Atmospheric Doom?

[perpetual motion]

Ok, heres another wild and woolly thought for all the brains out there.
Why is it that warm and cold air will not mix?
What would happen if they did mix? Would this planet turn into one giant Amazon or
into a giant Sierra? And whats with them not mixing, what is that? Is this some kind
of Mother Nature fixation thingy to make the planet habitable!
I can't find any reasonable answers (that I would like to know of) on this so called
internet. Nothing that I have found hasn't been updated since 2013-14. What is this
doesn't anybody use the internet and update their articles anymore!

[oz93666]

Well of course they do mix ... quite effectively , because hot air rises ....

when sunlight hits the surface of the earth , the air close to the surface will get hot and rise up in thermals ... rising columns of warm air ...it's what eagles and hang gliders look for to get a free ride up ...

this rising colomb will mingle and mix with the cooler air it is pushing through ...the central part of the colomb not so much ... it's also transmitting heat to the cooler air by radiation ... finally, many Km up, the rising colomb will run out of steam and petter out , when it's heat has been mostly transmitted to the surrounding air ...

Now suppose hot air was heavier than cool air , the air wouldn't move , temperature at ground level would get unbearably hot , that would be 'atmospheric doom'...