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Storms with vertical winds driven by thermal convection and condensation Come in four types Single Celled Multi-cell Cluster Multi-celled (Squall Line) Supercell Single Cell Multi-Celled Cluster Supercell Thunderstorms have three distinct phases: Cumulus Stage Starts with a warm plume of rising air The updraft velocity increases with height Entrainment pulls outside air into the cloud Supercooled water droplets are carried far above freezing level Thunderstorms have three distinct phases: Mature Stage The heaviest rains occur. The downdraft is initiated by frictional drag of the raindrops. Evaporative cooling leads to negative buoyancy. The top of the cloud approaches tropopause and forms anvil top. Thunderstorms have three distinct phases: Dissapating Stage The downdraft takes over entire cloud. The storm deprives itself of supersaturated updraft air. Precipitation decreases. The cloud evaporates. Also called “Air Mass” T-storms Form away from frontal systems Form where moist and unstable conditions exist in the atmosphere. The threat for severe is small. When they do reach severe limits, the thunderstorms may produce brief high winds or hail which develop because of high instability Each individual cell lasts about 30-60 minutes and has three stages. New thunderstorms may form as a result of parent thunderstorm outflows converging with warm, moist air inflow Multicell clusters indicate new updrafts are forming where the low-level convergence is strongest, usually at the right, or right-rear flank of existing cells. Multicell storms can form in a line known as a squall line, where continuous updrafts form along the leading edge of the outflow, or gust front. Often form ahead of strong cold fronts The cold air “forcing” the warm air up vertically drives the multi-celled line of storms Are the most powerful thunderstorms. By their definition, supercells are always severe. Supercells are responsible for a larger amount of damage and casualties. Supercells have a rotating updraft, or mesocyclone. This helps the longevity of a thunderstorm by allowing the flanking line to enhance the inflow into one main updraft, rather than helping create new updraft centers. The combination of rotation and longevity increase the chances for the development of strong or violent tornadoes.