What is the tornado scale?

Posted on April 29, 2026 by WeatherGuys Editor

A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. We classify the strength of a tornado after trained observers assess the damage it did to the area.

The “Modified” Fujita Scale uses damage caused by a tornado and relates the damage to the fastest 1/4-mile wind at the height of a damaged structure. (Image credit: NOAA/Storm Prediction Center)

All tornadoes are assigned a single number from the Enhanced Fujita Scale (EF) according to the most intense damage caused by the storm. When tornado-related damage is surveyed, it is compared to a list of damage indicators and degrees of damage to help estimate the range of wind speeds the tornado likely produced. The rating is assigned based on a set of 28 damage indicators, such as barns, schools and trees; the degree of damage to each one is used to determine the EF scale of every tornado. The breakdown and example damage of the Enhanced Fujita, or EF, scale is:

  • EF0 (weak) — 65-85 mph: Peels surface off some roofs; some damage to gutters or siding; branches broken off trees.
  • EF1 (weak) — 86-110 mph: Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors.
  • EF2 (strong) — 111-135 mph: Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed.
  • EF3 (strong) — 136-165 mph: severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown.
  • EF4 (violent) — 166-199 mph: Well-constructed houses and whole frame houses completely leveled.
  • EF5 (violent) — 200-230 mph: Strong frame houses leveled and swept away; steel-reinforced concrete structures badly damaged.

The National Weather Service is always looking for trained volunteers to provide severe weather reports, including reports of tornadoes. For more information, go to www.weather.gov/skywarn/wi-skywarn.

Steve Ackerman and Jonathan Martin, professors in the UWMadison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather

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Why do clouds turn green during some storms?

Posted on April 20, 2026 by WeatherGuys Editor

The visible light spectrum covers colors from violet to red. Each color corresponds to a different wavelength, with green sitting roughly in the middle. Blue and violet have shorter wavelengths. Light rays change direction when they hit particles — a process known as scattering. The sky looks blue because air molecules scatter shorter wavelengths more effectively.

The sky on Tuesday, April 14th, in Dane County. (Photo credit: Kaitlyn Richardson)

Clouds are made of water drops and ice crystals that scatter light from the sun in all directions. It is the multitude of drops and crystals that make a cloud look white during the day. Sometimes only a small amount of light escapes out the bottom, and so cloud bottoms often appear grayish.

At sunrise and sunset, clouds can appear orange or red. With the sun lower in the sky, the sunlight passes through a significantly longer atmospheric path, resulting in the removal of blue wavelengths before reaching the cloud. Only the longer wavelengths remain visible, accounting for the beautiful orange and red hues observed during sunsets.

Intense thunderstorms sometimes have a green tint. These storm clouds are packed with massive amounts of water and ice and often occur late in the day. The reduced presence of the short wavelength blue portion of the spectrum results from its being scattered out of the column during its long path through the atmosphere. Liquid water has a slight preference for absorbing the oranges and red colors. In the presence of so much liquid water in a thunderstorm cloud, the sligh absorption of red and orange is just enough to filter out some o those longer wavelengths. The middle visible wavelengths, the greens, are then able to dominate and become visible.

While a green sky often suggests severe weather, it does no guarantee you’ll encounter tornadoes or larger hail. The green colo is a visual indicator that the storm has the potential to be severe.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena, Severe Weather

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What is the shape of raindrops?

Posted on April 13, 2026 by WeatherGuys Editor

While cartoonists typically draw raindrops like a teardrop or a pear shape, raindrops are not shaped like that.

Shape of raindrops changes with size.
Shapes of raindrops (Image credit: https://www.usgs.gov/water-science-school/science/are-raindrops-shaped-teardrops )

They are drawn as teardrops to give the image of falling through the atmosphere, which they do. But as they fall, raindrops are flattened and shaped like a hamburger bun by the drag forces of the air they are falling through.

Raindrops are at least 0.5 millimeters or 0.02 inches in diameter. You will not find a raindrop bigger than about one-quarter of an inch in diameter. Larger than that, the drop will break apart into smaller drops because of the air resistance. Precipitation drops smaller than 0.02 inches in diameter are collectively called drizzle, which is often associated with stratus clouds.

The typical speed of a falling raindrop depends on the size of the drop. Gravity pulls everything downward. As an object falls it experiences a frictional drag that counters the downward force of gravity. When the gravity and frictional drag are balanced, we have an equilibrium fall speed that is known as the terminal velocity of the object. The terminal velocity depends on the size, shape and mass of the raindrop and the density of the air. Thus, it is worth talking a bit about the shape and size of raindrops.

A large raindrop, about one-quarter of an inch across or about the size of a house fly, has terminal fall speeds of about 10 meters per second or about 20 mph. That kind of speed can cause compaction and erosion of the soil by the force of impact. Since raindrops come in a variety of sizes, they fall with different speeds. The smallest raindrops fall at about 2 mph. Water droplets smaller than these smallest raindrops (known as cloud liquid water droplets) can resist falling in the atmosphere because there is upward moving air that overcomes the force of gravity and keeps them suspended in the cloud.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Meteorology, Phenomena

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How is air quality measured and what are the trends in Wisconsin?

Posted on April 6, 2026 by WeatherGuys Editor

The amount and density of pollutants in the air are converted into an Air Quality Index, or AQI. The Wisconsin Department of Natural Resources’ statewide monitoring network is operated following a federally approved plan. The DNR statewide network includes 30 ozone and 18 fine particle, or PM2.5, monitoring sites. PM2.5 describes particles with diameters that are generally 2.5 micrometers or smaller and thus inhalable.

Average Annual Population-Weighted concentration (PM2.5) trend in Madison (Image credit: State of Global Air.org)

Under the Clean Air Act, the Environment Protection Agency sets National Ambient Air Quality Standards for pollutants.

The DNR monitoring network is operated under a federally approved network plan, reviewed annually to ensure appropriate monitoring in all locations as required by federal regulations.

Continuous monitoring enables the determination of trends in air quality, which demonstrates how well air pollution controls and programs are working to improve our air quality. The monitoring also enables the DNR to rapidly inform the public when air pollution reaches unhealthy levels.

The data indicate that the concentrations of most pollutants regulated under the Clean Air Act have been decreasing across the state since the early 2000s, an indication that air quality is improving. However, in the most recent years, their report shows that ozone and PM2.5 concentrations have leveled off or recorded increases in concentrations.

The Endangerment Finding, established by the EPA in 2009, authorizes regulation of emissions from various sources, including vehicles and power plants. Without the Endangerment Finding, the U.S. would have fewer tools to curb emissions linked to greenhouse gases and poor air quality.

Recently, under the Trump administration, the EPA rescinded the Endangerment Finding, which could reduce regulatory oversight and worsen air quality. It will be important to continue collecting data on air quality to determine future impacts of this regulatory change.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc.edu.

Category: Climate, History

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What are ice balls and how do they form?

Posted on March 30, 2026 by WeatherGuys Editor

If you were lucky enough to be walking along Lake Mendota shoreline at Shorewood Hills and Frautschi Point on March 16, 2026, you might have seen ice balls, or ice eggs, floating in the water.  This was a rare occurrence of naturally formed balls of ice that form in cold water.

Shows grapefruit-sized balls of ice floating at Lake Mendota's shoreline
Linda Deith photographed this flotilla of grapefruit-size ice eggs bumping up against the Lake Mendota shoreline on March 16. (Photo credit: Linda Deith)

While they look a little like hail, the formation of ice balls is very different. The freezing temperatures form sheets of ice on the water surface. The wind and the resulting wave action causes the ice sheets to break up and form small pieces of ice and slush. The gentle tossing of the water by a breeze causes the pieces of ice to collide and stick together. Single balls of ice begin to form in the ebb and flow of water and grow as layers of water freeze over the surface of the ice balls. Eventually the wind and water currents move the ice balls to the shoreline where they collect.

The weather conditions must be just right for ice balls to form along a shoreline that is shallow with gentle slope. The temperatures must be slightly below freezing (32°F). The water must be calm with a light swell, and the wind must blow in a steady direction.

Ice balls have been observed in the Great Lakes and along several Arctic shorelines, including those of Estonia, Russia, and Northern Germany. Their sizes have been observed to range from golf ball size to 3 feet in diameter.  In 2016, ice balls washed up on a beach in Siberia with some measuring about 3 feet in diameter and covering an 11-mile stretch of coast.

The ice eggs along Lake Mendota were estimated to be the size of a grapefruit.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison department of atmospheric and oceanic sciences, are guests on WHA radio (970 AM) at noon the last Monday of each month. Send them your questions at stevea@ssec.wisc.edu or jemarti1@wisc. edu.

Category: Uncategorized

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