As the air flows down, waves are created. The high crest of the wave creates a distinctive cloud band parallel to the mountains called a chinook arch. They're found where prevailing winds cross mountain ranges situated parallel to them. They're particularly strong in southern Alberta and northern Montana. Chinooks also occur in other parts of the world including Germany, Switzerland, New Zealand and Argentina.
Chinooks can happen year-round, although the warming condition is more apparent in colder weather.
A chinook can last less than an hour or for several days. Not all east slope winds are Chinooks. Winds carrying warm, moist air from the Pacific glide up and over the western slopes of mountains - expanding, cooling and losing moisture. Once over the Coast and Rocky Mountains, the now-dry air plummets to the foothills, warming by compression. Environment Canada, 1995.