Violent, low-pressure storm, relatively small in diameter but with very rapidly rotating winds and an intense updraft near the centre. The relatively low pressure at the centre of a tornado's funnel-like vortex causes cooling and condensation, making the storm visible as a revolving column of cloud, which is called the funnel. Tornadoes normally travel at 30–40 mph (50–65 kph). The winds around the vortex may exceed 300 mph (500 kph) in extreme events. Tornadoes often occur in groups.