Tuesday, May 22, 2018

Little Cottonwood Terrain Shading

I'm on a short road trip to La Jolla, California where I'm visiting the Center for Western Weather and Water Extremes at the Scripps Institution of Oceanography.  There's a lot of great research happening here, so the trip has been quite invigorating.  Morning walks on the beach have been nice as well, although San Diego has been draped in "May Gray" since I got here. 

Flying out of Salt Lake City late Sunday revealed some curiosities about terrain shading in Little Cottonwood Canyon.  In particular, note how the high terrain north of the canyon is casting a shadow on the north facing sidewall. 

This can happen at our latitude between the spring and fall equinoxes when the sun sets north of west and is most pronounced at the summer solstice (late June).


  1. A field program that looked at thermally driven flows across the shade boundary would be super interesting

  2. I always thought that the sun would be south of west in Utah. Clearly, this isn't the case. But my understanding was that the tropics were defined as the region in which the sun would be at our above that latitude (which is effectively a line from east to west) at least once per year. Can you explain this discrepancy?

    1. The sun traces an arc in the sky. From the fall to spring equinoxes, at our latitude, the sun is always south of west (or east). However, from the spring to fall equinox, the sun rises north of east and sets north of west. At noon, however, the sun is south of east and west. This is a consequence of the earth being a sphere and the tilt of the Earth's axis. A sketch showing this is at the top of https://www-spof.gsfc.nasa.gov/stargaze/Secliptc.htm.

      I think your comment on the tropics would apply at solar noon. If you are on the tropic of cancer at solar noon on the summer solstice, the sun should be directly overhead. It would rise and set, however, somewhat north of east and west, respectively.