WMOD00274

• Title: Western Kansas Weather Modification Program 2006 - Final Report
• Prepared By: Walter E. Geiger III
• Date: 10/30/2006
• State: KS
• Weather Modification - Doc Type: Report

precipitate efficiently and is thought to be the most significant reason why large, damaging hail
can form in clouds. Except in the most complex, chaotic and unstable conditions, cloud seeding
is usually able to deliver the needed numbers of IN to correct the imbalances found in most
natural cloud systems.

Clouds can be made up of unfrozen water droplets, ic(: crystals or their combination.
Within a convective cloud having a portion of it colder than freezing, some of the sub-freezing
water droplets remain in a liquid state, termed "supercooled." Convective clouds often create a
condition in which both unfrozen water droplets and ice crystals simultaneously co-exist; in
Western Kansas this is critical to the formation of rain and hail. Super cooled water can remain
unfrozen to as low as -40 C (-40 F) before spontaneously changing to ice. When such
spontaneous freezing occurs, it is homogeneous nucleation.

Supercooled water droplets containing ice nuclei freeze first. The speed at which
supercooled water droplets convert into ice crystals increases as cloud temperature decreases.
That is, as cloud tops grow higher above the freezing level and temperatures decrease within the
cloud, the in-cloud ice crystal production increases. A process called vapor deposition starts to
have a significant effect within clouds when ice crystals and supercooled water exist in the same
medium. Surface pressures over ice crystals are lower than those over water droplets which
create a pressure gradient between them causing liquid to flow from the droplets to the ice
crystals, thereby reducing the cloud water. The ice crystals continue growing rapidly feeding on
the surrounding water vapor and cloud water from nearby water droplets. Continuous unequal
movements of water droplets arId ice particles inside convective clouds ensure random collisions
of ice and water droplets which promote the processes of coalescence, accretion and
aggregation to a greater or lesser extent, all of which increases ice multiplication in clouds:

Coalescence is a process in which the unfrozen water droplets collect other water droplets by
impact, the unfrozen water subsequently freezing upon impact.

Accretion, or riming, occurs when unfrozen water droplets freeze upon impact with cloud ice
particles.

Aggregation is the process in which ice particles collect, or attach to, other ice particles. In
advanced stages of cloud growth ice particles will shatter, coalesce, grow larger arId collide
repetitively in a complex manner through the processes just mentioned.

When the variously-sized ice particles eventually fall out of the cloud, dropping below
the freezing level, they begin melting. If melting is not complete, then hail, graupel (2-5 mm.
size snow pellets) or snow reaches the ground as precipitation instead of rainfall.

The sizes and concentrations of all nuclei present in the atmosphere as well as their
chemical and electrical properties all combine in important ways to determine how efficiently a
cloud system can produce precipitation. Although there is a massive amount of water vapor in
the atmosphere at any time, precipitation won't occur if certain conditions required for the
formation of precipitation are absent. In other words, not every cloud seen overhead is capable
of producing precipitation.

4

.!

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I