Due to the recent microburst event in Whately and Sunderland, MA on this past Thursday, July 3rd, I was asked by FIREGROUND360° to expand upon the differences between straight-line winds, microbursts, and tornadoes, as sometimes there is confusion as to what those terms actually mean.
To start, there are two types of wind events within the context of severe thunderstorms. You can have straight-line winds, or rotating winds.
Gusty straight-line winds can occur behind what is called a gust front, which is the boundary of air that separates warm humid air that the thunderstorm is entering into, and rain-cooled air along the leading edge of the thunderstorm, and under it as well. When a squall line is moving in, you often see dramatic looking clouds indicating the beginning of the thunderstorm, and the downward-arcing initial cloud is called a shelf cloud, which represents the dividing line of this air boundary change.
A downburst is the general term that describes both microbursts and macrobursts. Microbursts were described by meteorologist Ted Fujita (creator of the Fujita Scale for measuring tornado strength) as being 2.5 miles or less in size, and macrobursts being greater than 2.5 miles in size.
In more severe thunderstorms, microbursts can form, which are also straight-line wind events. The damage these events create is often mistaken for tornadoes, but microbursts are their own phenomena, which can cause damage similar to a tornado.
Severe thunderstorms are born when the following combination of factors are present: copious low-level moisture, air that is forced skyward, and atmospheric instability. Air being forced skyward is also known as lift, and is most commonly produced when cold fronts are approaching. Because the warm air ahead of them is less dense than the cold air behind them, as they approach, the air is forced skyward. Instability is usually represented by very warm moist air at the ground, and colder, drier air in the upper levels. Since warm air is less dense than cold air, this scenario fosters convection, which is a term that refers to a rising, buoyant warm air parcel through surrounding cooler air.
This setup produces very strong and powerful updrafts and downdrafts within the core of the thunderstorm. The rapidly descending downdrafts are formed from the substantial precipitation that is created, both in the form of liquid rain, and in some cases, frozen hail. This downdraft of rain and hail-cooled air, when organized enough, creates “drag” on the surrounding air, and begins to accelerate downward towards the ground. Upper air flows called jet streaks are thought to also be directed towards the ground as they get caught in these downdrafts as well.
As I saw on radar a few days ago, a classic bow echo (which looks like a C-curve) was present, which normally indicates the microburst.
As this rapidly sinking downdraft strikes the surface of the planet, it spreads out radially in all directions, with the most force and highest gusts being ahead of the storm, in the direction that it is traveling. For example, the gusts in the Whately/Sunderland microburst were determined to be 60 to 80mph, and indicated a northeasterly direction, which was the direction the thunderstorm was traveling. Microbursts can produce wind gusts in excess of 150mph, which is similar in strength to a Category 4 hurricane, or EF-3 tornado.
A microburst can also be determined from the wind patterns on the ground. You will see the direction from wind damage in a straight line across corn fields, or downed trees, whereas with tornadoes, the wind damage is circular, as was the case with the Springfield tornado of 2011. I personally drove past a road in Wilbraham, and you could literally see twisted trees and a scouring of the land in a twisted motion that was about 300 feet wide.
Photos by FIREGROUND360°
As for tornadoes, it should be noted that scientists don't truly understand how tornadoes are formed, but we do know some aspects of what causes development. Tornadoes are the opposite of microbursts, in that they are caused in the updraft regions of thunderstorms, and indicate inflowing, violently up drafting air. Wall clouds and funnel clouds are actually not technically lowering, per se, but are building down from the cloud base, due to condensing rising air.
For tornadoes to form you need strong updrafts in the rain-free portion of the thunderstorm (usually to the southwest of the cell), to combine with wind shear in the atmosphere levels above the cloud base. Wind shear references both changes in wind speed, and/or wind direction. In the example of speed shear, you could have a 10mph wind out of the west at 3000 feet, and a 30mph wind out of the west at 6000 feet. This causes the air above the cloud base to start “rolling”, and becomes a rotating horizontal column of air parallel to the ground. When this rotating column of air is close enough to the ground and encounters powerful updrafts, the column gets twisted, and part of it is directed down below the cloud base, which can form a funnel cloud, and if it continues to the ground, a tornado. Wall clouds are indicative of the area of strong updraft, and condense below the cloud base, through which funnel clouds and/or tornadoes can form.
Tornadoes usually form along dry lines, which are frontal boundaries where moist humid air meets and makes contact with dry arid air. This usually happens when air from the southwest deserts or Rockies meet with air from the Gulf of Mexico, but can also happen with dry Canadian air meets up with moist air from the Gulf, or from the Atlantic Ocean.
There are 28 damage indicators, and 8 scale degrees for each indicator in the Enhance Fujita Scale that was instituted in 2007 that help to define whether or not the tornado fits into one of six categories based on its severity that range from EF-0 to EF-5
Here is a link to the Enhanced Fujita Scale
Interestingly, microbursts are the opposite of tornados in many ways, in that microbursts are created through rapid descending air that has no rotation, whereas tornados are the result of rapidly ascending air that is rotating.
I hope this article has helped clear up some of the confusion regarding the difference between straight-line winds, microbursts, and tornadoes.
Thanks for reading!
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My name is Dave Hayes, I'm a self proclaimed weather nut, and I have been living in the Pioneer Valley of Western Massachusetts for over 25 years. I have had a passion for the weather since I was 12 years old growing up in Eastern Mass, when I first saw Bob Copeland on WCVB Channel 5 reporting on severe thunderstorms. Soon enough, all the bright colors on the radar corresponded to some big thunderstorms that moved-in overhead, and I was hooked.
Through my Facebook Page that I started back in 2011, my Twitter Feed, and now with WesternMassWeather.com, I curate and report on the weather of Western Massachusetts, and all of Southern New England when possible. I cull weather information from several non-local sources, the main one being NOAA and the National Weather Service, for which I am an official Severe Weather Spotter.
As a western Massachusetts weather curator, I put my own personal twist on weather reporting with a valley-centric focus by concentrating on the four westernmost counties of Massachusetts: Berkshire, Franklin, Hampshire, and Hampden counties.
Dave can be reached at:
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