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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 88 updraft boundary when the winds at low altitude relative to the storm motion veer markedly with height. Figure 7.7 Cross section through hailstone. Alternating clear and opaque layers mark growth under different conditions of temperature and rate of accretion of liquid water Photo courtesy of Charles Knight and Nancy Knight, National Center for Atmospheric Research. The properties related to these processesâvorticity, circulation, and angular momentumâare interrelated through considerations of area and distance. Thus the circulation in a region is the average velocity along the closed curve that defines the region times the length of the curve. This is the same as the average component of vorticity normal to the same region times the area of the region. The angular momentum of a particle or air parcel is measured about a reference point as the distance from the point times the velocity normal to the line connecting the particle and reference point. CONCLUDING REMARKS The thunderstorm entity is a result of thermodynamical, microphysical, and electric processes. All processes interact and must be observed contemporaneously in order to be well understood. The summary presented in the foregoing pages represents remarkable progress in understanding, a result of public investment in a focused use of new tools during the past two or three decades. Meteorological radar, artificial satellites, and marvelous new technologies for data processing, computing, and communicating have all been critically important aids and have facilitated impressive new methods for early identification of storms, dissemination of information about their location, movement and intensity, significant reduction in death rates from tornadoes, and marked decline in the rate of weather- related aircraft accidents. Now there are a host of new methods for study of the lightning process. As past is prologue, we confidently expect these new aids to contribute much toward clarification of the function of electric processes in severe-storm evolution and toward diminishing the still significant lightning hazard. ACKNOWLEDGMENT The author thanks Evelyn Horwitz, who typed this paper through several drafts, and Lindsay Murdock, who improved the paper editorially.