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2. FUNDAMENTALS OF SEEDING WINTER CLOUDS OVER MOUNTAINS Much of the discussion in this report requires an understanding of the basic principles of winter cloud seeding over mountainous terrain. Accordingly, a brief discussion follows. 2.1 Winter Orographic Clouds Winter clouds over mountains are referred to as "orographic clouds" because they are wholly or partially produced by the forced ascent of moist air over rising topography. The tenn "orographic cloud" in a strict sense refers to local clouds whose presence is detennined by the effects of terrain on the passing airflow. However, in this report, a broader interpretation is used which includes all winter clouds over mountains with any potential for precipitation. Such clouds range from small "cap clouds" enveloping the top of a mountain ridge to portions of widespread cloud systems that are enhanced by mountains. The latter systems are chiefly produced by gradual uplift induced by cold fronts or upper troughs and may extend for hundreds of kilometers. However, such systems have an orographic component embedded within them wherever additional uplift is produced by the modification of orography on the airflow. Rangno (1986) refers to such systems as "orographically enhanced." These uplift zones can produce large maxima in liquid water that are important for cloud seeding potential. Orographic clouds are thus defined as those portions of larger cloud systems that have mountain-induced liquid water and precipitation processes operating within them. If cooling of the air lifted over a mountain barrier causes the relative humidity to exceed 100 pct, tiny liquid droplets will condense from the water vapor in the air. These droplets, much like those found in fog, will form a visible cloud. If the cloud is colder than 0 oC, the droplets are supercooled. The droplets are colder than the freezing point of bulk water; but still liquid because of their small sizes. The very low terminal fall velocity of the tiny droplets allows even slight upward motion in the atmosphere to keep them suspended. These supercooled water droplets will not freeze unless they get very cold (around -40 OC), condense on or contact a small foreign particle (an ice nucleus), or freeze onto a larger and faster falling ice particle that collides with them. Natural ice nuclei are often effective in creating ice crystals in supercooled liquid clouds with temperatures colder than about -15 oc, but not at wanner temperatures. This limit is only a crude approximation because of ice crystal producing processes that depend on temperature and the spectra of droplet sizes in the cloud. At times, so called "secondary" ice crystal production mechanisms can lead to high concentrations of natural ice even in shallow, wann clouds. But, in general, the shallow, warmer orographic clouds are less effective precipitation producers than are deeper colder clouds. 2.2 Seeding of Winter Orographic Clouds Cloud seeding involves the enhancement of ice crystals in clouds that contain supercooled droplets even if natural ice particles are present. The most suitable clouds are between 0 oc and about -15 oc, though the colder limit can vary markedly. For precipitation enhancement to occur, a small fraction of the droplets must be converted to ice crystals over a substantial cloud volume. These crystals will grow rapidly into snowflakes if sufficient additional cloud droplets are present because the saturation vapor pressure is higher over water than over ice at temperatures below 0 oc. Consequently, water vapor molecules migrate to the ice crystals, which may result in evaporation of nearby droplets. Furthermore, the growing ice crystals may collide with many cloud droplets because ofthe higher fall velocity of the larger crystals, resulting in additional growth by freezing of the droplets onto the crystals. This 2