The Flickr pic you link to says "September...". And there is bright green grass. So clearly it was not taken on "4/16/09".

At 20,000 feet and 30,000, IN THE MIDDLE OF JULY, temperatures average 35F and 15F respectively. It is NEVER warm at those altitudes, thus contrails are always possible, year round.

I hope you're joking, or at least seeking professional help.

Duluth lies below seven different Low Altitude Enroute airways, and over a dozen High Altitude Enroute airways. Thus, there are a lot of jets flying over all of the time.

The past few days, we've had very high pressure (30.2 inches of mercury and higher) with very low humidity - ideal conditions for contrails (while pressure levels are much lower at altitude, they still correlate to surface air pressure due to a constant ambient lapse rate.)

I will sell you Chem - Trail® Insurance for $1,798 a year.

That's right genius. Some of the pics were taken last Fall. Actually about 4 of the pics are from September. Sorry to confuse you, or perhaps you already were.

Contrails do take place at 20,000 feet, but the pictured trail in not that high.  

Contrails dissipate in 2 minutes, they do not stay there for an hour or two, then turn the sky hazy.

Contrails generally do not dissipate in two minutes, and can last up to an hour. They can also "seed" the air around them (if it's dry enough), thus spreading into a localized haze.

Where are the leaves on the tree in the photo? No leaves seems to indicate that the photo was not actually taken in warm weather.