Cloud seeding is a technique used in cloud-free flight experiments and research projects. The purpose of this seeding is to “seed” clouds at various altitudes and concentrations in the atmosphere. Generally, it’s done in dry or cold weather with the goal of generating condensation that will help clouds form and grow. This technique can also be used to cold-seed the cumulus or low-pressure air flows over land or water surfaces. Here is an explanation from an article I wrote some time ago about why cloud seeding aircraft are required.
The originally designed cloud seeding aircraft were a Cessna340 fitted with hydraulically driven and hydrophobic dry/cold screens. These dry/cold screens act like a lubricant to minimize aircraft fuel consumption since they do not allow oil droplets to pass through them. The hydraulically operated dry/cold screens are also used for cloud seeding experiments at various airports.
In dry or cold climate conditions, these aircraft can be deployed for cloud seeding operations for periods ranging from one hour to one day. These periods of time vary according to the area and the altitude where the aircraft is deployed. Typically, the longest period of time for these types of deployment procedures is one day since there will still be warm weather where the droplets will freeze before descending to the ground. The longest period of time for these applications is usually around five hours.
In addition to using dry or cold air, cloud seeding aircraft can also make use of glideslopes, which are airflows much like cirrus clouds. These flows are cooler than the dry and low-pressure plume that they are emulating. During a period of warm and dry winds, the moisture from the air column emitted by the aircraft can be captured by the icing surface and drop to the ground as ice crystals. These ice crystals will grow and stick to the underside of cirrus clouds. They will continue to grow into clouds for as long as they are in the air and in a thermally stable environment.
Cloud seeding occurs when the aircraft is spraying granules of fuel into the upper levels of the cloud. Once these tiny particles have been seeded, they will begin to grow until they reach their critical point of attachment with the clouds. Once this point has been reached, they will start to grow together, creating what is known as updrafts. These updrafts are what will cause the meteorologist to state the cloudiness or lack of cloudiness in the area.
When it comes to the actual measurement of the amount of cloudiness or lack of cloudiness in the area, the meteorologist will use a Doppler radar tower to determine it. These towers have the ability to read the Doppler effect of the air rushing through them. The greater the speed of the air, the more pronounced the Doppler effect will be. A good rule of thumb for determining the proper amount of seeding is to apply it to a diameter of 200 feet per minute.
In order to use these methods of precipitation management, the FAA has placed several requirements on operators. Most of these requirements are located on page 4 of the Airport Operations Manual of the FAA. The rules and regulations regarding cloud seeding operations include the following: the aircraft must be certified and ready to spray; the cloud cannot be seen from the ground; the aircraft must be cleared from other airborne aircraft; the concentration of the seeding product should not exceed one-third of an inch, and the target area should not be within one mile of anything that is under cloud cover at any given time. The only exception to the rule on visibility is if a Cessna or helicopter is being used to spray clouds. Cloud seeding is only allowed when low levels of moisture are expected in the target area. When this is the case, the aircraft may not initiate the process unless visibility is a concern.
If you would like to test your abilities in terms of creating clouds, consider forming a little competition by spraying clouds of your own. There are several companies online that offer to help you create clouds to try out. These companies will then determine whether or not your seeding technique is sufficient enough to help you increase precipitation. They will provide you with their findings after analyzing your conditions.