It's Severe Weather Awareness Week in Nebraska and Iowa. Each day will focus on a specific weather hazard that neighbors in the Heartland deal with on a yearly basis. Today's topic is the type of severe weather that, historically, has done the most damage to our neighborhoods: hail and windstorms.
WHAT IS A SEVERE STORM?
The United States sees about 100,000 thunderstorms a year. Of these 100,000 storms, around 10% reach severe criteria. In Omaha, we experience 40-60 thunderstorms each year, meaning 4-6 severe storms. So, what are the criteria for a severe thunderstorm? Although any thunderstorm can be dangerous, severe thunderstorms are classified as storms that can threaten life or property.
For a thunderstorm to be severe, it needs to meet either of the following criteria: hailstones larger than the size of a quarter (1") or wind gusts of 58 mph or greater. This is where damage can be done to property.
It is important to note that a severe thunderstorm is NOT defined by the frequency of lightning OR the amount of rain it produces.
SAFETY IN SEVERE THUNDERSTORMS
When thunderstorms threaten, there are a few things you can do to keep yourself safe. For severe thunderstorms, the best way to protect yourself is to get away from windows. Hail or flying debris can break through windows and injure you. It is best to go to a lower floor in an interior space. If a tree falls, it can go through the roof, so it's best to avoid any top floors.
HOW THUNDERSTORMS/HAIL FORM
To get a thunderstorm, there are a few simple ingredients one needs: moisture, unstable air, and a forcing mechanism (i.e., a cold front). As the sun heats the Earth's surface, the warmer air, which contains plenty of water vapor, begins to rise, forming an updraft. As the air rises, it cools and eventually condenses to form a cloud. Depending on how "unstable" or energized the atmosphere is, air can rise at very fast rates and reach high in the atmosphere. If a lifting mechanism, such as a front, is present, the air can also rise quickly. Some clouds can tower over the Earth as high as 50,000 feet!
Once clouds form, more water molecules collect on each other until it gets too heavy for the storm updraft to hold, and thus they fall as rain. This is known as a downdraft. You now have a thunderstorm.
Inside the storm's updraft, the water molecules eventually freeze into ice crystals. These ice crystals begin to combine as they rise into the thunderstorm and tumble around like clothes in a dryer as they combine with more ice. At some point, the ice will become too heavy for the thunderstorm to hold, and it will exit out the downdraft. These chunks of ice that fall are what we call hail.
Hail can come in many shapes and sizes. The smallest hail size is around 0.25", or a pea. Other sizes of hail commonly seen are penny (0.75"), nickel (0.88"), quarter (1"), half dollar (1.25"), ping pong ball (1.5"), golf ball (1.75"), hen egg (2"), tennis ball (2.5"), baseball (2.75"), apple (3"), softball (4"), grapefruit (4.5"), and DVD (5"). Hail can grow even larger; the largest hailstone recorded in US history occurred in Vivian, South Dakota on July 23, 2010, which measured 8" in diameter! That is a bit less than soccer ball size hail!
Fun fact: the 2nd largest hailstone recorded in US history happened right here in Nebraska! In Aurora, east of Grand Island, on June 22, 2003. The hailstone measured around 7" in diameter!
