Hello, it's Rebecca Ladd again, This blog post might be a little more complicated than the others have been. We associate many storm elements with severe thunderstorms. Lighting and thunder, gusty winds, hail, flash floods, and tornados are the most well-known features, but we cannot forget their cousins , the microburst, mesoscale-convective systems (MCS), heat bursts, and derechos. This post will try and shed some light on these things.
Microburst and Macroburst:
A downburst is an area of rapidly descending air beneath a thunderstorm. When this downdraft hits the ground, it quickly spreads out in all directions, causing very strong, straight-line winds. These winds are commonly as strong as 40-60 mph but can exceed 125 mph at times. These downburst are broken down into two groups. The first is called a microburst; In order to be called a microburst the ground area impacted by the downburst is less than 2.5 miles in diameter. The other group is called a macroburst; a macroburst is physically the same thing as a microburst, but over a much larger space scale - Sometimes the area affected is greater than 5 miles in diameter. A downburst can last as long as 15 minutes.If you remember, in the thunderstorm life cycle. I said, rain aids in the creation of a downdraft. The process is the same here. Inside a thunderstorm, water vapor condenses into raindrops. On their way to the ground, these raindrops will fall through drier air which will make the drops start to evaporate. The evaporation process cools the air, causing it to become denser than the air around it. This rain-cooled air, along with the falling raindrops, accelerates downwards; it is this down-rushing air that eventually hits the ground and is forced to spread out in all directions causing the damaging straight-line winds. Microbursts are sub-divided as dry or wet, depending on how much rain accompanies the microburst when it reaches the ground.
Photo of a downburst.
A heat burst is an extremely rare event. A heat burst is a downdraft of hot and dry air that typically occurs in the evening or overnight hours after thunderstorms are ending. It is caused when rain falls into very dry air, high up in the atmosphere. The rain quickly evaporates as it falls through the dry parcel of air and that parcel cools rapidly. This dense mass falls rapidly toward the ground, heating up as it compresses. When this hot ball of air hits the ground it spreads out in every direction creating very strong, warm and dry winds. Wichita, KS was actually hit by one last week on Jun 9. National Weather Service meteorologist Stephanie Dunten says the heat burst hiked temperatures from 85 to 102 degrees in 20 minutes, beginning at 12:22 a.m. Thursday. She said a pocket of air in the upper atmosphere collapsed, and when it hit the ground it sent winds of more than 50 mph through parts of the city.
In the blog post on types of thunderstorms, I briefly mentioned squall lines, bow echoes, and MCS's. In this post, I will go a little more in depth on Bow echoes, MCS's, and especially the Derecho.
MCS:If anyone is interested, you can find more information here
Clearly a derecho is a dangerous storm. So if you hear that one is approaching you must act quickly to protect the lives of your family and yourself. And even if the severe thunderstorms are not a derecho, they are still deadly. There may actually be more deaths in regular severe thunderstorm, non-derecho, events.