A field of turbines spins gently over rolling hills at sunset, converting the kinetic energy of moving air into electricity. Real turbine blade tips can reach speeds of up to 290 km/h even though the hub itself turns slowly at only 10-20 rpm, because the blade tip travels a huge circle every rotation. Engineers target a tip-speed ratio of roughly 6-8 (tip speed divided by wind speed) for peak efficiency. A modern 2 MW turbine's three blades sweep an area comparable to a football field, and the whole farm's output scales with the cube of wind speed — double the wind, and available power increases eightfold.