WMOD00301 ‎(2)‎

• Title: Conceptual Plan to Develop Water Augmentation by Weather Modification in the Colorado River
• Date: 5/14/1979
• Weather Modification - Doc Type: Report

D R AFT

increased. This can be accomplished by increasing the number of ice
nuclei in the cloud when the number of natural nuclei is insufficient

to convert most of the available water to snow.

Orographic cloud systems (fig. 5), which produce most of the winter

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precipitation, are formed when moist air is forced and lifted over

mountain barriers. In a pure orographic situation, the cloud remains
stationary over the mountain as the air, cloud droplets, and ice

crystals flow through it. Frequently, however, the air is unstable
and convective updrafts form as cells and bands within the cloud.
These convective elements produce precipitation that is not only
extremely variable in both amount and intensity but also responsible
for a significant portion of snowfall that occurs during the winter.

Analysis of orographic storms in the San Juan mountains by Marwitz,
et al. 29/, indicates that a typical storm consists of four phases:
(fig. 6) stable, neutral, unstable, dissipating. Each of these phases
usually last from 6 to 9 hours. The stable phase and the storm often

begin with a stable orographic cloud forming over the mountain
barrier. It is characterized by low liquid water content and a
high natural ice concentration. Precipitation that forms is a result

of diffusional growth of ice and occurs in relatively small amounts.

During the neutral phase the airmass becomes more unstable* giving rise
to increased lifting and consequently more liquid water. The amount

* Instability as used in this discussion is defined to be convective
instability ~~~e~O) rather than conditional instability which is based
upon lifting a surface air parcel.

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