WMOD00259

• Applicant: Western Kansas Groundwater Management
• Project Name: Western Kansas Weather Modification Program
• Date: 1/1/1999
• Weather Modification - Doc Type: Report

II. THE PHYSICAL BASIS FOR CLOUD SEEDING

Much is still unknown about how some clouds grow, mature and dissipate over time,
especially very severe storms. During the crop-growing time of year rapidly growing convective
clouds can quickly become severe, producing highly destructive hail and surface winds which
. destroy crops and property, not to mention causing flooding and the occasional tornado. What
follows are simplified explanations of how convective clouds can grow to become severe and of
the generally accepted theory supporting the feasibility of seeding to reduce hail and increase rain.

A convective cloud forms when rising air containing water vapor cools by adiabatic
expansion to a temperature at which condensation occurs forming water droplets. Condensation
begins first upon microscopic aerosols, or particles, called cloud condensation nuclei (CCN).
CCN are relatively abundanfin the atmosphere and includes dust, smoke and salt particles. When
a collection of these water droplets have grown to sufficient size, they become visible as clouds.
In Western Kansas some of the mechanisms causing rising air to form convective clouds are:

(I) surface heating - returning solar radiation to the atmosphere --- warm air rising

(2) advancing cold and warm frontal systems. - forcing air to be lifted over its boundary

(3) relatively cold air in the upper atmosphere sinking into warmer air ahead of it, thereby
causing warm, moist air to be displaced, and forced upward

(4) upslope flow of air which moves moisture from lower altitudes in Eastern Kansas into
the higher altitude areas of Western Kansas and Eastern Colorado

(5) regions of horizontal convergence created by troughing at the earth's surface, or aloft,
forcing air to rise as it is squeezed together

(6) upper level low pressure systems (usually associated with low-level convergence,
sinking cold air and/or upslope effects)

(7) convective scale interaction resulting from thunderstorm outflows digging under
warm, humid air acting much like a mini-cold front forcing air ahead of it to lift
. rapidly --- a very important mechanism once storms mature and collapse.

(8) gravity waves---very small, internal perturbations traveling through the atmosphere,
many of which are created randomly and not always easily detectable in real-time

Other atmospheric aerosols are present in the atmosphere and known as ice nuclei (IN).
These are particles upon which, iffound in condensed water droplets, enhance droplet freezing.
Ice crystals also may form upon IN directly from water vapor. Despite the abundance ofCCN,

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