Have you ever wondered where the driest place on Earth is? The average person would probably guess that it is somewhere in the Sahara Desert. Personally, I would like to believe that Death Valley, California is the driest because it holds the record high temperature for North America. Technically, the real answer is the McMurdo Dry Valleys in Antarctica with an average of 0 inches (o millimeters) of rainfall per year, but since virtually no one lives in Antarctica, let’s focus on the driest non-polar desert in the world–the Atacama Desert.
Located along the coast of Chile in South America, the driest part of the Atacama Desert experiences an average of less than 0.004 inches (0.01 centimeters) of rainfall on an annual basis. It is not rare for the Atacama to go without rainfall for many years straight. As a cold desert, the Atacama has average daily temperatures ranging from 0 to 25 degrees Celsius, so any precipitation that does fall in the higher elevations will fall as snow. The Atacama Desert’s high altitude (roughly 2,500 meters) not only contributes to the cold temperatures (temperature lapse rate is about 6.5 degrees Fahrenheit per km), but it also contributes to very low humidity and dew point. While high latitude somewhat affects the south part of the Atacama, latitude is negligible due to the desert’s large vertical span.
What causes the Atacama Desert to get so dry? Well, I’ve already mentioned high altitude as a factor, but what really sets it apart from all the other non-polar deserts in the world? Two predominant factors are the Andes Mountains east of the desert and the ocean circulation patterns (gyres). It may be a bit puzzling why the Atacama Desert is so dry since it’s right by the Pacific Ocean. The trade or prevailing winds blow from the east (not the west), so when air rises due to higher terrain, the air cools and condenses on the windward side (Amazon Rainforest) and then precipitation falls. Once the air moves over the Andes Mountain Range, cooler air sinks and the clouds dissipate. On the leeward side (Atacama), the result is a rainshadow, which is an area with dry conditions and little precipitation.
Furthermore, ocean currents move counterclockwise (as opposed to clockwise) in the Southern Hemisphere, so very cold ocean air is brought from the Southern Pacific Ocean. Cold air holds less moisture than warm air, and any moisture that does make it over land condenses before it can move over the mountains to the west of the Atacama.
So far, this post has been about “severe climate,” but since the title of this blog is “severe weather,” I will bring up a recent atypical severe weather event. From March 24-26, 2015, the Atacama Desert flooded. Hold on, how is that even possible? Well, a low-pressure system from the southwest moved over northern and central Chile, dumping 1-2 inches of rainfall in a 24 hours on March 25! Considering that the average annual rainfall for the town of Antofagasta, Chile is 0.07 inches, it received about 14 years worth of rainfall in a single day!
Where did the moisture come from? The moisture from the humid tropics was dragged south and the low pressure system (clockwise rotation) was set up perfectly to bring that moisture into the Atacama Desert. Due to the hard ground and lack of vegetation, dry river beds turned into breeding grounds for flash floods, which devastated the cities of Copiapo and Antofagasta in Chile, led to a cost of $1.5 billion, displaced thousands, and claimed the lives of at least 26 people.
The recent floods in Chile show that weather is quite complex, unpredictable, and always changing. While the Atacama Desert may be the driest non-polar desert in the world climatically, one cannot depend on long-term statistics when living in a certain place. Thus, it is important to listen to meteorologists for the latest, most accurate forecasts because severe weather can happen anywhere at anytime.