At normal operating conditions of a wind turbine, the flow can be assumed to subsonic and consequently, the flow can be considered incompressible. Thus, air density is considered a constant. Conservation of mass reduces to the conservation of volumetric flux v_in*A_in=v_out*A_out . Aerodynamic forces are proportional to rho*v_in^2 (air density*square of reference velocity, in this case, the inlet velocity), thus denser air implies higher thrust on blades and higher wind turbine output. The standard model of the atmosphere tells us the air density decreases (quite fast) with altitude, thus it would be better to place wind turbines at sea level. However, stronger winds are more common (and constant) at higher altitudes. As the aerodynamic force is directly proportional to the square of the velocity, the gain is higher than the loss due to lower density. On the other hand, at sea level strong(er) and constant winds can be more commonly found at sea. Thus, offshore wind turbines: double gain from higher air density and stronger winds.