A Radar Climatology of Summertime Convective Clouds in the Black Hills

A radar climatology of the Black Hills has been prepared from data collected over the four summers of 1967–70. The basic data are time-lapse films of radar echoes obtained from the Nike-Ajax radar systems of the Institute of Atmospheric Sciences. Data were taken during the months of June, July and August and during the hours 0900–2000 on days on which cloud seeding operations were conducted somewhere in the area. Every hour on the hour the films were analyzed for echo location, size and characteristics (convective or stratiform clouds). The data were transferred to computer-punched cards. The echo location is related to a “cloud catcher” grid used in other aspects of the Institute's research program. The number of echo occurrences, called echo frequency, is determined for all years, individual years, direction of movement of echoes, time of day, and type of day. The results show double maxima in echo frequency located over the northeastern and southeastern sections of the Hills. The maxima are connected with airflow from the southwest and northwest, respectively, and are produced by upslope motion and convergence of the airflow on the lee side of the Hills combined with slope heating.