equator, it obviously gets less dense. I do notice though that the purple are is Greenland, northern Russia and Northern America. Which leads me to believe that it’s more mountain land and it’s density is compared to that of cooler air.
