An operational definition of a mixed phase environment, as ice and water, from the viewpoint of its role in aircraft icing is approached from a consideration that the physical properties of any accreted mix are necessarily dependent, through the aircraft penetration velocity, on the spatial distribution of such a mix. Aircraft measurement of such an environment depends on a high response (0.1seconds, some10m of flight path) instrument (the T probe) capable of independent measurement of ice and water and thus capable of distinguishing between all water and all ice environments. The physical properties of such a mix are ill understood, yet are critical to their surface flow and ultimate freezing behavior. Such highly resolved measurements are necessary, since although the frequency occurrence of such mixes may be less than ice and water alone, their consequences may be enduring should the aircraft trajectory happen to lie along a frontal or developing hurricane weather system. The ability to simulate such a mix in the laboratory under controlled conditions through a mixed phase wind tunnel yields insight into the physical processes as well as providing calibration for aircraft use.