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very small solid or liquid particles, dispersed in air.

see silver iodide.

the precipitation process through which ice formation and growth lead to precipitation development. This process is very common in continental clouds comprised largely of large numbers of very small cloud droplets, which result in an inefficient collision-coalescence precipitation process.

(CO₂) - Produced by the oxidation of organic material including fossil fuels and as a by-product of respiration (breathing), CO₂ is a strong greenhouse gas and has important radiative effects.

(CO) - a colorless, odorless, very toxic gas, it is the intermediate by-product in the oxidation of organic material. It is found everywhere in the atmosphere because of its widely distributed sources and fairly long lifetime. Carbon monoxide is oxidized to carbon dioxide by the hydroxyl radical (OH) and is a common player in local and regional air quality problems.

see cloud condensation nuclei.

hygroscopic aerosols that can serve as the nuclei of atmospheric cloud droplets, that is, particles on which water vapor condenses. Abbreviated CCN.

(CCNC) - an instrument for the measurement of cloud condensation nuclei (CCN). Known supersaturations are created under carefully controlled conditions, and the resulting number of cloud droplets are measured.

the microscale study of processes that govern all cloud and precipitation processes.

the process through which cloud and precipitation development is influenced by the measured introduction of agents that can affect cloud and precipitation processes. The agents are generally designed to encourage the initial growth of cloud droplets or the development and growth of cloud ice.

see carbon monoxide.

see carbon dioxide.

see Bergeron process.

those having at least portions in which temperatures are cold enough to support the development of cloud ice. Such clouds may be suitable for glaciogenic seeding.

also called the warm rain process, collision-coalescence is the process through with cloud droplets collide and coalesce, eventually producing drizzle and/or rain drops, drops that may be large enough to precipitate and reach the surface.

the process through which water vapor, an invisible gas, becomes liquid cloud droplets. The opposite of evaporation.

an aerosol particle forming a center for condensation of water at extremely high supersaturations, on the order of 400%.

clouds of vertical development, resulting from atmospheric instability. Most common during the warm seasons (spring and summer), but possible in many regions in all seasons. Precipitation from convective clouds is typically intense and of short duration. Lightning, hail, and tornadoes are produced by convective clouds. All classes of cumulus clouds are convective.

clouds of vertical development, resulting from atmospheric instability. This family of clouds includes the cumulonimbus, a.k.a. the thunderstorm.

towering cumulus clouds having white, cauliflower-like tops when illuminated by full sunlight, and a height typically three times the cloud width measured at cloud base. Cumulus congestus often evolve into cumulonimbus with continued growth.

the process through which water vapor, an invisible gas, becomes ice, a solid. The well-known symmetrical six-sided ice crystals result from this process. Deposition is the opposite of sublimation.

the temperature to which a given air parcel (volume) must be cooled (at constant pressure) and constant water vapor content for saturation to occur. Condensation then follows.

(sometimes popularly called mist) – very small and numerous water droplets that may appear to float while following air currents. Unlike fog or cloud droplets, drizzle droplets fall to the ground. Drizzle drops are by convention considered to be smaller that 0.5 mm (0.02 inch) diameter.

the numbers of cloud droplets of varying sizes that comprise the liquid portion of any cloud.

(also called vaporization) the physical process through which a liquid is transformed into the gaseous state. For water this is the conversion of liquid water to water vapor, invisible but sensed as humidity. The opposite of condensation.

a cloud present at the ground. There are three kinds of fogs: the ice fog, comprised of very tiny ice crystals that are too small to quickly precipitate; the "warm" water fog, comprised of cloud droplets but at temperatures too warm for ice development; and the supercooled fog, comprised of water droplets at temperatures cold enough to permit ice development.

the process through which liquid cloud water become ice, a solid.

see greenhouse gases.

the treatment of a cloud with the intention of initiating or stimulating the development of ice-phase precipitation.

(GHGs) – those gases, such as water vapor, carbon dioxide, ozone, methane, nitrous oxide, and chlorofluorocarbons that are fairly transparent to short wavelengths of solar radiation but efficient at absorbing the longer wavelengths of the infrared radiation emitted by the earth and atmosphere.

frozen precipitation, usually irregular in form, composed of solid ice and having a diameter of at least 5 mm (0.20 inch).

a measure of the water vapor content of the atmosphere. Most commonly humidity refers to the relative humidity, or RH, the ratio of the amount of water vapor actually in the air to the maximum the atmosphere can hold at a given temperature and pressure, usually expressed as a percentage. At relative humidities even fractionally greater than 100% condensation begins to occur.

any water particle (droplet or ice) in the atmosphere. This term includes both precipitation and sub-precipitation sized particles and drops/droplets.

having a marked ability to accelerate the condensation of water vapor.

the treatment of a cloud with the intention of stimulating the development of larger cloud droplets, which can lead to the activation of the warm cloud or collision-coalescence precipitation process.

the solid, crystalline form of water (H₂O). Ice is the lowest energy state for water (compare liquid water and water vapor). At normal atmospheric pressure ice can exist at temperatures up to 32°F or 0°C.

any particle that serves as a nucleus leading to the formation of ice crystals. Abbreviated IN.

an instrument designed to count ice nuclei (IN), commonly by placing the IN-laden air within a supercooled cloud environment and measuring the ice formation that results.

see ice nucleus.

Latin, meaning, in place. In situ measurements are made at the location of interest, that is, in situ cloud measurements are made in the cloud. Compare remote sensing.

water (H₂O) in liquid form. Each gram of liquid water contains 80 calories of latent heat, stored in the liquid during melting from ice. This energy is released to the atmosphere during freezing. This heat release contributes significantly to the energy of storm systems. Liquid water is the middle energy state for water; compare ice and water vapor. See also supercooling, supercooled water.

the process through which a solid becomes liquid. Ice melts at temperatures greater than 32°F or 0°C. Melting is the opposite of freezing.

pertaining to atmospheric phenomena having a horizontal scale of a few miles (kilometers) up to perhaps a hundred miles or more. Mesoscale features include thunderstorms, squall lines, precipitation bands, and topographically-induced weather such as mountain waves and land breezes.

an instrument that passively measures water vapor and liquid water in the atmosphere. Energy emitted at microwave wavelengths (specific wavelengths between 1 mm and 1 m) by atmospheric water vapor and liquid water is measured and integrated to provide total water vapor and liquid water along the direction (path) the antenna is pointed.

see drizzle.

(NO_x) - a family of compounds in which nitrogen (N) is bound to oxygen (O). The most abundant of these is N₂O, which is relatively unreactive. Nitric oxide (NO) and nitrogen dioxide (NO₂) are highly reactive and of concern for air quality reasons.

see nitrous oxide.

the initiation of a phase change of a substance to a lower thermodynamic energy state. Examples: vapor to liquid: condensation; vapor to ice: deposition; liquid to ice: freezing.

see ozone.

stratiform clouds formed over mountains by the forced lifting of moist air as it rises to pass over the blocking mountain. Such clouds account for the majority of the cold season precipitation (snow, or rain at latitudes closer to the equator). Cloud water and ice that fails to grow large enough to precipitate while over the mountain evaporates and sublimates when the air sinks into lee-side valleys.

(O₃) - a nearly colorless gas having an odor like that of weak chlorine. It is formed in the reaction between of atomic oxygen (O) and molecular oxygen (O₂). It is a very strong absorber of ultraviolet radiation. It is produced by photochemical reactions. In the troposphere it is regarded as a pollutant as it can result in respiratory distress and crop damage. In the stratosphere it is beneficial, blocking harmful ultraviolet radiation.

the tracking or detection of aerosol or gas plumes by using instruments to detect the aerosol or gas itself, or a surrogate aerosol/gas co-released with the gas/aerosol of interest. Plume tracing is commonly conducted to confirm how and/or where aerosols/gases move from their sources.

precipitation in the form of liquid water drops larger than 0.5 mm (0.02 inch) diameter. Compare drizzle.

balloon-borne instrument to measure vertical profiles of temperature, dew point, winds, and pressure.

instrumentation designed to make measurements from afar. Common examples include radar and satellite imagery.

see sulfur hexafluoride.

(AgI) - the most common compound used for glaciogenic (ice-forming) cloud seeding. The shape of silver iodide crystals is very similar to that of water ice, and it therefore functions as an ice nucleus at temperatures as warm as -4 or -5°C (+25 or +23°F).

particulate matter in the atmosphere resulting from the combustion process.

see sulfurous oxide.

a vertical profile of atmospheric temperature and pressure, often obtained by balloon-borne instrumentation. Wind measurements are often obtained concurrently.

clouds in layers. Stratiform clouds often produce precipitation in maritime situations, over mountainous terrain, and in association with synoptic-scale low pressure systems.

the region of the atmosphere extending from the top of the troposphere (10 to 17 km, depending upon latitude) upwards to the mesosphere, roughly 50 km above the earth’s surface. Ozone is beneficial within this layer.

the process through which a solid becomes a gas (vapor), without melting. For water, the solid (ice) becomes water vapor, invisible but sensed as humidity. Sublimation is the opposite of deposition.

(SO₂) - Sulfur dioxide (SO₂) is formed in the combustion of sulfur-laded fossil fuels. In the atmosphere it can combine with oxygen to form sulfurous acid (SO₃), the primary cause of acid rain. See also sulfurous oxide.

(SF₆) - an extremely unreactive gas that is entirely of anthropogenic origin. It is commonly used as a chemical tracer in tropospheric field experiments.

(SO_x) - a family of gases in which sulfur (S) is bound to oxygen (O). Of these, greatest interest/concern is produced by sulfur dioxide (SO₂, above).

water that remains in liquid form at temperatures colder than 0°C (+32°F). This extremely common occurrence in the atmosphere results from a lack of ice nuclei that activate (form ice) at the temperature at which the supercooling is observed. See also nucleation. Supercooled water is a required ingredient for the formation of hail, and is also responsible for all aircraft icing.

the condition existing in a given portion of the atmosphere when the relative humidity is greater than 100%, that is, when it contains more water vapor than is needed to produce saturation. Humid air, purified of all foreign nuclei, can be expanded in cloud chambers to relative humidities on the order of 400% without any condensation taking place.

a specific scale of atmospheric motion that includes features having a scale of several hundred miles (kilometers). Weather features commonly included in this scale of motions and processes include high and low pressure centers, fronts, and tropical cyclones.

any material tracked from one location to another to reveal information about atmospheric transport or dispersion. Tracers can be pre-existing materials such as exhausted gases, or materials released specifically to be tracked such as sulfur hexafluoride (SF₆) or ice nuclei.

that portion of the atmosphere from the earth's surface to the tropopause; that is, the lowest 10-20 km (6-12 mi) of the atmosphere; the portion of the atmosphere where most weather occurs.

general size and/or numbers are observed and recorded, but information about the composition is not measured.

a cloud that has not cooled sufficiently to allow the development of cloud ice.

see collision-coalescence.

water (H₂O) in the gaseous phase, the highest energy state (compare liquid water and ice). Each gram of water vapor contains 680 calories of latent heat, 80 stored in the liquid during melting from ice, and another 600 during the evaporation from liquid to water vapor. This energy is released back into the atmosphere, 600 calories per gram during condensation, and another 80 during freezing. If the water vapor skips the condensation phase and goes directly to ice (see deposition), all 680 calories per gram are liberated at once! This heat release contributes significantly to the energy of storm systems. Water vapor is the highest energy state for water; compare ice and liquid water.

weather modification may refer to any process deliberate or inadvertent, that affects the weather. Planned (deliberate) weather modification is most often designed to affect precipitation, and is accomplished through cloud seeding. Inadvertent weather modification commonly results from gases and aerosols added to the atmosphere through volcanism, fires, industrial processes, the combustion of fuels, land use changes (especially rural to urban) and other human activities. See also cloud seeding.