A remote pilot is planning a flight in a region of rolling hills. The forecast wind at the planned altitude is 20 knots from the west. When operating on the downwind (east) side of a hill, what wind condition should the pilot anticipate?

Wind flowing over a hill behaves like water over a rock. On the windward side the flow is compressed and accelerates smoothly up the slope, producing an updraft. Over the crest it separates, and on the downwind side it cannot follow the terrain down cleanly — it breaks into eddies and rotors, producing a region of reduced mean wind speed with strong, disorganised turbulence embedded in it. That combination is what makes the lee side deceptive for a small unmanned aircraft: the reported wind speed is lower there, which reads as the safer place to operate, while the actual air is unsteady and includes downdrafts and gusts that can exceed the aircraft's control authority. A multirotor holding position in that flow works its motors hard against constantly shifting inputs, which drains the battery faster than planned as well. The stronger the wind and the steeper the terrain, the further downwind the disturbance extends.

Air separating over the crest cannot follow the terrain down cleanly, producing a lee-side wind shadow of reduced mean speed containing eddies, rotors and downdrafts.

Wind flowing over terrain accelerates and produces updrafts on the windward slope, separates at the crest, and forms turbulent lee-side eddies, rotors and downdrafts extending downwind in proportion to wind speed and terrain steepness.

Up the Front, Broken at the Back. The lee side is calmer on average and rougher in reality.

Remember that reduced mean speed and increased turbulence coexist on the lee side. Options offering only one of the two are incomplete.

• •Choosing the lee side because reported wind is lower
• •Expecting smooth flow behind terrain
• •Underestimating how far downwind the disturbance extends