"He is to take the aircraft to an altitude of 27,900ft (8,500m) by late Wednesday, when the sun's rays stop being strong enough to supply the solar cells with e

"He is to take the aircraft to an altitude of 27,900ft (8,500m) by late Wednesday, when the sun's rays stop being strong enough to supply the solar cells with energy." I'm just a bit confused by this. Wouldn't the sun's rays be stronger at higher altitudes because of having to travel through less atmosphere, and therefore less diffraction from the air?