An aneroid instrument, the airspeed indicator measures the dynamic pressure of the outside air entering a pitot tube. At sea level, and an atmospheric pressure of 1013.2 mb, and with no wind effect, the airspeed indicated is the true speed of the aircraft relative to the surface. As the aircraft climbs, the air density decreases and the indicated speed will be less than the True Air Speed (TAS). However, when it comes to controlling the aircraft, because the flight characteristics of the aircraft also alter with reduction in atmospheric density, the indicated airspeed is of greater importance than the true airspeed. This is why control speeds (e.g. V1, V2, etc) are given as KIAS (Knots Indicated Airspeed).