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July 8, 2011 / Dave Gorham

Driest of Deserts Experiences Rare July Snowfall

In one of Mother Nature’s oldest tricks, those of us “here” are suffering through the blistering heat of summer, while those of us “there” are in the grips of winter. In this case, “there” is south of the Equator and winter is not only in full effect, it’s snowing in places that haven’t seen snow in 20 years.

This BBC image from the Atacama Desert shows stranded vehicles, workers with shovels — and significant snow in one of the driest places on Earth.

The Atacama Desert is one of the driest deserts known on the planet. In fact, experts call it hyper-arid. Like the Columbia Plateau of the Pacific Northwest and Washington State, Atacama finds itself between two mountain ranges which results in a rain shadow no matter from which direction a moist wind may blow.

A rain shadow is formed when moisture-laden winds deposit heavy rain (or snow) on the windward side of a mountain as orographic lift condenses the moisture into precipitation. As the wind rushes over the mountain top, any residual moisture is evaporated as the wind heats up and rushes downhill.  Regions on the lee side of a mountain range are in a rain shadow and typically receive significantly less precipitation than those areas on the windward side, just a few miles away. A double rain shadow occurs when a region is located between two mountain ranges. No matter which way a moist wind may blow, by the time it reaches the lee side of the range, the moisture is gone. This is Atacama’s predicament, as it it located between the Andes to the east and the Chilean Coastal Range to the west. Additionally, Atacama finds itself situated just far enough north so that it misses out on the intense and widespread weather systems passing over southern South America as they transition from the South Pacific to the South Atlantic.

The Rain Shadow effect. Orographic lift condenses moisture into precipitation on the windward side of the mountain, leaving no moisture for precipitation on the leeward side. Image: Wikipedia

That’s not to say moisture can’t come from another direction. However, it’s the prevailing or predominant wind that is responsible for annual rainfall. If the predominant wind is always squeezed free of precipitation, the area will likely find itself classified as a desert.

What has accounted for the Atacama snow then? Not what you might expect. As surface weather features are influenced by the region’s topography, precipitation must then come from a source that would have to be something other than a surface system, and that is indeed the case today. ImpactWeather’s Gmaps-based surface frontal forecast maps indicate the Atacama Desert to be located between two (dry) high pressure systems. Any student of meteorology will tell you that where you have two high pressures, there must be a low pressure somewhere in between. However, due to the dramatic mountain ranges of this part of the world, surface features such as cold fronts, warm fronts and troughs are not only difficult to analyze on a weather map, they have a difficult time maintaining their identity as move over, around and through the canyons and mountains. Yet surface features are not independent features left to wander on their own but rather a reflection of a more organized pressure system in the middle and upper atmosphere. These mid- and upper-level systems typically come with plenty of moisture and, if everything lines up just right, rain (or snow) can fall into rain shadows from systems too high in the atmosphere to be influenced by mountains and orographic lift.

Surface features indicate Chile's Atacama Desert between two (dry) high pressure centers. Image: ImpactWeather Gmaps

Indeed this is the case over Atacama. Notice the ImpactWeather Gmaps surface features to the left — two high pressure systems, yet nothing in between. As mentioned previously, surface features are difficult to track and analyze through the mountains, but a low pressure area is indeed present. Now notice the same ImpactWeather Gmaps image below, with the overlaid infrared satellite image. The mid-level low pressure area, along with the heavy clouds (indicating likely precipitation – there’s no weather radar in this region of the world) is easy to see directly over the Atacama Desert.

With infrared satellite imagery, it's easy to see the mid-level storm system responsible for the Atacama snow. Image: ImpactWeather Gmaps

Finally, snow in July? Let’s not forget that when the Northern Hemisphere tilts toward the sun for summer, the Southern Hemisphere enters its winter phase. Southern Hemisphere snow, especially in July, is not uncommon — except in the Atacama Desert.





















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