This article looks at the northwestern corner of Wisconsin, in the crook of the St. Croix River, before and just after a tornado cut a west-to-east swath half a mile wide and more than 20 miles long.

These images are shown in simulated natural color; green fields and forests are interspersed by glacial lakes, country roads, and a few popcorn clouds in the May 18 image. (The lakes look much brighter on June 19 than on May 18, most likely from the sun glinting off waves.)

The Siren storm

The tornado touched down on a Monday evening, 18 June 2001, about 5 miles west of Siren, a town of about 900 people. This tornado was later classified as severe, an F3 on a scale from F0 to F5, its winds of 158—206 mph placing it among the 10 percent most damaging tornadoes. "It was purple, it was green, it was white," one resident said. "It was the most amazing thing I've ever seen in my life, and I couldn't stop watching it."¹

As a lakeside town, Siren had many houses without basements, and it had short warning that night since lightning had knocked out its tornado siren about a month earlier. The tornado hit the town at about 8:20 p.m., damaging or destroying about 200 houses and 40 businesses. No one died in Siren, but 16 people were injured that night, and three people were killed east of Siren in the small town of Dewey. This was Wisconsin's deadliest tornado since 1984.

Just before noon the next day, as recovery crews poured into the area, Landsat 7 passed overhead, getting a nearly cloudless image. The Environmental Remote Sensing Center at the University of Wisconsin rush-ordered this scene from the USGS and produced a "change" image comparing May 18 to June 19. This image gave recovery workers a more synoptic view of the damaged area than attempts to map it by airplane, helicopter, or ground survey.²

Red or green?

You may have noticed that these images look different from most images in Earthshots, which are mostly red. Why? The Garden City / Help article explains band combinations in some depth, but here is the short explanation.

Satellites detect not only visible light but also various "bands" of infrared light. Infrared tends to dominate satellite images, and it yields excellent information not available to the naked eye. But the human eye can't see infrared, so how should we display it?

In Earthshots we mostly use "red" false-color images, in which infrared is represented by red. This traditional method dates back to color infrared film. It makes particular sense when showing images from Landsats 1 through 3. And it makes it obvious that these are not true-color photographs.

There are also "green" false-color images. In these images, which often include new bands not available on the older Landsats, infrared is shown as green. Since plants are very infrared-reflective, this has the pleasing "natural" effect of making vegetated lands appear green. Some mapping websites use more-realistic "green" false-color images, created by mathematical algorithms from many bands. People may confuse them with actual true-color, so truth-in-labeling is important.

For this article the visible bands showed the tornado swath well, so we decided to sidestep the infrared issue and use real natural color. But what would that color really be? Red? Green? When red green and blue represent red green and blue, how does the resulting image look? (See the answer below.)

Footnotes

Thanks to Dr. Thomas Lillesand, Marcia Verhage, and Nancy Podger of the Environmental Remote Sensing Center of the University of Wisconsin, Madison, for assistance and ground photos.

1. Information on the Fujita Tornado Damage Scale from the Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin, at cimss.ssec.wisc.edu/oakfield/Fscale.htm. Siren storm information from Vikki Ortiz and others, "Tornado Devastates Village; Storm Claims 3 Lives, Flattens Much of Area," Milwaukee Journal Sentinel, 20 June 2001, at www.jsonline.com/news/state/jun01/siren20061901a.asp as of 2 August 2001.

2. Personal communication from Dr. Thomas Lillesand, 5 July 2001.

Satellite images

18 May 2001: Landsat 7 ETM+, path 27, row 28.

19 June 2001: Landsat 7 ETM+, path 27, row 28.

Apollo photograph showing the Arabian Peninsula, Africa, and Antarctica: from Apollo 17, 7 December 1972.

For the following three images, the same linear stretch was performed on all three bands (i.e. the "RGB" channel), with only a trace of outlier pixels clipped.

Mexico: 31 March 2001, Landsat 7 ETM+, path 27, row 47.

Mount St. Helens: 22 August 1999, Landsat 7 ETM+, path 46, row 28.

Florida: 27 January 2000, Landsat 7 ETM+, path 16, row 41.

Maps

U.S. Geological Survey, 1972 (compiled 1967, revised 1972), National Atlas: Northern Great Lakes States, scale 1:2,000,000, Albers Equal Area projection.

U.S. Geological Survey, 1980 (1953, revised 1980), Stillwater, scale 1:250,000.

Photographs

Thanks to Dr. Thomas Lillesand and the Environmental Remote Sensing Center, University of Wisconsin at Madison, for the ground photos of the pond near Siren and the foundation near Dewey, taken on 19 June 2001.

Thanks to Wisconsin Emergency Management for the remainder of the ground photos, taken in Siren on 19 June 2001.

Answer to the question above

As you may recall from the famous "blue marble" photo, when viewed from space the Earth is of course neither red nor green. It is a hazy, ambiguous blue. The atmosphere, especially down in its damper depths, makes blue light scatter more than red light. As a result, Landsat 7 looks down through a blue sky just as we look up through one. This effect varies, but for most areas the visible bands (of Landsat 7, in this case) yield an image ranging from bluish pink (from the dry highlands of Mexico), to bluish green (from the forest around Mount St. Helens in summer), to just a bluish blue (from misty central Florida).

But we thought a blue Wisconsin would be too natural-looking for our readers, so we damped down the blue to a nice grass green.

How to cite this article

Campbell, Robert Wellman, ed. 2008. "Siren, Wisconsin: 2001." Earthshots: Satellite Images of Environmental Change. U.S. Geological Survey. http://earthshots.usgs.gov. This article was released on 14 August 2008.