I suggest you take a soaring ride in a two seat glider with an experienced glider pilot. Preferably during a day with favourable, thermal, weather conditions. In comparable regions, eg. Namibia, South Africa, USA, Australia similar heights will easily be obtained on a good day in a similar flight pattern. (A modern glider however transits between thermals with a more enjoyable speed) With a bit of luck you encounter some falcons, storks or other feathered friends as well.
See for glider example traces
www.onlinecontest.org and search for above mentioned regions.
My best guess it's the weather making it possible and birds just optimizing their energy use in choosing its flight path. Flight Visibility in my experience is almost always above 20-50km in your mentioned region at altitude. Closer to the tropics the humidity rising up during the day may spoil it a bit. Depending on the rain season it usually clears soon after a good shower.