WMOD00196

• Contract/Permit #: 2003-01
• Sponsor Name: Gunnison County, City of Gunnison, Crested Butte
• Project Name: WM Evaluation Report
• Title: The Development, Conduct and Evaluation of a Cloud Seeding Program for the Gunnison River Basin, CO During the 2003-04 Season
• Date: 8/1/2004
• County: Gunnison
• State: CO
• Weather Modification - Doc Type: Reports

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2.4 Analysis of the Significant Storm Events

Upper-air soundings (National Weather Service, twice daily balloon observations
of temperature, dew-point and winds) from Grand Junction, Colorado (GJT) were used to
determine the 700-mb (approximately 10,000') wind velocity and temperature, as well
the stability of the atmosphere during storm events_ Although Denver is approximately
the same distance away from the project area, the Grand Junction soundings were
primarily used because of the greater similarity of terrain than Denver, which frequently
experiences air masses primarily affecting the Great Plains_ Denver soundings were used
in a few cases when the sounding from Grand Junction was not available, The 700-mb
wind flow is used to represent the generalized transport of potential seeding material, and
700-mb temperature is used to gauge the seedability of the storm events. Silver iodide,
the chemical commonly used to conduct cloud seeding projects, becomes an active ice
nucleant at temperatures colder than -5 oc. Winter cloud seeding research has shown that
ground releases of silver iodide upwind of mountain barriers can rise to heights of
approximately 1000' above the top of the barrier, depending upon atmospheric stability.
Therefore, the 700-mb temperature, in conjunction with the project's mean barrier height,
can be used as an index of whether the silver iodide particles are likely to reach effective
temperatures in a given storm_

Table 2-3 shows an analysis of 700-mb data for these significant storm events by
month, and Table 2-4 breaks the data down by 700-mb (approximately 10,000') wind
direction sectors. Wind roses for the 700-mb level were prepared for individual months
and for the entire season_ These wind roses, provided in Figures 2_1 to 2.8, are plots of
the frequency of the direction and speed of the 700 mb winds. The direction is reported
as that from which the wind is blowing (for example, a 225 0 wind would be a wind
blowing from the southwest). The results of the analysis are summarized in Table 2-3_
The 700-mb wind during these storm events strongly favored a southwesterly direction,
with a direction between southerly and westerly in approximately 64% of the 166
soundings examined. Wind direction was between north and west about 23% of the time,
between north and east in 8% of the soundings, and between south and east in only 5% of

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