A Chinook Arch

Thanks to a Chinook Arch, we experienced a fantastic sunset on October 18. Chinook Arches are unique to Alberta, Canada, and Montana. These distinctive cloud displays and their formation are closely tied to the unique geography of the region.

Chinook Arches occur near the Rocky Mountains and are characterized by their distinctive arch-like shape, spanning across the sky. These arches are formed as a result of the interaction between moist, warm air from the Pacific and the towering Rocky Mountains. Running north to south, the mountains act as a barrier to incoming Pacific weather systems. Eastward-roving moist air masses are forced to ascend as they encounter the mountains. As the air rises, it cools and condenses, forming clouds.

Descending on the eastern slopes of the mountains, the clouds sometimes create a distinct arch-like cloud formation, known as the Chinook Arch. The clouds that make up the arch typically have a lens or saucer-like shape. The arch can stretch for miles across the sky and is often backlit by the setting or rising sun, producing stunning visual effects. Chinook Arches are most prevalent during the fall and winter months in Alberta and Montana.

I am sharing two photographs of the arch from the sunset on October 18.

—Mitchell Hegman