Why go airborne?
Simply put, the reason is that the winds are stronger and steadier at higher altitudes above terrain, and the further one rises above terrain, the less difference location makes.
This is well known, but it is reassuring to examine some real data. A "wind speed profile" - is a plot of windspeed vs. elevation above terrain averaged over many geographic locations. Fortunately, there are examples of data from the U.S. National Oceanic and Atmospheric Administration (NOAA) being distilled in this way: http://showcase.netins.net/web/wallio/MZW.html