Fly your kite at sea-level, in 30deg temperature air.
Now go fly it at the same altitude near Palm-Springs in 110deg temperature.
The force of the kite felt on your harness in both places will be the same
The guy flying his kite in Palm-Springs will need to fly his kite "Faster" to get
that same pull, because there's not as much air occupying a given cubic measurement
And here's Why :
Take say a cubic yard of air at 30 degrees caged in a wire mesh cage.
drive out toward palm springs in july with that cage in the back of the truck.
As it heats up the air expands out the sides of the cage. You still have a cage
full of air but there's less of it in a given space now that its hotter.
the hot air inside that cubic yard cage should now weigh less because there's less of it.
In order for your glider plane or kite to fly,
the force of the air hitting it is the weight of the air times the speed the kite hit's the air at.
So Ya gotta fly Faster to achieve the same force on the wing. Drag won't increase, even though
your flying faster
because there's less air in a cubic measurement of aria when it's Hot.
OK -- That's just temperature's affect on air density.
Elevation , and Relative humidity and barometric pressure,(Hg) ---also have an effect on the amount of air molecules that
occupy a given amount of space.
Play with this calculator and maybe you can see the affect that each have on air density :http://wahiduddin.net/calc/calc_da_rh.htm