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Peter V. Hobbs

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Peter V. Hobbs

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Simultaneous measurements of the concentration of ice particles and ice nuclei in natural clouds have shown that the concentration of ice particles can be several orders of magnitude greater than the concentration of ice nuclei effective at the cloud top temperature. However, the ratio of ice particles to ice nuclei appears to decrease sharply with decreasing cloud top temperature, and approaches a value of unity for cloud top temperatures in the neighborhood of −25C. These results suggest that the freezing of supercooled droplets is responsible for the multiplication of the number of ice particles in clouds.

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Peter V. Hobbs

Wherein is revealed the trials, tribulations, and triumphs of operating an aircraft for atmospheric research at a university over the past 20 years.

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Peter V. Hobbs

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Peter V. Hobbs

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Three case studies are described in which detailed airborne and ground observations were made in order to evaluate the effects of artificial seeding from the air on cloud structure and snowfall on the ground in a small predetermined target area in the Cascade Mountains.

On 31 January 1972, stratocumulus clouds just west of the Cascade crest were heavily seeded with silver iodide for 72 min. Airborne observations showed that the clouds were glaciated by the artificial seeding, new crystal types appeared, and there was an increase in convective activity. Particle trajectory analysis showed that snowfall in the target area should have been affected by the seeding. On the ground in the target area near the Cascade crest new snow crystal types appeared, riming decreased, freezing nuclei in the snowfall increased, and the snowfall rate decreased during the predicted period-of-effect (PPE) of seeding. Further east in the target area, new crystal types appeared, freezing nuclei in the snowfall increased, and the only precipitation of the day fell during the PPE.

Stratocumulus clouds over the Cascades were heavily seeded with silver iodide and Dry Ice for 77 min on 19 January 1973. Clouds, glaciated by the seeding, were tracked over the target area from the aircraft. A 2.5-fold and 10-fold increase in precipitation occurred in the target area at Snoqualmie Pass and Kachess Dam, respectively, during the PPE. At the same time the amounts of silver in the snowfall increased. At Kachess Dam graupel particles were replaced by aggregates of dendrites and stellars and double crystals during the PPE. The increase in precipitation at Kachess Dam during the PPE was caused mainly by an increase in the number concentration of ice particles rather than an increase in their average mass.

Twenty-six cumulus clouds over the Cascades were seeded with silver iodide on 27 March 1973. There were sharp increases in the ice content of these clouds and snow fell from them over the target area during the PPE causing snow showers at Snoqualmie Pass and Hyak. At Snoqualmie Pass graupel particles were replaced by aggregates of unrimed stellars and dendrites during the PPE. At Hyak, snow which fell during the PPE contained 100 times more silver than normal. The concentrations of freezing nuclei in the snow increased at both stations during the PPE.

These detailed physical evaluations support predictions, based on observations of the natural clouds and precipitation and theoretical computations, that snowfall across the Cascade Mountains can be redistributed and increased by artificial seeding.

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Peter V. Hobbs

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This is the first of three papers describing field investigations, carried out from 1969 to 1974, of winter clouds and precipitation in the Cascade Mountains of Washington State, and physical evaluations of their modification by artificial seeding. The present paper describes airborne and ground observations of the natural clouds and precipitation.It has been observed that (i) in pre-frontal conditions ice particles dominate over water droplets above the −10°C level, but the ratio of ice to water is lower in post-frontal conditions; (ii) the passage of an occluded or warm front causes a sharp lowering of the diffusional growth layers of the ice crystals, so that particles reaching the ground change from unrimed crystals which grow at low temperatures to rimed crystals which form at higher temperatures; (iii) the maximum ice particle concentrations in the clouds are often several orders of magnitude greater than measurements of ice nuclei would suggest; (iv) the growth of precipitation particles by riming and aggregation is particularly rapid in the last kilometer of fall; (v) snow particles reaching the ground originate 10 to 100 km upwind; and (vi) on the western slopes of the Cascades the degree of riming and precipitation rates increase with increasing wind speed and water vapor content at 3 km, but this is not the case on the eastern slopes of the Cascades.

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Peter V. Hobbs
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Peter V. Hobbs

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Peter V. Hobbs

It is now thirty-four years since the 1st International Conference on Cloud Physics was held in Switzerland. The 1988 conference marks the tenth in this series of conferences. On this occasion it is perhaps appropriate to glance back to our origins, to reflect on the current situation, and to look to the future.

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