Imagine I want to make a wind turbine, what should be the best height if techology wasn't any problem to make one, like the Altaeros BAT.

Now I heard that most winds get lessened on the ground due to friction with the Earth obstacles. But higher in the sky the density of the air is much less, so the forces of those high winds are less.

So at what altitudes are the winds the fastest and where do you get the best mixture of fastness and force?