City Creek Nature Notes – Salt Lake City

September 20, 2016

August 18th

Filed under: Cricket, Insects, Weather — canopus56 @ 11:07 pm

Bernoulli Katabatic

6 a.m. Although the days are hot, as we move towards the equinox, the mornings have become cooler. This morning jogging up the canyon, the air has become truly cold. It is not a simple invigorating cold that chills the skin. This cold penetrates and reaches the bone. After so many days of endless summer heat, it is a welcome, refreshing change to be truly cold and it is a reminder of the winter to come. Two factors combine to make it this cold in the lower mile of a canyon. The first is the Bernoulli effect. To mile post 1, the canyon narrows and as winds come down canyon, they are constricted and pick up speed. The second cause is the morning, descending katabatic wind. Both of these increase wind speed and raise the wind chill factor. The lower temperature also affects the insects. During the heat of the day the canyon is a buggy place. It is better to keep moving and not give flying insects a place to land. But in the cold morning air, there are no flying insects. In the morning twilight, crickets have not finished their nightly chorus, but their song is oddly slowed down. Counting “one Mississippi”, “two Mississippi”, you can time the cycle of their croaking. During the hot evening, it lasts a half-second. Now it is stretched out to just short of a full second.


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