Unfortunately, however, thisunpredictablephenomenonalsocausesturbulence, and means thata solarplanehas toclimbhigh in the air to get the energy-efficientsmooth ride that it needs.
In 2010, the solarplane flew non-stop for 26 hours to demonstrate that the aircraft could soak up enough sunlight to keep it airborne through the night.
But for a solar eternal plane to be exactly that it needs an efficient energy harvesting, conversion and storage system that will work when the Sun has set.
And last year, defence firm Qinetiq flew a solar-powered plane for 54 hours, smashing the official world record for the longest-duration unmanned flight.