An airborne wind turbine is a design concept for a wind turbine that is supported in the air without a tower. A tether would be used to transmit energy to the ground. These systems would have the advantage of tapping an almost constant wind and doing so without a set of slip rings or yaw mechanism, without the expense of tower construction. The main disadvantage is that kites and 'helicopters' come down when there is insufficient wind. Also, bad weather conditions, such as lightning and/or thunderstorms, could temporarily suspend use of the machines, probably requiring them to be brought back down to the ground and covered. These schemes require a long power cable and, if the turbine is high enough, an aircraft exclusion zone. As of 2008, no commercial airborne wind turbines are in regular