WMOD00057

• Contract/Permit #: #90-3
• Applicant: Western Kansas Groundwater
• Project Name: Kansas Weather Modification
• Date: 9/13/1990
• Weather Modification - Doc Type: Report

l

Within a cloud that has some part colder than freezingl there
may be some water droplets still in the liquid state; th~se are
termedH supercooled" droplets., It is in the supercooled part of the
cloud that a suspended mixture of both supercooled w~ter droplets
and ice crystals can be found. Often there may be only supercooled
water droplets. The supercooled cloud volume 1s most important in
creating rain and hail. Supercooled water can remain unfrozen as
low as -40C (-40 F) before spontaneously changing to lcej when it
happens in this manner, it is called homogeneous nucleation.

I

I
I

Supercooled water droplets containing ice nuclei begin
freezing first. The speed of droplets converting from supercooled
water into ice gener~lly increases as cloud temperatures decrease
<typically, when clouds grow in heigh~) and the physical properties
of ice and water start interacting. An important process known as
vapor deposition begins having a significant effect within clouds
when ice crystals and 6upercooled water exist in the same
environment. Since surface pressures over ice crystals are lower
than those over water droplets, the pressure gradient between them
causes water molecules to flow from the droplets to the ice
crystals. Ice crystals then grow larger at the expense of water
droplets, eventually using up all the supercooled water and
producing larger ice crystals. Likewise, after ice crystals develop
within a supercooled claud they continue growing rapidly using up
surrounding water vapor and cloud water from nearby supercooled
water droplets. Inside growing clouds the unequal continuous
JllOvement of water droplets and ice particles insure inevitable
collisions. These collisions soon create a sort of chain reaction
process. called coalescence. in which the unfrozen water droplets
collect ather water droplets and eventually freeze or they may grow
rapidly and freeze upon impact with cloud ice particles in a
process called accretion. The ice particles may also shatter, grow
larger and again collide in a complex repeating manner. Eventually,
the various sizes of ice particles fallout of cloud suspension,
warm. up and melt while falling to the ground. If melting isn't
complete, hail, graupel or snow results instead of rainfall. One
idealized conception of how a complex precipitation process evolves
is shown in Fig. 2 on the next page.

Sizes, concentrations, and chemical properties of all nuclei
present in the atmosphere combine 1:0 play an important role in
determ.ining the efficiency with which a part icular cloud system
can produce precipitation. At any given time there are massive
amounts of water in the atmosphere in the form of water vapor.
Precipitation often will not occur because certain required
conditions far the formation of precipitation are absent.

ihere are two natural cloud types which produce all
precipitation, the" warm cloud" and the" cold claud".

5