| OCR Text |
Show to the activation temperature region (criterion 4) is pointed out by St. Amand, et. al. (1968). According to them, dissolution of the seeding material may occur if the material is exposed to temperatures > -4C. They also reported that contact nucleation or condensation followed by freezing are the dominant nucleation processes from -4C to -12. 5C, while the sublimation process takes over below -12. 5C. Thus, it would be helpful to know the climatological frequency of occurrence of exposure temperatures > -4C and between -4C and -12. 5C. To obtain this information, it was necessary to assume moist adiabatic lapse rate from 700 mb to the Mt. Harris altitude (7400 ft. MSL). Also, based partially on diffusion experiments, the altitude to which the Agl reaches was assumed to be 12, 000 ft. MSL. It was also stated previously that for optimum seeding effectiveness, the Agl should he dispersed in the region containing the -15C isotherm. Thus, with the above assumptions and conditions, the first two columns of Table 18 were computed. The frequency of highly seedable conditions (from the Mt. Harris site) is indicated to be quite low during November and April due to relatively warm air, whereas about 40% of the cases should be ground seedable from Mt. Harris from December through March. Airborne seeding could be used for the cases too warm for ground seeding. Since Table 18 used 700 mb temperature, plus the assumption of moist adiabatic temperature stratification, to estimate the climatological frequency of various isother·m s at specified levels, the various temperature limits on seedability could just as easily be delineated by 700 mb temperature (or Rabbit Ears temperature insofar as it approximates 700 mb temperature), and Table 19 shows these limits, along with the targeting limits. An example of how these seedability criteria could be used to stratify data, either ex post facto or in concurrent time (given almost concurrent time analysis of the three-dimensional temperature and cloud patterns), is shown in Figures 218 and 219, and Table 2 0. Critical isotherms depicted on Figure 219 are: -4C, for warm limit of Agl nucleation (dissolution of seeding agent also possible at (T > -4C). (1) (2) -12. 5C, for changeover from contact nucleation (~ -12. 5C) to sublimation nucleation ( < -12. 5C). (3) -15C, for optimum crystal growth rate isotherm. (4) -20C, as lower limit of cloud top temperature for highly productive seeding. 323 |
