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<br />2. FUNDAMENTALS OF SEEDING WINTER CLOUDS OVER MOUNTAINS <br /> <br />An understanding of the fundamentals of winter cloud seeding over mountainous terrain is necessary to <br />much of the discussion in this report. Accordingly, a brief description is given in this section. <br /> <br />2.1 Winter Orographic Clouds <br /> <br />Winter clouds over mountains are often referred to as "orographic clouds" as they are wholly or partially <br />produced by the uplift of moist air Y'hich can occur when airflow is forced over rising topography, The <br />tenn orographic cloud in a strict sense refers to local clouds whose fonn is detennined by the effects of <br />terrain on the passing airflow. However, in this report, a broader interpretation will be used in which all <br />winter clouds over mountains with any potential for precipitation are considered orographic. Such clouds <br />range from small "cap clouds" just above or enveloping the top of a mountain ridge to those portions of <br />widespread cloud systems that are associated with mountains. The latter systems are chiefly produced by <br />gradual uplift from cold fronts or troughs and may extend for hundreds of kilometers. However. such <br />systems have an orographic component embedded within them wherever additional uplift is produced by <br />the disturbance of orography on the airflow. Rangno (1986) refers to such systems as "orographically <br />enhanced." These uplift zones can produce large maxima in liquid water production that are very <br />important for cloud seeding potential. In this report orographic clouds will include those portions oflarger <br />cloud systems that have mountain-induced liquid water and precipitation processes operating within them. <br /> <br />If the associated cooling of the lifted air over a mountain barrier results in the relative humidity exceeding <br />100 percent, tiny liquid droplets will condense from the water vapor in the air. These droplets. much like <br />those found in a fog. will result in a visible cloud. If the cloud is colder than 0 OCt the droplets will be <br />supercooled. that is. colder than the freezing point of bulk water but still liquid due to their small sizes. <br />The tenninal velocity of the tiny droplets is very low so even slight upward motion in the atmosphere can <br />keep them suspended, These supercooled water droplets will not freeze unless they get very cold (around <br />-40 oC). condense on or contact a small foreign particle (an ice nucleus) that can cause them to become <br />a ice crystal. or freeze onto a larger and faster falling ice particle that collides with them. Natural ice <br />nuclei often are effective in creating ice crystals in supercooled liquid clouds colder than about -20 oC <br />but not at wanner temperatures. The -20 oC value is only an approximation as the concentration of <br />natural ice nuclei. effective at any given subfreezing temperature. can vary significantly with space and <br />time. Moreover. processes that are not highly temperature dependent may be producing ice crystals <br />(Hobbs and Rangno. 1988), But. in general. obselVations have indicated that winter orographic clouds <br />with wann top temperatures tend to less effective producers of ice crystals than clouds with cold tops, <br /> <br />2.2 Seeding of Winter Orographic Clouds <br /> <br />Ooud seeding involves the fonnation of ice crystals in clouds that are supercooled but have few natural <br />ice nuclei (natural ice nuclei are typicall)' very tiny clay particles). In general. such clouds are between <br />o and about -20 OCt though the colder limit can vary markedly. In order for precipitation to occur. a <br />small fraction of the droplets must be converted to ice crystals. The crystals will grow rapidly into <br />snowflakes at the expense of surrounding droplets inside a supercooled liquid cloud. That is because the <br />saturation vapor pressure is higher over water than over ice at any temperature below 0 oC. Consequently. <br />molecules of water vapor migrate to the ice crystals. which may result in evaporation of nearby droplets. <br />Furthennore. the growing ice crystals may collide with many cloud droplets due to the higher fall velocity <br />of the larger crystals. resulting in additional growth by freezing of the droplets onto the crystals. This <br />growth process is called accretion or riming and can result in snow pellets (also called soft hail or <br /> <br />3 <br />