On the evening of August 25, 2011 smoke from forest fires in the western United States moved into western Wisconsin. The smoke is discernable over western Wisconsin in the visible satellite image below, captured at 6:15 PM CDT on August 25th. The satellite image shows a shadow being cast by the smoke as well (note the darker shading to the east of the arc shaped smoke plume). Looking to the west this evening you may have seen a yellow haze or what looked like thin cloud cover, this was the smoke. This can be seen in the picture below on the right, taken in Monona, courtesy of Ben Miller.
The curving appearance of the smoke may be attributed to an upper level disturbance, near 15,000 to 20,000 feet. It is likely that the rotation within the disturbance gave the smoke its curving demeanor. Typically, in a moister environment, this disturbance would result in clouds and possibly precipitation, but due to the dry air over the region no clouds formed.
It is possible that this smoke originated from a forest/grass fire in Shannon County, South Dakota, although we cannot be sure. Linked below is the Active Fire Mapping program. It shows the forest fires that are occurring at this moment, for more information click on the forest fire in question.
In addition you can see a wispy veil from western Minnesota down to central Oklahoma. This is also smoke from the forest fires out west.