Purple line is typical summertime conditions in Boulder.
Green line is conditions measured during Fontanelle fire smoke event in Boulder.

The Fontenelle fire started on June 24, 2012 in western Wyoming, 100s of miles from Colorado. Hot, dry conditions allowed the fire to grow significantly. On July 19th 2012, firefighters managed to reach 87% containment. The fire did not burn any structures, but smoke from the fire was transported south and east to Colorado's Front Range, making July 4 in Boulder, CO a very hazy day.

This image (see lower right pane) from the Naval Research Laborotory shows the smoke forecast over North America for the evening of July 4 (Colorado time). You can see the large smoke 'blob' over Wyoming and Colorado.

Smoke plume from Fontanelle fire taken from International Space Station.