The Earth Story

Alabama Hills and the Sierra Nevada Mountains

Mobius Arch in California is hardly remarkable as far as natural arches go, but it draws photographers anyways because it offers the opportunity to photograph Mount Whitney, the tallest peak in the lower 48 states, framed by the arch. The jagged peaks of the Sierra Nevada Mountains contrast beautifully against the rounded contours of the 2-meter (6 feet) tall Mobius Arch and the other rocks of the Alabama Hills. Despite what their drastically different appearances might suggest, the rocks that make up the two neighboring formations are the same age - both were the result of geologic uplift 100 million years ago. Different geologic processes have shaped them since.

Sometime after the uplift, the Alabama Hills became a moist, highly vegetated region, meaning the granite was chemically weathered by percolating water. This type of erosion creates the rounded rock.

About 5 million years ago, fault-block uplift (the crust is pulled apart and some large blocks are pushed upwards while others collapse downward) created the Sierra Nevada Mountains. The mountains were shaped initially by the uplift, and more recently by glaciers.

The mountains put the Alabama Hills in a rain shadow that killed the vegetation. Over time, the soil eroded away revealing the weather rocks.

Photo Credit: Barb Ignatius http://bit.ly/1ydIpZV

References: http://bit.ly/1Fmrj98 http://bit.ly/1CpxbNb

Incredible view of clouds ending at the high peak of Nevado de Huandoy, Peru

Rain shadow

Mountainous areas usually induce precipitation by causing masses of air flowing past them to rise, cool and precipitate their moisture as clouds. The Alps and Andes get their snow and ice caps in this way, as the moist marine air of the Atlantic and Pacific respectively gets blown onto the continents by the prevailing winds. Even lower lying hilly areas such as the Welsh hills in western Britain seem to congeal the Atlantic's wetness out of the air, resulting in Britain's reputation for general soggyness. Similarly in the high Himalaya, the first chain of peaks and their foothills (still an impressive 2,500 metres high) is abundantly watered by the monsoon during the months of August to October. As the heat on the Indian plains builds up during 'summer', enhanced by the absorption of the huge areas of flood basalt known as the Deccan Trapps, the air rises, pulling in moist air off the Indian Ocean that gathers into the monsoon, assuring the subcontinent's food supply since time immemorial.

During the monsoon, the first chain of the Himalayas essentially shuts down as torrential rains cause landslides, cut roads and turn the foothills (made of unconsolidated marine muds) into a morass. In Hindu mythology this chain of peaks and hills are the dreadlocks of Siva, who collects the water poured down by Ganga, goddess of the Ganges, and channels it into the sacred rivers of India that snake down the plains toward the Bay of Bengal and Arabian seas. The water that falls as snow and compacts into ice in the fast retreating glaciers provide a store, that is released during the hot dry months allowing life below to continue.

On the other side however, it is a different story. The Tibetan Plateau is rising up piston like behind the first chain in response to the tremendous tectonic forces involved in the slow motion collision of the Subcontinent with Asia. Perched at an average 4-5000 metres, the rainfall in these barren lands is alot more scant, since all the moisture has either rained onto the Indian side or fallen as snow over the first big chain of peaks, a phenomenon known as a rain shadow. Other well known one is the Atacama desert, shielded from the moist Atlantic winds blowing across the South American landmass by the chain of the Andes.

There is some precipitation in Tibet, but the region depends on the glaciers that release the frozen monsoon down their side of the chain, forming rivers like the Brahmaputra, Yangtse and Mekong. The landscape in the photo is typical of the plateau, with a green splashed valley in which a braided river is transporting runoff and glacial melt water through a barren world, with powerful peaks rising all around.

Loz

Image credit: Coolbie Re

Why is the Atacama so dry?

The Atacama Desert is a fascinating place. It is one of, if not the driest place on the planet Earth, with some locations never having recorded precipitation. It sits in northern Chile and perhaps into some surrounding countries, and is a very unique place for geoscientists to work.

Given these properties, what makes the Atacama Desert so dry? That’s a really interesting question combining atmospherics, geology, and ocean science.

When we discussed the polar vortex a few weeks ago (https://www.facebook.com/TheEarthStory/posts/626004947460553), we described how air, heated by the sun, rises near the equator and comes back down to the north and to the south, a circulation known as the Hadley Cell pattern. Typically, air in this pattern descends close to 30 degrees north and south of the equator – very close to where the Atacama sits.

This sinking air creates deserts worldwide at these latitudes – the Sahara, the Mojave, and the Australian Outback also sit at similar latitudes north and south of the Equator.

When air rises near the equator, it cools and moves to lower pressure, causing water to condense out and rain – making the areas near the equator some of the rainiest on Earth. By the time that air starts to sink, it has mostly dried out. Sinking, dry air is one component of why the Atacama is so dry.

On top of the atmospheric circulation, there is a big contribution from ocean circulation. All of the Earth’s major oceans circulate in big, hemisphere-sized patterns called gyres. These patterns are driven by strong winds known as the trade winds near the equator. They blow consistently west and take water near the equator with them.

Warm equatorial waters are driven west – making places like the Western Pacific or the Caribbean warm. Those waters then are pushed up the coasts, moving in giant circles – west across the equator, north/south along the continents, and then back towards the equator.

In the South Pacific the waters flowing along the coast of South America are coming from Antarctica as a consequence of this global circulation. Cold water doesn’t evaporate easily, so the ocean offshore of the Atacama isn’t contributing much moisture. That’s the second ingredient in this desert.

Finally, topography plays a role. The Atacama is a basin sandwiched in-between mountains. To the east of the Atacama sits the mighty Andes mountain range, with peaks in excess of 6000 meters. Moisture carried over South America from the Atlantic runs into this giant mountain range and rains out in the Amazon basin. The eastern side of the Andes stays wet, the western side winds up as dry as any place in the world.

The western side of the Atacama doesn’t help either. The Andes are created by subduction taking place offshore. Right along the shoreline, stresses caused by the subduction zone are pushing rocks upwards as well, creating a mountain range called the Coastal Cordillera. It’s not nearly as tall as the Andes, but in places it can also be over 1000 meters in height. So, whatever water does evaporate from the cold Pacific Ocean offshore runs into this first topographic wall and is kept out of the Atacama.

Pinned in on both sides, at the perfect latitude for a desert, and close to a very cold ocean, the end result is hyper-arid conditions – conditions so dry that plant life simply can’t live there.

-JBB

Image credit: Terry Feuerborn (creative commons license) http://www.flickr.com/photos/travfotos/6799007936/

Rain in the sky and topography

Rain shadow in the Cascade Mountains.

The new LANDSAT satellite, whose entry into operations we covered back in March (http://tinyurl.com/caxyuqg) imaged the Cascades at the end of that month. The image shows the national forests to the west of the range, and the semi-desertic lands beyond, in the rain shadow of the mountains. As moist warm air blows in from the Pacific Ocean, it rises, forcing its water content to condense and fall out as rain or snow.

The Operational Land Imager snapped the area about 60 kilometres southwest of Bend, and 70 Km north of Crater Lake. This was part of a four day period when the newest LANDSAT was on the same orbital path but a few miles below LANDSAT 7. With both birds imaging the same place simultaneously, a calibration between their instruments can be done, allowing for robust data continuity between these two generations of satellite.

Loz

Image credit: NASA.

http://earthobservatory.nasa.gov/IOTD/view.php?id=80998

Andean rainshadow

Snapped looking northwards from the space station window somewhere over Chile, this stunning photo reveals why the strip between mountains and Pacific Ocean is mostly harsh and arid desert. In a usual year at these latitudes the trade winds blow over the South American continent from the Atlantic, bearing the moisture evaporated from the sea. As you can see, most of the resulting snow has fallen on the Argentinean side of the watershed, and most of the cloud lurks there too. When the oceanic air hits the mountains, it rises, and sheds its moisture as rain and snow before passing over and flowing onwards towards the Pacific.

On the western side of the chain most of the moisture has gone, and such rain as there is falls intermittently. Parts of the Atacama have not seen any recorded rainfall since the Spanish first arrived there. The clouds above the Pacific form well offshore, because a cold current of deep oceanic water is rising where the Pacific meets South America. These waters are cold enough to form fogs, but do not produce much in the way of rain bearing clouds. The barren eroding badlands that compose much of northern Chile are evident by their drab brown colours.

In an El Nino year like this one (see linked posts below) the trade winds stop and the cold current is inhibited, allowing rain to reach the coast, which often causes flash flooding. Argentina has a drier year and my home city of Montevideo is much less windy (though the trades are slowly picking up after a long absence, today they howl and dash rain against my window and I guess I'll just have to get used to their familiar presence again).

The EL Nino at work series: http://bit.ly/29YS6ps http://bit.ly/29EsXw6http://bit.ly/207KZt0, http://on.fb.me/1P1BV2O,http://on.fb.me/1OSg0dH, http://on.fb.me/1JEC5La, http://on.fb.me/1SjYm8e http://on.fb.me/1PuX6OQ, http://on.fb.me/1NUmrwU http://on.fb.me/1RT7l0M, http://on.fb.me/1mtXgKv, http://bit.ly/1SBlTkP

Loz

Image credit: Tim Peake

Rain shadow Mountainous areas usually induce precipitation by causing masses of air flowing past them to rise, cool and precipitate their moisture as clouds. The Alps and Andes get their snow and ice caps in this way, as the moist marine air of the Atlantic and Pacific respectively gets blown onto the continents by the prevailing winds. Even lower lying hilly areas such as the Welsh hills in western Britain seem to congeal the Atlantic's wetness out of the air, resulting in Britain's reputation for general soggyness. Similarly in the high Himalaya, the first chain of peaks and their foothills (still an impressive 2,500 metres high) is abundantly watered by the monsoon during the months of August to October. As the heat on the Indian plains builds up during 'summer', enhanced by the absorption of the huge areas of flood basalt known as the Deccan Trapps, the air rises, pulling in moist air off the Indian Ocean that gathers into the monsoon, assuring the subcontinent's food supply since time immemorial. During the monsoon, the first chain of the Himalayas essentially shuts down as torrential rains cause landslides, cut roads and turn the foothills (made of unconsolidated marine muds) into a morass. In Hindu mythology this chain of peaks and hills are the dreadlocks of Siva, who collects the water poured down by Ganga, goddess of the Ganges, and channels it into the sacred rivers of India that snake down the plains toward the Bay of Bengal and Arabian seas. The water that falls as snow and compacts into ice in the fast retreating glaciers provide a store, that is released during the hot dry months allowing life below to continue. On the other side however, it is a different story. The Tibetan Plateau is rising up piston like behind the first chain in response to the tremendous tectonic forces involved in the slow motion collision of the Subcontinent with Asia. Perched at an average 4-5000 metres, the rainfall in these barren lands is alot more scant, since all the moisture has either rained onto the Indian side or fallen as snow over the first big chain of peaks, a phenomenon known as a rain shadow. Other well known one is the Atacama desert, shielded from the moist Atlantic winds blowing across the South American landmass by the chain of the Andes. There is some precipitation in Tibet, but the region depends on the glaciers that release the frozen monsoon down their side of the chain, forming rivers like the Brahmaputra, Yangtse and Mekong. The landscape in the photo is typical of the plateau, with a green splashed valley in which a braided river is transporting runoff and glacial melt water through a barren world, with powerful peaks rising all around. Loz Image credit: Coolbie Re

This is a false color image from Landsat 5 showing the rain shadow in Oregon caused by the Cascade Mountain Range. A rain shadow is the result of warm moisture laden air being pushed up the windward side a tall terrestrial body such as a mountain range, a condition known as “orographic lifting”. As the air rises it becomes cooler and cooler until it precipitates on the windward side. After the air loses its moisture it continues up and over the mountain range by which time there is little to no moisture left to precipitate. One of the most extraordinary rain shadows on Earth is on the South Island of New Zealand. On the western side of the Southern Alps anywhere from 6,300-8,900 mm (250-350 in) of water (or snow at higher altitudes) precipitates per year. On the eastern side of the mountain range, yearly precipitation falls to less than 760 mm (30 in) and in some areas its half that. Do you live in a rain shadow? -NF http://earthobservatory.nasa.gov/IOTD/view.php?id=79247&src=fb