ENGINEERING WATER DISTRIBUTION

Cloud seeding: how humans can change the weather

WHAT IS CLOUD SEEDING?

• Cloud seeding is a form of weather modification. It is the process of altering the volume or variety of precipitation that falls from clouds, by dispersing substances into the air that serve as cloud condensation (ice nuclei), which alter the microphysical processes within the cloud.

WHEN DID IT START?

• Modern cloud seeding began in the 1940's.  General Electric (GE) labs in Schenectady, New York made the discovery, through a series of laboratory trials, that flecks of dry ice converted supercooled water droplets  to ice crystals .  Trials in the atmosphere soon followed, and operational and research cloud seeding projects began in the late 1940’s/early 1950’s.

WHERE HAVE THE PROJECTS HAPPENED?

There are two major types of project areas; mountainous areas and crop or range lands. Projects in the mountains are usually conducted to increase winter snowpack. Other projects are conducted directly over crop lands during summertime or drought to provide more precipitation for agricultural crops or range land vegetation. 

WHO SPONSERS THE PROJECTS?

The typical types of sponsors include agricultural organizations, municipalities and hydroelectric utilities. 

WHAT AGENTS ARE USED?

Two different types of seeding agents are used, depending on the temperature of the cloud volume to be treated. The most common agents used in the projects are silver iodide and dry ice (frozen carbon dioxide).  Other agents like liquefied gases (liquid nitrogen for example) can also be used. 

HOW DO THE AGENTS MAKE IT RAIN OR SNOW?

 The combustion of silver iodide acetone solutions or burning pyrotechnic flares produces siler iodide ice nuclei. Both methods produces trillions of microscopic sized particles of silver iodide. Silver iodide nuclei (aerosols) increase the probability of ice crystals forming in a cloud at temperatures of approximately -50 C or lower. The dry ice cools the air down so quickly that either unfrozen cloud droplets at temperatures of -50 C or lower quickly freeze or water vapor molecules become so cold that they fuse together forming very tiny ice crystals.  The conversion of supercooled water to ice results in the release of heat to the cloudy air that can cause the cloud to grow further and last longer.  The ice crystals that form grow at the expense of the surrounding cloud moisture and they fall through the cloud, collecting unfrozen cloud drops and smaller ice particles in the process. They then fall as rain or snow. 

WHAT ARE THE BENEFITS?

Increased precipitation to arid areas

WHAT ARE THE CONCERNS?

It is often ineffective, and there are some concerns over the affect of silver iodied on the environment.