 
Silver Iodide vs. Liquid Propane Seeding
For both airborne and
ground-based seeding, silver iodide has become the dominant
method, while dry ice and liquid propane are less used. The
reason for this situation has several historical and technical roots.
Nevertheless, the release of liquid propane into gaseous form has
several advantages that make it a desirable alternative for ground use,
as discussed below.
Silver Iodide (AgI)
Even with the best chemical solutions available, the air
temperature must be -5°C or colder for AgI to begin forming ice
crystals by artificial nucleation of supercooled liquid water (SLW), and perhaps -8°C
or colder to produce enough crystals to generate significant snowfall.
Since natural ice nuclei dramatically increase at -10°C
or colder, there is a narrow temperature "window" in which AgI can
augment precipitation where nature cannot. This narrow window may
reduce the period for crystals to grow in the proper temperature
and SLW conditions, thereby decreasing the precipitation increase
from seeding. AgI generators
can clog, so AgI-in-solution flow should be monitored to ensure release
of the material, particularly if the generator is remotely
controlled.
Liquid Propane (LP)
Release of liquid propane as a gas from a LP dispenser chills the air to as cold as -100°C. This immediately condenses water
vapor and then freezes it, effectively creating vast
numbers of tiny ice crystals that act
as seeding material. Because of the tremendous local
chilling, LP release can generate ice crystals at temperatures as warm
as -0.5°C, and enough crystals to produce significant precipitation (downwind) at -2°C. This means that there is a 4.5 to 6°C
bigger temperature window of effectiveness for LP versus AgI.
Since water vapor and therefore SLW increases with temperature,
this warm temperature window will usually possess more of the SLW
"fuel" necessary for more crystal growth and resultant precipitation.
This LP technology is used to disperse cold fog at airports and is
consistently successful. LP
dispensers are simpler than AgI generators, in large part because no
chemical solution or flame are required. This simplicity
also makes for greater reliability of operation and lower cost compared
to AgI generators. The propane gas release has low
flammability and is safe for the environment because of rapid oxidative
decay. Propane is colorless and odorless, does not harm plant or animals, and is released in very small quantities (~ 1.9 g/sec).
Implications of these Comparative Facts for Choice of Ground Seeding Method
The main consideration in the choice between LP and AgI is the
greater warm temperature window of opportunity for seeding - see
figure below. There are many mountain ranges that, either because
of relatively low elevations or latitudes, do not frequently experience
temperatures of about -6°C or colder. Examples
of such ranges would be the Sierra Nevada of California and the
Mediterranean Sea region. Moreover, the current period of climate
warming is decreasing the cold temperature episode frequency. So
even locales that currently have frequent cold conditions, such as the
northern/central Rocky Mountains and the Alps, will likely see shifts
toward the warm temperature window. This trend has already been
shown by research that indicate more long-term rainfall than snowfall
at given elevations, and earlier snowmelts in spring. The result
is major negative impacts on water supplies and activities like snow
skiing. Given these trends, its simplicity and lower costs, we
recommend the use of LP dispensers for all but the coldest and highest
(above ~ 3 km) mountains to take full advantage of the "water window." 
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