Where the wind blows: Northern Illinois windier than southern part of state

SYCAMORE, Ill. — The peak wind speed in Illinois happens in March and April and it is the least windy in July and August.

“The wind happens because we have a pressure gradient and what causes the pressure gradient dictates how strong the wind is and where the wind is blowing the strongest,” said Trent Ford, Illinois state climatologist.

“Between now and mid-spring, we see the most frequent passage of mid-latitude cyclones and the center of the low pressure of those mid-latitude cyclones can be a lot stronger than normal summer and daytime heating will create,” said Ford during a presentation at the Illinois Crop Management Conference, hosted by University of Illinois Extension.

There are big differences in the wind speed across Illinois.

“The northern parts of the state are windier than the southern parts of the state and that pretty much holds true for winter, spring, summer and fall,” Ford said. “Part of that is due to Lake Michigan and in southern Illinois there are a lot more trees and hills than in northern Illinois.”

On a warm day when the sun is heating the air near the surface, the air begins to rise, the state climatologist said.

“When we get enough moisture in the air and instability, we get the formation of small-scale storms,” he said. “When the air rises off the surface, we get an area of low pressure because there is less air there.”

Since nature does not like a gradient, it tries to fill the void and balance what is unbalanced.

“There is an area of low pressure and the air comes in to fill that void and that’s essentially what creates wind,” Ford said.

“We can also get wind by the passage of a mid-latitude cyclone,” he said. “That’s a big area of low pressure that’s not created by the sun warming the air, but it is created by how the jet stream flows from the north to the south or east to west to fill the void and balance out the pressure gradient.”

The location of the low pressure dictates weather conditions.

“If the area of low pressure is sitting over the Memphis area and we’re in DeKalb, we get a lot of very cold and dry air streaming in from the north,” the climatologist said. “If the area of low pressure is situated where we are, we’re going to get a different weather pattern.”

Ford provided data on the number of thunderstorm and non-thunderstorm related wind damage reports in Illinois for every month starting in 2004.

“We don’t have very many wind damage reports from January to April and from September through December is also a pretty weak time for wind damage reports,” he said. “Roughly 80% of the wind damage reports come during May through July and that’s exactly when we’re at our least windy time.”

The reason the majority of the wind damage to infrastructure and crops happens during this time is because the types of weather that bring the windy conditions in the winter and spring are distinctly different than the conditions that cause windy conditions in the summer and the effects are distinctly different.

“The biggest thing is that in the summertime we get small-scale thunderstorms that can produce tremendously strong wind in small areas,” Ford said.

“The backbone of a thunderstorm is an area we call the updraft,” he said. “Because the air is buoyant, it rises relatively quickly and in the strongest thunderstorms in Illinois we can get the air moving upward in excess of 100 mph.”

As the air moves up in the atmosphere, it cools down and begins to fall and produces downdrafts on the backside and front side of the storm, Ford said.

When air flows in from the back of the storm it can combine with the downdraft region and result in some really strong straight-line winds.

“About 90% of thunderstorms are going to pop up, rain out, create winds of 30 to 40 mph and die without any incidence at all,” the climatologist said.

“But every once in a while, we’ll get a storm where the air coming in from the back of the storm combines with the downdraft to create some really strong winds that can exceed 50 to 60 mph and sometimes 70 to 80 mph.”

Ford showed the radar images from 8 a.m. to 7 p.m. of the 2020 derecho that started in South Dakota and ended in Ohio.

“This is probably the strongest and most developed derecho I’ve ever seen,” he said. “We probably have to go back 30 to 40 years to see a feature like this.”

The storm was at its strongest when it got to the Cedar Rapids area.

“It had 140 to 150 mph winds, it had weakened a little when it got to the Quad Cities, and by the time it gets to DeKalb, it’s starting to fall apart,” Ford said. “It produces about a dozen tornadoes in the Chicagoland area and it creates quite a bit of damage all the way into Ohio.”

“Those of you who live north of I-80 in Illinois live in the most derecho-prone region of the whole world,” he said.

Over the past 35 years, Ford said, there has been about one derecho per year.

“A derecho is a widespread storm over 100 miles that has winds over 40 to 45 mph,” he explained.

The evolution of storms like the derecho that occurred in 2020, Ford said, is unable to be forecasted, even hours ahead of the event.

“It depends on the right conditions and this is not something that happens normally,” he said.

Planting trees or hedgerows can provide some protection from straight-line winds that occur during thunderstorms or derechos.

“There is not a whole lot we can do to limit building damage,” Ford said. “Preventative maintenance is No. 1 because rusted areas and foundation cracks are points of weakness for strong winds.”

He advises farmers to shut all doors, windows and vents on farm buildings before a thunderstorm.

“With open doors, when the wind blows through, that creates an area of low pressure in your building,” he said. “That forces more air in and that can collapse the building on itself.”

Documenting the condition of buildings is important and can help with insurance claims and farmers may want to plant windbreaks around their buildings.

“Studies show steel bins with vertical stiffeners perform better during windstorms,” Ford said. “They seem to do better than wind rings.”