The Secondary Cause of North American Deserts

[Please see the three videos posted in order, a bit earlier this evening.]

From the time that I took a Weather and Climate course while in grad school, during the Fall of 1987 until I started teaching Environmental Science in junior college in 2001, I didn't give much thought to Hadley Cells. 

After I met my first wife, Marla, in early 1983, I gained so much more focus and purpose than ever before.  I made an "A" in that Weather and Climate class and in the Physical Geography Lecture and Lab courses, as well, for a "4.0 semester".

Though we did cover the concept of Hadley Cells, I don't recall if it was given more than a passing comment as being responsible for vertical air-circulation patterns that produce most of the world's mid-latitude deserts, a process that can locally be enhanced by the Rain Shadow Effect.  (As briefly covered, yesterday.)  

For the sake of review, in 1735, George Hadley proposed a single, large vertical atmospheric circulation cell (in each latitudinal hemisphere) as a major factor behind global wind patterns.  Uplift provided by Equatorial Heating was the driving force behind each circulation cell, with the atmospheric winds cooling and falling as they reached the Poles.

Over time, further study revealed that Earth's circulation likely prevented the existence of a single large circulation cell, making the 3-cell model more plausible.  With the acceptance of that model, the term "Hadley Cell" was re-applied to the two cells adjacent to the Equator and driven by persistent Equatorial Heat in the "Equatorial Low-Pressure Zone".

In both cases, the upper Troposphere Winds "fall" at approximately 30 degrees North and South, creating a persistent zone of High Pressure, now referred to as the "Mid-Latitude High-Pressure Zone" or the "Subtropical Ridge" (of High Pressure).  As with the Rain Shadow winds, the falling air warms and dries, suppressing cloud formation and growth.  Between 30 and 60 degrees Latitude (North and South), this cell is now referred to as the "Farrell" or "Ferrel" cell.

At approximately 60 degrees North and South of the Equatorials are weak zones of Low-Pressure and rising air, called the High-Latitude Low-Pressure Zone."  Between there and the North (or the South Pole) is referred to as the Polar Cell, in each hemisphere.

[References will be given tomorrow.  I didn't plan on both blogs posting the same things, but the hubbub of the holiday weekend made that a temporary necessity.]