Scientists still do not fully understand how a tornado forms and why. The general theory is that tornadoes form from cloud systems that are slowly spinning, usually in supercell storms, but weaker systems can produce them as well. The current belief is that a mesocyclone within a storm system is surrounded by pronounced variations in air temperature and downdraft. However, tornadoes have been known to form in cloud systems where strong winds and variations in temperature do not exist. Once a funnel begins to form, however, it can gain speed and strength in much the same way as ice skaters gain rotation speed as they pull their arms close to their bodies.