The following weather and cloud conditions should exist in order to initiate or continue cloud seeding operations in any area where DRI has a seeding project. Operations can also be initiated based on a 0-3 h forecast of these conditions existing in an operational area. Seeding suspension criteria will always override seeding operations criteria.
1. Cloudiness of sufficient areal extent to cover at least 50% of the intended target area. Verification is by means of GOES visible or infrared satellite images and NEXRAD radar images.
2. Clouds of sufficient depth, with cloud bases at least as low as the highest mountain peaks, to provide the potential for precipitation over the target areas. Verification of these conditions can be obtained by one or more of the following:
- NWS hourly reports of cloud conditions and precipitation from reporting sites within a cloud seeding target area.
- Visual observations and/or reports of cloud conditions by the project meteorologist, other project staff, contacts in any of the project areas, or internet links to webcams with views of target areas.
- Observation of precipitation from any automatic recording gauge whose data are telemetered by telephone modem or the Internet to DRI. The DRI has access to many such gages in the Sierra Nevada and other mountain ranges throughout the western U.S.
- WSR-88D (NEXRAD) radar images showing echo coverage of any DRI project area.
3. Wind directions that are conducive to transporting seeding material toward the target areas. This criterion will vary by area and will be verified by data from upper air soundings, NEXRAD radar wind measurements, mountain top weather stations, and wind data from instruments at seeding sites. Examples of wind direction criterion for specific ongoing projects appear below:
- Truckee-Tahoe area: Wind direction at 700 mb (~10,000 feet), as measured by the Reno NWS soundings or measured by weather stations close to the 10,000 ft altitude level, should be from 180 degrees clockwise through 330 degrees.
- Ruby Mountains: Wind directions in the cloud layer should range from 190 degrees clockwise through 330 degrees as verified by the NWS Elko upper air sounding, the Elko NEXRAD radar, or remote weather station data.
- Tuscarora area: Wind directions in the cloud layer should range from 120 degrees clockwise through 270 degrees as verified by the NWS Elko upper air sounding, the Elko NEXRAD radar, or remote weather station data.
4. Wind speeds at or near 700 mb should not exceed 30 m s-1 (~60 kts) to ensure that adequate time is available for growth and fallout of seeded ice crystals onto the target area. Mountain top weather stations, NWS upper air soundings, and NEXRAD radars will provide verification of wind speed.
5. The existence of supercooled liquid water in clouds is a condition necessary for successful cloud seeding. This quantity is not routinely measured over all the target areas, but the observation of icing at a mountain top site, or the observation of liquid water from one of DRI's microwave radiometers will be given strong consideration in the decision to initiate a seeding operation in any area where these data are available. When available these data will also be used in postseason evaluations of seeding operations.
6. To increase the likelihood of ice crystal formation by AgI seeding aerosols from ground generators, the temperature near mountain top level should be -5o C, or colder, as verified by data from mountain top weather stations, or upper air soundings. Operations may be initiated at a temperature as warm as -3°C, provided the -5°C threshold is forecast to be met within 0 to 3 hours.
7. For aircraft seeding winds can have a wide range of wind directions. The airborne seeding contractor, in coordination with the DRI meteorologist, will determine suitable wind conditions based on radar observations, soundings, or NWS upper air charts. Flight levels will be selected to ensure that seeding material is released at temperatures colder than -5o C. The presence of supercooled liquid water must be verified for aircraft seeding operations to be initiated or continued.
The project meteorologist is responsible for forecasting and verifying seedable conditions, and also for initiating and terminating operations. Logs documenting the weather conditions during an operation will be kept on file at DRI by the meteorologist or other staff members.