Wind turbines use the same principles of physics as airplanes.
Like airplane wings, turbine blades are curved to create two unequal flows of air. The pressure differential between the top and bottom surfaces results in a force, called the aerodynamic lift. Since the wind turbine is stuck to the ground, the lift force determines the rotation speed of the blade. This means that the amount of time a wind turbine produces a given power output is just as important as the level of power output itself. The capacity factor is simply the wind turbine's actual energy output for the year divided by the energy output if the machine operated at its rated power output for the entire year.