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SEVERE WEATHER Thunderstorms and Lightning Thunderstorms Form in warm, moist, unstable air when a “trigger” such as a mountain or cold front forces the air to rise quickly. Storm clouds may be up to 20 km high in the atmosphere. Life Cycle of a Thunderstorm Cumulus Stage – Air rises and a cumulonimbus cloud forms. The rising air is an updraft. The updraft prevents precipitation from reaching the ground. Mature Stage – the precipitation becomes heavy enough to fall through the updraft and reach the ground. The falling precipitation creates a downdraft. Life Cycle of a Thunderstorm Dissipating Stage – the downdraft weakens the updraft, cutting off the supply of moist air rising in the cloud. The cloud begins to evaporate. Lightning Lightning is a discharge of electricity Typically occurs during thunderstorms In the atmospheric electrical discharge, a bolt of lightning can travel at speeds of 60,000 m/s (220,000 km/h), and can reach temperatures of 30,000 °C (54,000 °F). This is hot enough to fuse sand into glass There are some 16 million lightning storms in the world every year Lightning The top of the cloud becomes (+) charged and the bottom (–) charged from friction and other processes. This charge is then discharged as lightning Lightning Discharge of electricity. The temperature may reach 50,000oF. (5X hotter than the sun’s surface) Thunder is produced by the shock wave sent out due to the heated air expanding so violently. Hail Formation Hail is produced during some thunderstorms under these conditions: Condensation nuclei is available (dust) Super cooled (<0oC) water droplets are carried through the updrafts and downdrafts of a cumulonimbus cloud. Types of Thunderstorms Single Cell: A single thunderstorm with one main updraft Rarely severe Result from local unstable atmosphere Typical summer thunderstorm Types of Thunderstorms Multicell cluster: Formed from a cluster of storms Each cell is it’s own separate updraft Can develop into squall lines Squall Line (Multicell line): Form when multicell clusters from a line Can be hundreds of miles long Supercells Largest and most severe thunderstorm that can last for hours Have a mesocyclone, a rotating updraft Produce large amount of rain, hail, and strong winds About 30% produce tornadoes Most supercells and tornadoes rotate counter-colockwise in the Northern Hemisphere Supercell Video