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penetration. In all instances where safety is a factor, the option to penetrate the cloud will be that of the pilot. Each seeding pass through the cloud by the seeding airplan,e will be in a cross-wind direction, and all cloud penetrations by the cloud physics airplane will be along the wind direction. When the Lear Jet is used for seeding, cloud physics measurements will be made on the seeding pass. Occasionally following the seeding pass, the Lear Jet will climb to the -20oC level and make as many cloud penetrations as time perririts along the wind direction to gather cloud physics data, while the Navajo is gathering cloud physics data at the -lOoC level. While the Navajo and Lear Jet are taking measurements at the -lOoC and -20oC levels, respectively, the Aztec will be taking measurements below cloud base. For small clouds, the Aztec will make successive passes along and perpendicular to the mean wind direction. Each pass will be r~de through the center line of the cloud as judged by the pilot. For large clouds, the Aztec will make successive passes along the wind direction, beginning near one edge of the cloud with each successive pass separated by approxi- mately 1.5 km. When the opposite side of the cloud has been reached,the process will be repeated starting on the side of the cloud where the aircraft is located. The pilot of the Aztec will note the height of the base and changes in height of each cloud seeded. When the decision has been reached to terminate measurements immediately under the cloud, the Aztec will make successive passes through the precipitation column at intervals of 300 m (1000 ft) during descent. These passes will be made along the mean wind direction. In those instances when the Aztec is to perform cloud-base seeding, the pilot of the Navajo cloud physics airplane will select the cloud to be seeded and notify the pilot of the Aztec of his choice. The Navajo cloud physics airplane will then sample the cloud at the -lOoC level while the Aztec will sample the cloud at its base. Following sampling of the natural cloud, the Aztec will seed the cloud at its base (seeding pass perpendicular to mean wind direction) without revealing the type of seeding if any (the pilot of the Aztec will be given an envelope before leaving the ground which contains the type of seeding to be performed). After seeding, the Aztec will proceed to make passes below the base of the cloud to gather cloud physics data in the manner described above, while the Navajo cloud physics airplane continues to make passes through the cloud at the -IOoC level. Both airplanes will collect cloud physics data until the cloud dissipates or becomes too intense for further safe penetrations. 25