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1. Introduction
In a mature thunderstorm there are both updrafts, where air is sucked into the storm, and downdrafts where cold air sinks toward the ground and extends ahead of the storm as a gust front. These areas of updraft and downdraft occur in distinct parts of the storm so that much of the air circulates from the bottom to the top of the storm several times. The top of the cloud will be over 12 km tall and has temperatures well below freezing. The constant freezing and melting of cloud droplets as they move within the storm system creates hail. In severe storms the hail produced may be the size of oranges. If I were to slice hail open I would see a pattern of concentric rings, each created as the outer part of the hail melted and was re-frozen on its trip. The weight of an individual raindrop or hail particle is a clear indication of the forces that it overcame in order to fall out of the cloud - therefore, the size of the hail is a clear indicator of the magnitude of the updrafts.
A necessary component to the formation of tornadoes is that a rotation around a vertical axis begins within the cloud. While it is common to experience increased windspeeds as you ascend upwards (vertical wind shear), horizontal wind shears are less common. Current theory suggests that a thunderstorm with strong updrafts can realign the normal vertical shear into a horizontal shear, thereby producing rotation. The most severe tornadoes are therefore associated with atmospheres with strong updrafts and strong vertical shears - both of these are common along an area of the central states of the US called tornado alley.
