GLOSSARY OF IMPORTANT TERMINOLOGY Source: Department of Atmospheric Sciences University of Illinois at Urbana-Champaign (http://covis.atmos.uiuc.edu/guide/clouds/html/glossary.html#eddy) Air Parcel -- An imaginary small body of air that is used to explain the behavior of air. A parcel is large enough to contain a very great number of molecules, but small enough so that the properties assigned to it are approximately uniform throughout. Anvil Cloud -- The spreading of the upper portion of a cumulonimbus cloud that takes on the form of an anvil, typically fiberous or smooth in appearance. Bouyancy -- That property of an object that enables it to float on the surface of a liquid, or as in the case with air parcels, to ascend and remain freely suspended in the atmosphere. Cloud Base -- For a given cloud or cloud layer, it is the lowest level in the atmosphere where cloud particles are visible. Cold Front -- A narrow transition zone separating advancing colder air from retreating warmer air. The air behind a cold front is cooler and typically drier than the air it is replacing. Condensation -- The process by which water vapor becomes a liquid; the opposite of evaporation, which is the conversion of liquid to vapor. Sublimation is the process by which a solid forms directly from vapor. Convection -- In general, mass motions within a fluid resulting in transport and mixing of properties of that fluid. * In Meterology: Convection is referred to as the atmospheric motions that are predominantly vertical, such as rising air currents produced by surface heating. * Forced Convection is the ascent of air induced by some external force. An example of this convection is the lifting of lighter, warmer air by an advancing cold front. Other examples include convection resulting from orographic lifting and convergence. * Free Convection is the rising of heated air and the sinking of cooler air without the need of external forces. Convergence -- An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region. Divergence is the opposite, where winds cause a horizontal net outflow of air from a specified region. Eddy -- A small volume of air that behaves differently from the predominant flow of the layer in which it exists, seemingly having a life of its own. An example of such would be a tornado, which has its own distinct rotation, but is different than the large-scale flow of air surrounding the thunderstorm in which the tornado is born. Evaporation -- The physical process by which a liquid is transformed into a gaseous state. Front -- The transition zone between two distinct air masses. These air masses could be different in temperature or moisture content. The most commonly known examples of fronts are the: Cold Front Warm Front Glaciation -- The transformation of cloud particles from water droplets to ice crystals. Thus, a cumulonimbus cloud is said to have a "glaciated" upper portion. Halo -- A ring or arc that encircles the sun or moon when seen through a cloud of ice crystals, such as cirrostratus. Halos are produced by the refraction of light. Hot Spot -- Typically large areas of pavement, these "hot spots" are heated much quicker by the sun than surrounding grasses and forests. As a result, air rises upwards from the relatively hot surface of the pavement, reaches its condensation level, condenses, and forms a cloud above the "hot spot". Lapse Rate -- The rate at which atmospheric variables decrease with height, (temperature for example). Lifting Condensation Level -- The level in the atmosphere where a lifted air parcel reaches its saturation point, and as a result, the water vapor within condenses into water droplets. Nucleus -- a particle of any nature upon which molecules of water or ice accumulate. Orographic Lifting -- It is the forced ascent of air by topographical features, such as a mountain. Refraction -- The bending of light as it passes from one medium to another. Relative Humidity -- The amount of water vapor in the air compared to the amount of water vapor the air can actually hold at a given temperature and pressure. When the air has a relative humidity of 100%, it is saturated. Saturation -- An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure. If the pressure or temperature were to decrease, the water vapor would then condense and form water droplets. Saturated air has a relative humidity of 100%. Stratosphere -- The layer of atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km) generally characterized by an increase in temperature with height. Supercooled -- Liquid droplets that exist in an environment where the temperature is colder than the freezing point (0 degrees Celcius). Thermal -- A small rising parcel of warm air produced when the earth's surface is unevenly heated. How they form: Air molecules bounce against the earth's hot surface, acquiring energy, and as a result, the heated air expands, becoming lighter than the surrounding air, which allows it to rise. These thermals "bubble" upwards and transfer energy to higher levels. This is an example of convection. Topography -- Generally, the lay-out of the major natural and man-made physical features of the earth's surface. Bridges, highways, trees, rivers and fields are all components that make up this topography. Tropopause -- The boundary between the troposphere and stratosphere. Updraft -- A current of ascending air. Warm Front -- A narrow transitions zone separating advancing warmer air from retreating cooler air. The air behind a warm front is warmer and typically more humid than the air it is replacing. Wave Crest -- The highest point in a wave. Wave Trough -- The lowest point in a wave. Wind Shear -- The rate of change of wind speed or wind direction over a given distance. Vertical Shear -- is simply the rate of change of wind speed or direction, with a given change in height.