Measurements of Water-Ice Budget Changes at −5C in AgI-Seeded Tropical Cumulus

R. E. Ruskin U.S. Naval Research Laboratory, Washington, D.C.

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Abstract

During the 1965 Project Stormfury experiments a combination of six cloud physics aircraft instruments were used to measure the changes in the water-ice budget in a large tropical cumulus which had been seeded. Pyrotechnic “Alectos” were used to produce an estimated 180 AgI nuclei per liter effective at −5C in the seeded area. Measurements showed an 80 per cent conversion to ice at the −5C level (17,000-ft pressure altitude) in a 1 km region of the cloud 5 min after seeding, together with a 1.5C rise in temperature. At the same time the adjacent unseeded region of the same cloud decreased in percentage ice at that level, but increased about the same amount in temperature, probably because of increased updraft induced by the heat of fusion energy released in the seeded region.

On two passes 10 min apart 300-m length of “wisp” visible outside the main cloud produced many 10–20 μ replicas of ice particles in air which had a measured 75 to 90 per cent relative humidity, including the moisture from the cloud particles which were vaporized in an instrument which measures the cloud total water content.

Abstract

During the 1965 Project Stormfury experiments a combination of six cloud physics aircraft instruments were used to measure the changes in the water-ice budget in a large tropical cumulus which had been seeded. Pyrotechnic “Alectos” were used to produce an estimated 180 AgI nuclei per liter effective at −5C in the seeded area. Measurements showed an 80 per cent conversion to ice at the −5C level (17,000-ft pressure altitude) in a 1 km region of the cloud 5 min after seeding, together with a 1.5C rise in temperature. At the same time the adjacent unseeded region of the same cloud decreased in percentage ice at that level, but increased about the same amount in temperature, probably because of increased updraft induced by the heat of fusion energy released in the seeded region.

On two passes 10 min apart 300-m length of “wisp” visible outside the main cloud produced many 10–20 μ replicas of ice particles in air which had a measured 75 to 90 per cent relative humidity, including the moisture from the cloud particles which were vaporized in an instrument which measures the cloud total water content.

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