The famed Chinook Arch of Alberta. The Rocky Mountains lift the warm, moist Pacific air, leading to orographic lift which wrings the clouds dry on the front slopes. Then, as the air continues to stream over the slopes it descends, drying out almost completely and the strong winds shape the remaining cloud masses as they interact with the ridges and arêtes of the easternmost ranges. The Arch is always a sign of warm, dry, and windy weather for Albertans – especially those living closer to the mountains. The effect is reduced the further east you go.
This arch was photographed in Calgary, where temperatures this week have been up around 8 – 10 Celsius.