The invention resides in apparatus for directly measuring density altitude in an aircraft and includes a housing in communication with ambient conditions of air temperature, pressure and humidity outside the aircraft. A rotor having dynamically balanced blade elements is positioned rotatably within the housing. An electric motor is connected so as to drive the rotor within the housing against the resistance therein of the air under the ambient conditions of temperature, pressure and humidity. An operational characteristic of the motor, such as the current which it draws, is continuously sampled, amplified and changed in shape and applied as the input to a display device which is calibrated to display the input in terms of density altitude.