How Severe Storms Are Formed

The more unstable the air mass and the stronger the lifting mechanism, the stronger the thunderstorm updraft becomes and the more likely the storm will be severe. Increasing winds with height also help to the storms ability to maintain itself. The longer a storm lasts, the greater chance it has of becoming severe. The National Weather Service defines a severe thunderstorms as a storm producing three-quarter inch or larger hail and/or winds greater than 58 mph. When thunderstorm updrafts reach speeds of 70 mph, they can support the growth of hailstones.
A hailstone is a lump of ice that falls from a thunderstorm. It can range from pea size to the size of grapefruit. Such large hail can impact the ground at nearly 100 mph demolishing crops, breaking windows, and damaging roofs, cars and airplanes. Hail begins as rain droplets which are carried by strong updrafts to high altitudes (well above the freezing level) where they are frozen into ice pellets. The ice pellets collide with more water droplets which freeze to the surface of the developing hail stone increasing its size. The stone continues to grow until the updraft can no longer suspend its weight and the hail falls to the ground.
Long-lasting thunderstorms, sometimes referred to as supercells, are more likely to be severe. For a thunderstorm to last, it must be able to sustain both its updraft and its downdraft. One way that this occurs is with increasing winds with height. If the horizontal wind, blowing into the storm, is stronger in the mid and upper reaches of the storm, the rising updraft becomes tilted. Now the rain is carried downwind of the updraft instead of collapsing upon it. Another important factor is if the horizontal wind, blowing into the storm, veers with height (changes direction in a clockwise motion), the storm’s updraft may begin to rotate. The combination of veering and increasing winds with height can produce a tilted and rotating updraft. This rotating thunderstorm, called a mesocyclone, is able to maintain its updraft and warm inflow region independent of the storms rain-cooled outflow. The rotating updraft of this type of thunderstorm is where the tornado can form and descend to the ground

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