You probably could not help but to notice that skies were quite hazy this evening across the area. Unlike a recent morning when fires across the southern plains were the source, it is hypothesized that this feature has a much more distant origin. Volcanic activity on the Hawaiian Islands may be to blame!
On the early evening visible satellite picture beneath, you can see a "hazy" area extending from west to east across the Northern Plains. The area lines up very well with winds between 15,000 and 18,000 feet. While this haze was not visible for a good portion of the day during high sun angle times, as the sun became lower in the sky, the scattering of light due to presence of the elevated particles became much more evident, giving the sky a milky white color, instead of the familiar blue.
Here is an excerpt which describes the feature above:
DESCRIPTIVE TEXT NARRATIVE FOR SMOKE/DUST OBSERVED IN SATELLITE IMAGERY
THROUGH 1615Z April 22, 2008
Area of haziness moving through the Northern Plains and into MN seen in morning visible imagery. While origin is unknown for certain, it is likely an area of vog from HI that got caught in weather systems located in the Northern Pacific.
According to a USGS Fact Sheet, "vog" is simply volcanic smog, a result of the high quantities of sulfur dioxide and other volcanic gases which combine and interact chemically with atmospheric oxygen, moisture,dust, and sunlight.
For reference in future events, descriptions of smoke plumes from significant fire activity (or as in this case above, vog) are provided within the following text products: