Geoffrey E. Hill

A Balloonborne Instrument for the Measurement of Vertical Profiles of Supercooled Liquid Water Concentration

Abstract A vibrating wire placed in the humidity duct of a standard U.S. rawinsonde is used to measure vertical profiles of the concentration of supercooled liquid water in clouds. The natural frequency of vibration varies according to the mass of ice accumulated by contact freezing. By monitoring the natural frequency and the airspeed relative to the wire, the supercooled liquid water concentration can be found. Suitable electronics are developed for both individual expendable rawinsonde units and a ground-based receiver so that the vibration frequency can be recorded. Calibration of the frequency change versus mass accumulation is done theoretically along with measurements made in a wind tunnel with supercooled water present. Further verification is found by the use of paint, uniformly sprayed on the “upwind” side of an exposed wire. For exposition of the instrument, a sample sounding is described.

Fog Effect of the Great Salt Lake

Abstract The effect of the Great Salt Lake on the frequency and geographical extent of wintertime fog is analyzed by use of fog reports, precipitation, and temperature records over a 25-yr period, during which time the size of the lake has more then doubled. Fog reports at Salt Lake City on days when precipitation other than snow grains did not occur are analyzed to find a relationship between lake size and fog frequency. While a large winter-to-winter Variability is found, there is also a strong relationship with lake size. To analyze the geographical effect of the lake on fog, the daily range of temperature is first related to the occurrence of persistent fog; it is shown that when fog is present throughout a day the difference between the maximum and minimum temperature will be low. Analysis of the geographical distribution of these persistent fog days shows the strong effect of the lake. Also, the occurrence of persistent fog was much less when the lake was small compared to when the lake was large.

Analysis of Precipitation Augmentation Potential in Winter Orographic Clouds by Use of Aircraft Icing Reports

Abstract Aircraft icing reports from northern Utah and southeastern Idaho along with radiosonde and precipitation data for six winter seasons are utilized in an analysis of precipitation augmentation potential in winter orographic clouds. According to this analysis clouds with top temperatures warmer than a critical value are primarily composed of supercooled water and colder clouds are primarily composed of ice. This critical cloud-top temperature varies from ∼−20°C at zero to 10 m s−1 cross-barrier wind speeds (measured near the altitude of the barrier crest) to about −26°C at cross-barrier wind speeds between 10 and 20 m s−1. Deep convective clouds are excluded from the analysis. It is concluded that the critical cloud-top temperature is governed both by the temperature dependence of active ice nuclei, and the time available for glaciation, which is directly related to the cross-barrier wind speed. For cloud-top temperatures warmer than the critical value, the time available for glaciation is of second...

Dispersion of Airborne-Released Silver Iodide in Winter Orographic Clouds

Abstract Two flight plans were used for a seeding aircraft and a measurement aircraft in the study of silver iodide plume dispersion. An instantaneous line source and a continuous point source plume generation were simulated approximately by the seeding aircraft. Plume interceptions were made about an hour later by another aircraft equipped with an acoustical type ice nucleus counter. Results of the measurements are that both the vertical and horizontal diffusion of silver iodide released from airborne generators in the northern Wasatch Mountains during winter orographic storms are much lower than that desired for effective seeding. Plume-edge outward velocities are only about 0.5–1 m s−1 in the horizontal and about 0.1 m s−1 in the vertical directions, respectively.

Seedability of Winter Orographic Clouds

This article is a review of work on the subject of seedability of winter orographic clouds for increasing precipitation. Various aspects of seedability are examined in the review, including definitions, distribution of supercooled liquid water, related meteorological factors, relationship of supercooled liquid water to storm stage, factors governing seedability, and the use of seeding criteria.