Shredding weather. Weather conducive to shredding one's face off. Preferably on a board but skates, skis, and other shredable items will suffice. Activity dependent. For instance, Surfing shreather would be warm and still air with big swells. Skateboarding shreather would be moderately cool. Snowboarding shreather would be light snow.
Grab your board, the Shreather is awesome.