RAPID CITY, S.D. (KOTA/KEVN) – Thunderstorms are common here on the plains. The threats that come with storms range from hail, to lightning and tornadoes. We will take the time to explain how all of these form within a thunderstorm.
Tornadoes are pretty rare, though it seems we see them the majority of days in the United States when there is the threat for severe weather. The setup needs to be nearly perfect for tornadoes to form. On a hot sunny and humid day, air parcels rise up to form clouds. As they do so rapidly, they create thunderstorms.
One of the important factors of tornado formation is wind shear. There are two types – speed shear and directional shear. Speed shear is the increase in wind speed from the surface to the higher portions of the atmosphere. This causes the thunderstorm to “tilt” and be able to last longer throughout the day, allowing intensity to build. Directional shear is where the wind direction changes with height. This allows the storm to rotate and take a step towards tornado formation.
While rotating, the storm will start to show a feature called a “wall cloud”, which is a lowering from the original base of the cloud and that is the area you want to keep an eye on for tornado development. As the storm continues to rotate and other variables are lined up, a funnel cloud will form, which looks like a tornado, but isn’t because the rotation has not reached the ground. The funnel will continue to lower until there is circulation at the surface, and once you see that you can officially say a tornado has formed.
Another threat with severe thunderstorms, especially around here, is hail. Hail is formed when frozen water droplets continue to bounce above and below the freezing level of a thunderstorm. This allows the small frozen droplet to accumulate more liquid, then refreeze as it enters the freezing layer. This process will continue until the hailstone is too heavy for the storms updraft to hold it, proceeding to fall until it reaches the surface. Hailstones can range from pea size, all the way up to the size of a grapefruit, or larger. As a matter of fact, the largest hailstone to fall in the world was recorded two hours east of Rapid City, in a place called Vivian. This happened on June 23, 2010. The hailstone was 8.0” in diameter, had a circumference of 18.625” and weighed 1.9375 lbs!
Lightning is a common thunderstorm threat all across the world. There are positive and negative charges in a thunderstorm. Typically the positive charges are in the higher parts of the storm, while the negative charges are near the bottom. Positive charges are also at the surface. As the charges build, the positive charge will find a taller object and start to move up toward the base of a storm, which attracts the negative charge to drop down from the cloud. These eventually meet up and cause a lightning strike.
There are many different types of lightning strikes. The common ones are cloud to ground and intracloud. You will also occasionally see cloud to sky lightning strikes. Some of the more uncommon lightning strikes are cloud to cloud and also cloud to ground, but in a positive electron form. The most uncommon type of lighting is ground to cloud, though we do see that more here in Rapid City than many other places in the world due to the TV towers up on Skyline Drive.