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CGlossary These definitions were generated and modified by the Committee and report reviewers and from the American Meteorological Society glossary, o'er edition (20004; the latter are denoted with an asterisk God. *Accretion: In cloud physics, the growth of an ice l~'drometeor by collision with supercooled cloud drops that freeze wholly or partially upon contact. Aerosol: Suspension of solid or liquid particles in ails or gas (as smokes fog, of mist). *Anthropogenic: Hu~na~-i~duced or resulting from human activities Bin models: Cloud models in which the size distribution of particles is specified over discrete intervals (bins). Blocking (or block design experimentation): Separating experimental units that are known before the experiment to be similar in some way (e.g., the same type of cloud in two different locations, say the windward and leeward side of a mountain, where each location is considered a block); randon~iza''on of experiments there is carried out ire each block. Blocks restrict randomization by accounting for important outside variables (e.g., location) by incorporating those variables into tl~e experimental design. Cloud condensation nuclei (CCN): Particles, either liquid or solid, upon which water vapor condenses and forms cloud drops in the atmosphere. Cloud liquid water: The amount of non-precipitating liquid water in a cloud, usually measured in gm~3. * Cloud seeding: The introduction of agents into a cloud to alter the phase and size distribution of cloud particles for the purpose of modifying its development or increasing its precipitation. The most frequently used agents are silver iodide, granulated solid carbon dioxide (dry ice), and salt. 114

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A PPEIN7DIX C 115 * Coalescence: In cloud physics, the merging of two water drops into a single larger drop after collision. * Cold (supercooled) cloud: A cloud composed of s~pe~^cooled wate'A drops. * Condensation: The physical process by which a vapor becomes a liquid; the opposite of evaporation. Covariates: Measurement of two or more variables against each other over time to see how they vary together. Cross-over: A technique in which the same site is used alternately in a randomized scheme both for experimentation and control to minimize location-specific bias. Deposition: The physical process that occurs in subfreezing air when water vapor changes directly to an ice without Seconding a liquid first; the opposite of sublimation. Double-blind: A type of experiment in which neither the experimenters nor the evaluators know which subjects were treated; this is done to remove all human bias in evaluation. Specifically in weather modification, both the experimenters and the evaluators are unaware of which clouds are being seeded until after the experiment is completed and the results have been evaluated. Dynamic seeding: Seeding to increase a cloud's potential for rainfall by causing it to grow larger and last longer than it would have grown without seeding. Transformation of water droplets to ice crystals is sought to release the latent heat of fusion to enhance buoyancy and invigorate cloud growth. Glaciogenic seeding: Process of enhancing ice content in clouds either by nucleating new crystals or freezing cloud droplets. Ground generators: In weather modification, usually refers to silver iodide smoke generators that are operated from the grouted (as opposed to airborne equipment). * Graupel: Heavily rinsed snow particles, often called snow pellets. Homogeneous nucleation: Nucleation that occurs without the intervention of a pre- existing foreign particle. * Hydrometeor: Any product of condensation or depositions of atmospheric water vapor, whether formed in the free atmosphere or at the Earth's surface; also, any water particle blown by wind from the Earth's surface. Hygroscopic: The ability of condensation nuclei to absorb water and thus to accelerate the condensation of water vapor.

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I16 A PPE.NDLY C Hygroscopic seeding: Process of enhancing water droplet size distribution in clouds by introducing hydroscopic nuclei with the objective of rain enhancement or hail suppression. Mixed-phase cloud: A cloud in which ice particles are intermingled with s'~pe~coole~ water drops. * Negative cloud-to-ground lightning: A lightning Isle or stroke between a cloud and the ground that lowers a negative charge to the ground. * Nowcast: A short-term weather forecast, generally for the next few hours. * Nucleation: The initiation of a phase change of a substance to a lower themodynamic energy state (i.e., vapor to liquid condensation, vapor to solid depositions, or liquid to solid freezing). Nuclei: A particle of any nature upon which, or the location at which, molecules of water or ice accumulate as a result of a phase change to a more condensed state; an agent of 'nucleation. Null hypothesis: The statement being tested in a test of significance which is designed to assess the strength of evidence of a claim; the null hypothesis often is the reverse of what the experimenter believes, put forth to be contradicted by the data. Orographic cloud: A cloud whose form and extent is determined by the disturbing effects of orography (i.e., mountains), which causes lifting and condensation in the passing flow of air. Because these clouds are linked to the terrestrial relict, their location changes very slowly, if at all. Overseeding: Condition in a cloud where an excess of nuclei are available, thereby creating a competition for the available cloud droplets or water vapor, possibly preventing any of them from growing to the appropriate size necessary to reach the ground. * Positive cloud-to-ground lightning: A lightning flash off stroke between a cloud and the ground that lowers a positive charge to the ground. Pre-screening: The removal of some weather or closed conditions for consideration in the design of an experiment before randomization on the balance is made. This is done to focus the experiment on the conditions of interest. Randomization: The use of chance to determine experimental units to minimize the sources of bias on the results. Specifically in weather modification, the design of experiments by dictating that, for example, "seed" or "don't seed" decisions be made purely randomly.

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A PPEA~DIX C 117 Replication: Repeating each experiment on a large enough number of subjects to allow the systematic effects of the experiments to be seen; it reduces the role of chance variation and makes the experiment more sensitive to differences among experiments. Re-randomization: Also known as resampling or Monte-Carlo tests, it is the construction of artificial datasets using a collection of real data on which experiments are rerun with seed and no-seed allocations selected at random. The percentage of such re- randomized seeding effects float exceed the actual real result is the probability of the real result occurring by chance. Riming: The rapid f eezing of supercooled water droplets as they impinge upon an exposed object and accrete to it. Snowpack: The amount of annual accumulation of snow at higher elevations. Static seeding: A strategy for optimum nucleation; exploiting the preexisting situation where less-than-optimal ice crystal concentrations exist, which leads to prolonged periods of supercooled watet, with no attempt to modify the dynamics of the seeded clouds. halt.] Influencing precipitation formation processes by changing the microphysics of the cloud. * Supercooled water: Liquid water at temperatures below the freezing point (0C of 32F). Variance: A measure of the spread about the mean if the mean is a measure of the center of a group of observations; it is defined as the average of the squared deviations of a value from its mean. The variance also is the standard deviation squared. Warm cloud: A cloud composed of liquid water drops at temperatures above the freezing point (OC or 32F).