October 2013

Meet Mr. Hadley: A Brief Atmospheric Understanding of the Southwest


Sam Hinkle


Ever heard of a Hadley Cell? Know what it does? Perhaps it’s time to become familiar with it, because it plays a critical role in our lives and will become even more critical in coming years.


A Hadley Cell is a type of atmospheric bombardment from the tropics. Living in the Southwest, you are no doubt familiar with its effects, including drought and destructive rain events. But are you familiar with why this aerial barrage happens in the first place? I’m not talking about the monsoons, by the way, though the destructiveness of such storms may be exacerbated by the bombardment.


Basically, the Southwestern states lie just north of a part of the Earth that experiences a constant supply of dry air from a global air circuit called the Hadley Cell. The cell is driven by differing air temperatures. Air heats up near the equator, and the warm air holds more moisture as it rises. But as the warm air rises into the colder atmosphere, it begins to cool, reducing its ability to hold more moisture. As a result, the moisture condenses in the air and eventually falls to the Earth as precipitation. The Hadley Cell air, now cold and dry, continues in its cycle, falling to the Earth at about 30 degrees North and South latitude.


Earth’s hot and dry deserts are located at and around these latitudes: The Sahara; the Atacama; the Outback country of Australia; and in North America, the Chihuahuan, Sonoran and Mojave deserts. The American Southwest, just above these dry latitudes, is either a part of these deserts or largely influenced by their presence, giving rise to arid and semi-arid climates.


That system is not an issue in and of itself, of course. However, current research shows that the Hadley Cell is expanding. That is, the drop point for that cold dry air is moving to higher latitudes. Consider, for a visual, a north-central Mexican landscape in southern Colorado. Luckily, according to research conducted by local author Bill DeBuys, the Hadley Cell expansion seems to be occurring slowly enough that it is unlikely that any of us will see a landscape change of that magnitude in our lifetimes. That being said, it is reasonable to expect the cell’s expansion will make itself known in two ways. First, the Southwest will experience increased aridity. Droughts will occur with increased severity and frequency. It is likely that the droughts of today will become the norms of tomorrow. Second, the Southwest will experience greater variability of precipitation. A general rule for what Southwesterners may see is dry events getting drier and wet events getting wetter. That means more concentrated rainfall that could lead to more powerful, destructive and frequent flooding. We have already started to see such events occur in towns like Magdalena, NM, having lost its source of water, and Manitou Springs, Colo., flooding beneath the viscous byproduct of brief, powerful rain events and recent bare fire scars.


But now we are back to the things we know: the drying, the flooding, the drought. New knowledge of the artillery that is drying this region does not change what happens when it hits the ground, but may, at least, give us an expectation of what to prepare for—a base from which we can mount a mitigation effort. In the meantime, while the bombing continues, we can take some advice from our friends, the Brits: keep calm, and carry on.


For anyone interested in further research and more detailed accounts of the impacts of Hadley Cell expansion and desertification, check out Bill DeBuys’ book, A Great Aridness. You can also visit the website: http://www.boqueteweather.com/climate_article.htm#winds



Sam Hinkle was an intern with the Quivira Coalition during the summer of 2013.





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