Abstract: A cylindrical tube is open at both ends but divided into two portions by a barrier having an aperture therein. Side branches are connected to the tube either side of the barrier respectively, the side branches leading to a pressure transducer for deriving an airspeed indication. One open end of the tube is arranged to face into an airflow so that a pressure difference exists between the two portions, the pressure difference being measured by the pressure transducer through side branches. The barrier may have no aperture, the portions then being connected by a restricted passage disposed between the sides of the portions or between the side branches. The outside air temperature may be derived by the provision of a resistive element within the restricted passage.