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e e Direct detection experiments require measurements of all key physical processes from release of seeding material to snowfall on the ground. Recent instrumentation developments make this practical, using a combination of surface-based systems to directly and remotely monitor clouds and precipitation, and using specially instrumented aircraft that directly measure the seeded cloud volume and adjoining natural cloud. However, as discussed below, careful experimental site selection is also necessary. Comprehensive direct detection seeding experiments are not practical over many mountainous areas with existing instrumentation. 2. SITE SELECTION The choice of a suitable experimental area is a critical factor in the success of direct detection experiments. It is crucial to select an area nractical for both surface sampling within the target, and low-level aircraft monitoring above it. Review of a number of recent investigations reveals that most of the SL W is within 1 km (3300 ft) or less of the mountain surface. Moveover, plume tracking studies have shown the bulk of ground- released AgI is usually within 0.6 kIn (2000 ft) of the surface. It follows that most of the ice crystal nucleation, growth and fallout due to ground-based seeding will be near the mountain surface. It is, therefore, essential that measurements for direct detection experiments be concentrated within this layer. The FAA (Federal Aviation Administration) regulations for aircraft flight within clouds specify that IFR (instrument flight rules) apply, with the aircraft maintaining at least 600 m (2000 ft) vertical separation above the hillhest terrain within 8 km (5 mi) of the flight path. Research aircraft sometimes have been able to operate under special waivers, depending upon the aircraft type, experience of the pilot, the terrain in question and other factors. Special wavers usually have allowed flight down to within 300 m (1000 ft) of the highest terrain, which places them approximately 600 m above the averalle high terrain for relatively flat mountain regions. However, 300 m above highest terrain translates into much greater vertical separation above the average elevation of rugged moun~ regions. Even a few isolated peaks can limit aircraft sampling to altitudes well above both the SLW zone and ground-released seeding plumes. In such situations aircraft measurements are of very limited use in direct detection experiments. Thus, one characteristic of a suitable experimental mountain area is that it have a relatively smooth top with no isolated peaks more that a few hUndred meters above the general terrain. Such mountainous areas often are called plateaus or mesas. Operation of surface instruments is very important in direct detection experiments. Instruments that directlv measure the near-surface environment include icing rate meters for SL W detection, IN and tracer gas detectors, snowfall rate (precipitation) gauges, ice crystal monitoring equipment, wind and temperature sensors and others. Surface instruments that remotely probe the environment include microwave radiometers for SLW and water vapor, radars for cloud structure and chaff plumes, lidars for the base of the SL W cloud and profllers for the vertical wind, temperature and moisture structure. The best location for these and other measurement devices often is on top the mountain barrier within the seeding target area. Some devices, such as precipitation gauges, need 2