Weather 101 '" a Look at Cloud Motion

Jeff, Julie and Jane of the Banner Marketing Group wrote to me a while ago asking about variations in low-level cloud motion between the California and Oregon coastal regions. According to the troika, motion of stratus clouds, the low-lying clouds in question, was much slower in California. Now they reside at an altitude of about 500 feet above sea level in Sutherlin, OR.

The group offered several possible reasons for the difference in the speed of cloud motion:

Is it due to the fact that we are further north?
Is it because we are further inland?
Is it because we are at a higher elevation above sea level?
Is it a combination of any or all of the reasons above?

Actually any of the three proposed answers could be a factor and there are even some other possibilities.

First, let's address climatology. Large-scale wind and storm patterns favor stronger winds along the Oregon coast than the California coast, although there can be significant local variations. Sometimes these variations can occur over very short distances due to the coastal geography of the U.S. West Coast.

Low cloud motion in California, however, can be quite fast. I've lived in the San Francisco Bay region and can attest to the fact that when the sea breeze pushes ashore on occasion, stratus clouds can race across the sky as fast as they do in a northeast winter storm. This situation occurs most often when Central Valley temperatures are very warm (much warmer than average), while coastal readings are quite cool (as they usually are).

Typically, cloud motion is usually faster along the coast, through narrow coastal valleys and at places near the tops of coastal hills. As air is forced past the coastal hill peaks, it speeds up due to the Bernoulli Effect. This is much like what happens when air flows over the wing of an aircraft. In urban settings, flow through narrow valleys is akin to wind flow between buildings. This is known as the "concrete canyon" effect.

Also, the closer one is to the clouds, the faster they seem to move. Doing some right triangle geometry - a.k.a. trigonometry (see Figure 1) - a cloud 500 feet above the ground (stratus), moving at 3.5 miles an hour, will transcend an angular distance of about 30 degrees. A cloud at 5,000 feet above the ground (cumulus), moving at the same speed, would move a distance of about 3 degrees. A cloud at 35,000 feet (cirrus), again, moving at the same speed, would barely move 1 degree.

If you allow clouds at all altitudes to traverse the same angle of view, the high clouds, due to their greater distance from you, will have to move much faster. This comes from viewing a nested array of similar triangles.

Looking up

Now watch clouds of all types as they move across the sky. Chances are you will say that the lower clouds move faster (even though the higher clouds may actually be moving the fastest).

To visualize this, extend your arm ahead of you and move it horizontally across your field of view. Which part of your arm (hand, wrist, elbow, upper arm moves fastest? It has to be your hand because it covers more distance as it moves across the same angular measure in the same time.

Hence, given the same angle of motion, clouds further away from you have to be moving faster.

Back to the question

Unfortunately, the actual elevation of the clouds in both settings proposed by Jeff, Julie and Jane is unknown, other than the clouds are stratus and likely somewhere around 1000 feet above the ground.

Since the trio is likely closer to cloud base in Oregon, the clouds should appear to be moving faster there than they did in California.

© H. Michael Mogil, 2010