From Glossary of Meteorology
A mountain-wave cloud form that occurs along the east slopes of the Rocky Mountains, particularly in the Montana–Alberta area, where the range runs roughly north–south.
The arch cloud has a sharp western (upwind) edge and a great cross-wind extent in the lee of and parallel to the Rockies. Arch clouds can stretch a few hundred km or more north–south as measured from the ground and from satellite. The arch refers to the shape of the western edge of the wave cloud as seen by an observer looking westward from the ground on the plains east (to the lee) of the mountains. The arch is especially noticeable when the blue sky to the west of it is completely clear of other cloud layers. Glider pilots sometimes refer to the clear area as the "window." The distinctive shape of the arch is due to the cloud height (usually middle or high cloud), its large cross-wind extent along the horizon, and the perspective of the observer, who sees the cloud and the mountain range converge with distance to the north and south. The arch cloud often presages a chinook. Because of the rapid temperature rises associated with a chinook, the arch has an important place in local weather lore as a predictor.