The Chinook Arch of December 13, 2018

March 01, 2019

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Photographer: Rod Benson
Summary Author: Rod Benson 

The photo at top shows a classic chinook arch that appeared on December 13, 2018, in Helena, Montana. The camera is facing west toward the Continental Divide. This phenomenon is called an arch because an observer standing below it sees a curved patch of clear sky between the band of clouds and the mountains. As air flows over the Rockies it may develop a wavy up and down motion like water flowing over rocks in the rapids of a river. Although the air flows downward once it crests the highest summits, it can continue to oscillate up and down as it flows away from the mountains for several hundred miles. The upward-flowing part of this mountain wave is what forms the long arch of clouds. The clear slot between the arch and the mountains exists because the air is down-sloping here.

Air that moves down mountain slopes, in this case the east slopes of the Rockies, is warmed by compression. Then, as the wave action continues, and the air begins to rise again, it cools by expansion. If there's enough water vapor present, the arch of clouds forms as the vapor condenses into cloud droplets (or ice crystals). Typically, an extended area of clouds will form near the crest of the first wave that's then pushed eastward by higher level winds. The bottom image shows a satellite view of the distinct cloud boundary that formed along the eastern flanks of the Rockies in Montana and Alberta, during the December 13, 2018 chinook event. 

Photo Details: Top - Camera: Panasonic DMC-ZS60; Software: iPhoto 9.6.1; Exposure Time: 0.0080s (1/125); Aperture: ƒ/4.0; ISO equivalent: 80; Focal Length (35mm): 24.