A tornado is a funnel cloud, or vortex, of spinning air, water vapor and sometimes dust and debris. The word tornado is usually reserved for the funnel cloud spawned by supercell storms; other funnel clouds are called spouts. Tornadoes can form out of supercell storms if the updraft (the mesocyclone) rotates and creates enough air vorticity with respect to the flanking downdraft area. This vorticity, if the airflow pattern in the storm is balanced well, can be enhanced and concentrated, forming a low-pressure vortex at the perimeter of the mesocyclone and ultimately the tornado. The tornado becomes visible below cloud base due to condensation of the lower pressure air.
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