The Nature of Winter Clouds and Precipitation in the Cascade Mountains and their Modification by Artificial Seeding. Part II: Techniques for the Physical Evaluation of Seeding

Abstract

Airborne, ground and radar techniques used for evaluating the effects of artificial seeding on winter clouds and precipitation over the Cascade Mountains are described. The clouds were seeded for 1 or 2 h with silver iodide and/or Dry Ice, dispersed from an aircraft, at locations which particle trajectory analysis, based on field data, indicated would affect precipitation in a small (90 km2) predetermined target area straddling the Cascade crest. The effects of seeding on the clouds were determined from the aircraft through visual observations, ice nuclei measurements, and measurements of the type and concentrations of cloud particles. A Doppler radar located near the Cascade crest was used to measure the spectra of fallspeeds of the precipitation particles. At manned stations within the target area on the ground, measurements and observations were made before, during and after seeding of precipitation rates, the types, concentrations and degrees of riming of snow crystals, and the concentrations of freezing nuclei and silver in the snowfall.The effects of heavy seeding on the clouds were generally pronounced and measurable. Good physical evidence for artificial modifications of snowfall on the ground within the target area was not as common, but was obtained in a number of detailed case studies.