۱۳۹۱ اسفند ۹, چهارشنبه
Criss-Crossing Contrails
The condensation trails that form behind high-altitude aircraft,
or contrails, are one
of the most visible signs of the human impact on the atmosphere. On February
15, 2013, the Moderate Resolution
Imaging Spectroradiometer(MODIS) on NASA’s Terra satellite
captured this view of numerous contrails over Portugal and Spain.
The composition of these long, narrow clouds is virtually
identical to naturally-forming cirrus
clouds. However,
while naturally high levels of humidity cause the clouds, contrails form when
airplanes inject extra water vapor into the atmosphere through their exhaust.
In order for contrails to develop, air temperatures must be -39°C (-38°F) or
below.
The humidity of the air affects how long contrails last. When
air is dry, contrails last just seconds or minutes. But when the air is humid,
contrails can be long-lived and spread outward until they become difficult to
distinguish from naturally occurring cirrus clouds. Satellites have observed
clusters of contrails lasting as long as 14 hours and traveling for thousands
of kilometers before dissipating; however, most remain visible for only a few
hours.
Contrails have an impact on climate. Long-lived, spreading contrails like
the ones shown here are of particular interest to researchers because they
reflect sunlight and trap infrared radiation.
A contrail in an otherwise clear sky reduces the amount of solar radiation that
reaches Earth’s surface, while increasing the amount of infrared radiation
absorbed by the atmosphere. These opposing effects make it difficult to sort
out the overall impact on climate.
However, a group of scientists at NASA’s Langley Research Center
have made progress. They have developed a computer algorithm that searches
through data from MODIS and automatically distinguishes between natural cirrus
clouds and young- to medium-aged contrails. This has made it easier to estimate
how much contrails contribute to overall cirrus and cloud coverage. In a study published
in 2013, the group estimated that contrails cover between 0.07 percent to 0.40
percent of the sky in a given year. They also concluded that contrails produce
a slight net warming effect on the Earth.
There are still problems the researchers are working to solve.
“Detecting the older, wider contrails, like many of those in this MODIS image,
remains a challenge and we are still unable to estimate their coverage and
impact on climate as well as we would like,” noted Patrick Minnis, a NASA
Langley scientist.
·
References
· Duda, D. (2013, Feb. 11). Estimation
of 2006 Northern Hemisphere contrail coverage using MODIS data. Geophysical Research Letters.
· Spangenberg, D. (2013, Feb. 11). Contrail radiative forcing over the Northern Hemisphere from
2006 Aqua MODIS data.Geophysical Research Letters.
NASA image by Jeff Schmaltz, LANCE/EOSDIS MODIS Rapid Response. Caption by Adam Voiland, with information from Patrick Minnis
and Douglas Spangenberg.
Instrument:
Terra -
MODIS
Mount Etna Boils Over
After maintaining a low simmer for ten months, Italy’s Etna volcano boiled
over on February 19–20, 2013, with three outbursts in 36
hours. According to the Italian Istituto Nazionale di Geofisica e Vulcanologia,
each outburst (paroxysm) featured “emission
of lava flows, pyroclastic flows, lahars, and an
ash cloud.”
The Advanced Land Imager (ALI) on the Earth Observing-1 (EO-1)
satellite captured Mount Etna on February 19 at 9:59 a.m. Central European
Time, about 3 hours after the end of the first paroxysm. The false-color image
combines shortwave infrared, near-infrared, and green light in the red, green,
and blue channels of an RGB picture. This combination makes it easier to
differentiate between fresh lava, snow, clouds, and forest.
In the image, fresh lava is bright red, as the hot surface emits
enough energy to saturate the instrument’s shortwave infrared detectors but is
dark in near-infrared and green light. Snow is blue-green because it absorbs
shortwave infrared light, but reflects near-infrared and green light. Clouds
made of water droplets (not ice crystals) reflect all three wavelengths of
light similarly and appear white. Forests and other vegetation reflect
near-infrared more strongly than shortwave infrared and green, and so appear
green. Dark gray areas are lightly vegetated lava flows, 30 to 350 years old.
1. References
2.
Abrams, M, Bianchi, R,
and Pieri, D. (1996 December) Revised Mapping of Lava Flows on Mount Etna, Sicily.Photogrammetric Engineering & Remote Sensing. 62(12), 1353–1359.
3.
Cooperative Institute for
Research in the Atmosphere. (n.d.) Snow/Cloud Discriminator (3-color technique)—Basic Information. Accessed February 20, 2013.
4.
Istituto Nazionale di
Geofisica e Vulcanologia, Sezione di Catania. (2013, February 19) Etna and Stromboli update, 19 February 2013. Accessed February 20, 2013.
5.
Istituto Nazionale di
Geofisica e Vulcanologia, Sezione di Catania. (2013, February 20) Etna update, 20 February 2013.Accessed February 20,
2013.
·
Further Reading
NASA Earth Observatory image by Jesse Allen and Robert Simmon,
using EO-1 ALI data from the NASA EO-1 team.Caption
by Robert Simmon.
Instrument:
EO-1 -
ALI
Bushveld Igneous Complex
With uses ranging from jewelry to catalytic converters, platinum ranks
among the most prized and most expensive metals. About 70 percent of the
world’s platinum is mined in the Bushveld Igneous Complex in South Africa—a
geological formation roughly the same size as West Virginia. The Bushveld also
supplies significant quantities ofpalladium, rhodium, chromium, and vanadium.
The Advanced Spaceborne
Thermal Emission and Reflection Radiometer (ASTER) on NASA’s Terra satellite acquired this image of the Bushveld Igneous Complex on
October 24, 2006. It shows part of the Bushveld Complex, an area around the
Bospoort Dam.
ASTER combines infrared, red, and green wavelengths of light to
make false-color images. In this image, water appears in shades of blue, with
darker blue indicating greater water depths. The pale blue polygonal shapes
indicate reservoirs and tailings ponds associated with mining operations. Bare
rock and sparsely vegetated land appear in shades of red and red-brown.
Vegetation is green.
Bushveld is an example of a large igneous province, a massive
assemblage of rocks formed by volcanic activity. The Bushveld is not only big
in land area; its rock layers are several kilometers thick. The complex
consists of multiple “suites” of rocks, each of which in turn holds multiple
layers. Different layers are sources of different types of valuable metals;
some favor platinum, for example, while others are rich in chromium.
Bushveld is unusual in that, despite its great age—more than 2
billion years—it has not been significantly deformed by subsequent tectonic
activity. (It has undergone extensive erosion.) Despite years of extensive
study in the area, geologists have not reached a consensus about how this
igneous complex formed. One hypothesis is that the complex is a single, massive
feature shaped like a giant bowl. Others suggest that it consists of discrete,
disconnected structures. In either case, the complex might have received
multiple infusions of magma from different sources.
Although there is little agreement about precisely how it
formed, dating of the rocks from the Bushveld indicates that the igneous
complex formed over a relatively short time period, perhaps less than 10
million years. Before dating techniques constrained the ages of the rock layers
to a time around 2.06 billion years ago, many geologists suspected that the
complex formed over a period that might have exceeded 100 million years.
1. References
2.
ASTER. Bushveld Igneous Complex, South
Africa. NASA
Jet Propulsion Laboratory. Accessed February 21, 2013.
3.
Buck, J.S., Maas, R.,
Gibson, R. (2001) Precise
U-Pb titanite age constraints on the emplacement of the Bushveld Complex, South
Africa. Journal of the
Geological Society, 158(1), 3–6.
4.
Kinnaird, J.A. The Bushveld Large Igneous Province. School of Geosciences, University of the Witwatersrand. Accessed
February 21, 2013.
5.
Schouwstra, R.P.,
Kinloch, E.D., Lee, C.A. (2000). A short geological review of the Bushveld Complex. Platinum Metals Review, 44(1),
33–39.
6.
State of the Planet. Bushveld Igneous Complex. The Earth Institute. Columbia University. Accessed February 21,
2013.
NASA Earth Observatory image by Jesse Allen and Robert Simmon,
using data from NASA/GSFC/METI/ERSDAC/JAROS, and U.S./Japan ASTER Science Team. Caption
by Michon Scott.
Instrument:
Terra -
ASTER
۱۳۹۱ اسفند ۸, سهشنبه
Iran’s Great Salt Desert
Roughly 300 kilometers (200 miles) east-southeast of Tehran lies
Iran’s Dasht-e Kavir, or Great Salt Desert. To the untrained eye, Dasht-e Kavir
looks like a place that has been bone-dry since the dawn of time. But to the
well-trained eyes of a geologist, this desert tells a tale of wetter times.
Tens of millions of years ago, a salt-rich ocean likely occupied this region,
surrounding a microcontinent in what is now central Iran.
The Thematic Mapper on the Landsat 5 satellite
captured this natural-color image of Dasht-e Kavir on October 15, 2011. The top
image is a wide-area view, and the area outlined in white is then shown in the
close-up view below.
Dasht-e Kavir is a complex landscape, but it can be mostly
explained by the invasion and subsequent evaporation of an ancient ocean. As
the ocean dried up, it left behind a layer of salt as much as 6 to 7 kilometers
(4 miles) thick. Salt has a fairly low density, so if a layer of new rock
buries the salt layer—and if that overlying rock is soft enough—the salt can
slowly push up through it and form domes.
As its name implies, the Great Salt Desert is rich in salt
domes, or diapirs. Geologists have identified about 50 large salt diapirs in
this region. Like any other surface feature, a salt dome is subject to erosion.
Wind and rain scrape away particles of rock, gradually wearing away the top of
the dome and exposing it in cross-section.
But erosion is not the only force at work in this region. In the
close-up view, we can see north-south-trending structures, some raised and some
lowered. Callan Bentley, a geologist at Northern Virginia Community College,
identifies them as folds or fault zones that
run parallel to the trend of the region’s mountains. Bentley attributes the
deformation of the salt domes to plate tectonic activity that has occurred
since the salt domes formed. Bentley describes the landscape as a “a palimpsest tale
that helps constrain the age of the diapirism to pre-folding.”
1. References
2.
Arian, M. (2012) Clustering
of diapiric provinces in the Central Iran Basin. Carbonates and Evaporites, 27(1),
9–18.
3.
Rahimpour-Bonab, H.,
Shariatinia, Z., Siemann, M.G. (2007) Role of
rifting in evaporite deposition in the Great Kavir Basin, central Iran. Geological Society, London, Special Publications, 285,
69–85.
NASA Earth Observatory image by Jesse Allen and Robert Simmon,
using Landsat data from the U.S. Geological Survey. Caption by Michon Scott with information from Callan Bentley, Northern Virginia Community College.
Instrument:
Landsat
5 - TM
Where the Danube Meets the Black Sea
The Danube River is the largest in the European Union, its
watershed draining 801,463 square kilometers (309,447 square miles) of land
across 19 countries. Where that great river reaches the Black Sea, a remarkable
delta has formed—the “Everglades” of Europe. The Danube Delta is home to more
than 300 species of bird and 45 species of freshwater fish.
The Danube Delta has been home to human settlements since the
end of the Stone Age (the Neolithic Period), and the ancient Greeks, Romans,
and Byzantines all built trading ports and military outposts along this coast.
Today, the border between Romania and Ukraine cuts through the northern part of
the delta. The area is a United Nations World Heritage Site, both for its
natural and human history, and for the traditional maritime culture that persists
in its marshes. All the while, the landscape has been shaped and re-shaped by
nature and man.
The image above was acquired on February 5, 2013, by the
Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1)
satellite. The Danube Delta has a number of lobes formed over the past several
thousand years, and this image is focused largely on the northernmost Chilia
(or Kilia) lobe. It is the youngest section of the delta—somewhere between 300
to 400 years old—and lies mostly within Ukraine. Much of the land in the image
above is officially considered part of the Danube Biosphere Reserve. (To see
more about how the delta formed, click
here.)
Near the center of the image, the small city of Vylkove is known
as the “Ukranian Venice,” due to its canals. To the lower left, the older
Sulina lobe of the delta stretches to the south and further inland into
Romania. White and brown curved lines reveal beach ridges and former
shorelines, with the whiter ridges composed almost entirely of pure quartz sand
in high dunes. To the east of the ridges, most of the landscape is flat
marshland that is mostly brown in the barren days of winter.
The Bystroye Canal through the center of the Chilia lobe has
been the subject of heated debate over the past two decades. Over the
centuries, damming and channeling of the Danube throughout Europe has reduced
its water flow and sediment load to roughly 30 percent of what it once was,
according to coastal geologist Liviu Giosan of the Woods Hole Oceanographic
Institution. In recent years, the Ukrainian government has dredged some delta
channels (including Bystroye) and proposed extensive dredging of others in
order to provide navigational channels for large ships. Proponents argue for
the economic needs of water transportation routes. Opponents note that deeper,
faster channels mean less mud and sand is deposited in the delta; in some
places, more is carried away by swifter currents. Both affect the sensitive
ecosystems and the ability of the delta to restore itself and grow.
In a 2012 report led by Giosan, scientists noted that the shape,
water chemistry, and biology of Danube Delta was being altered long before the
modern Industrial Era. Land use practices—particularly farming and forest
clearing—added significant amounts of nutrients into the water and reduced
salinity in the Black Sea, changing the dominant species of phytoplankton and
sending a ripple of effects through the entire food web.
1. Related Reading
2.
Carlowicz, M.J.,
(2005, July 11) The
Once and Future Danube River Delta. Oceanus. Accessed
February 15, 2013.
3.
Der Spiegel (2007, October 4) The
Shipping News: Construction Threatens Danube’s Natural Paradise. Accessed February 15, 2013.
4.
Giosan, L. et al. (2012) Early Anthropogenic
Transformation of the Danube-Black Sea System. Scientific Reports 2,
582.
5.
International
Commission for the Protection of the Danube River.Working for Danube River Basin
and Its People. Accessed
February 15, 2013.
NASA Earth Observatory image by Jesse Allen and Robert Simmon,
using EO-1 ALI data provided courtesy of the
NASA EO-1 team and
the U.S. Geological Survey. Caption by Mike Carlowicz.
Congratulations to Kevin
Martin,CEO/senior meteorologist for TheWeatherSpace.com, for being the
first person to solve the puzzler.
Instrument:
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