At present there is no means of measuring the external temperature directly from a fast aircraft. This temperature is calculated by using the formula To = Ts/(1 + 0·2M2), using the mach number M given by the static and dynamic measurements of pressures, and the ram temperature Ts, measured by a resistance or thermocouple element. At high altitudes and when M is high, precision ranges from 1° to 4°, and the time constant from 3 to 4 seconds.