Information sought for flight conditions in the Sahel or Sahara
 

My colleague at the university of Amsterdam has successfully tracked Eleanora's falcons on their migration through Africa and has reveled some curious flight behavior.

Eleanora’s falcons are raptors that migrate from Alegranza to Madagascar and back, preferably with soaring flight. During dawn they ascent to altitudes varying from 2000m-5000m AGL (5000m only occurs with profitable winds). We think that there must be a strong reason to ascent and that this has to do with visual orientation.

He is particularly interested to hear from other pilots who have operated in this region to better understand flight conditions en route, inparticular about the sight distances at high altitudes in the tropics and Sahel or Sahara.. Even if you have never flown in Africa, the soaring and ascending flight behavior is fascinating to any airman..please take a look at the attached and feel free to distribute widely.

For an example of a soaring EF use Chrome or Firefox to see:
Migrating Eleanora's Falcon 20130419_11:00-13:30_t1013 - Doarama

Prof. Willem Bouten Computational Geo-Ecology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam Sciencepark 904, 1098 XH Amsterdam, The Netherlands Home - UvA-BiTS

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.

I would guess that raptors navigate by ground features, so must have sight of ground to migrate, and probably can't fly without a horizon as reference (unlike geese, which can navigate above clouds and even fly in zero visibility). I fly a light survey plane mostly in West Africa over the Sahel and southern Sahara. The visibility is mostly determined by Harmattan conditions. At its worst, horizontal visibility (to human eye) can drop to under 1000m, the horizon is blurred or even occasionally sloped, and vertical visibility 3000ft or so. However, our survey camera can still take pictures through thick Harmattan, even when the ground is not visible, because its sensors are sensitive over a wider frequency band than the human eye. I believe most raptors can also see in UV frequencies, so I would imagine that the falcons in your study are able to see ground features more easily than pilots can, and are probably rarely troubled by visibility.

The whole area has very strong thermal activity. If I am flying medium level orthophoto (4000ft agl etc) over rocky areas the thermals are so strong that the camera stabilisation cannot cope and I have to abandon the flight. I can feel thermals easily often at 18000 ft. The falcons would have plenty of opportunity for thermal flying.

However, I suspect the main reason for the changes in migrational altitude are due to the very variable wind patterns at different altitudes. It is not unusual to have the wind blowing in opposite directions as little as 4000ft altitude change. I suggest you download a good GRIB viewer (I highly recommend MeteoEarth for Mac platform) and use it to look at the wind variations with altitude.