Apollo 11 Landing Site Panorama
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Supercooling may sound exotic, but it occurs pretty routinely in Earth?s atmosphere. Altocumulus clouds, a common type of mid-altitude cloud, are mostly composed of water droplets supercooled to a temperature of about -15 degrees C. Altocumulus clouds with supercooled tops cover about 8 percent of Earth?s surface at any given time.
Supercooled water droplets play a key role in the formation of hole-punch and canal clouds, the distinctive clouds shown in these satellite images. Hole-punch clouds usually appear as circular gaps in decks of altocumulus clouds; canal clouds look similar but the gaps are longer and thinner. The natural-color (top) image shows hole-punch and canal clouds off the coast of Florida, as observed on December 12, 2014, by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA?s Terra satellite.
Both types of cloud form when aircraft fly through cloud decks rich with supercooled water droplets and produce aerodynamic contrails. Air expands and cools as it moves around the wings and past the propeller, a process known as adiabatic cooling. Air temperatures over jet wings often cool by as much as 20 degrees Celsius, pushing supercooled water droplets to the point of freezing.
As ice crystals form, they absorb nearby water droplets. Since ice crystals are relatively heavy, they tend to sink. This triggers tiny bursts of snow or rain that leave gaps in the cloud cover.
The second image was made from infrared and visible light, a combination that makes it possible to distinguish between water and ice clouds. Ice clouds appear cyan; water clouds are white. Notice that the hole-punch and canal clouds consist of an ice cloud surrounded by a halo of clear sky where the water has frozen and fallen away.
Whether a cloud formation becomes a hole-punch or canal depends on the thickness of the cloud layer, the air temperature, and the degree of horizontal wind shear. Both descending and ascending aircraft?including jets and propeller planes?can trigger hole-punch and canal clouds. The nearest major airports in the images above include Miami International, Fort Lauderdale International, Grand Bahama International, and Palm Beach International. (Credit: NASA/Jeff Schmaltz, LANCE/EOSDIS Rapid Response)
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Hole-punch and canal clouds form when aircraft pass through altocumulus clouds that are rich with supercooled water droplets.
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The magnetic field along the Galactic plane
While the pastel tones and fine texture of this image may bring to mind brush strokes on an artist?s canvas, they are in fact a visualisation of data from ESA?s Planck satellite. The image portrays the interaction between interstellar dust in the Milky Way and the structure of our Galaxy?s magnetic field.
Between 2009 and 2013, Planck scanned the sky to detect the most ancient light in the history of the Universe ? the cosmic microwave background. It also detected significant foreground emission from diffuse material in our Galaxy which, although a nuisance for cosmological studies, is extremely important for studying the birth of stars and other phenomena in the Milky Way.
Among the foreground sources at the wavelengths probed by Planck is cosmic dust, a minor but crucial component of the interstellar medium that pervades the Galaxy. Mainly gas, it is the raw material for stars to form.
Interstellar clouds of gas and dust are also threaded by the Galaxy?s magnetic field, and dust grains tend to align their longest axis at right angles to the direction of the field. As a result, the light emitted by dust grains is partly ?polarised? ? it vibrates in a preferred direction ? and, as such, could be caught by the polarisation-sensitive detectors on Planck.
Scientists in the Planck collaboration are using the polarised emission of interstellar dust to reconstruct the Galaxy?s magnetic field and study its role in the build-up of structure in the Milky Way, leading to star formation.
In this image, the colour scale represents the total intensity of dust emission, revealing the structure of interstellar clouds in the Milky Way. The texture is based on measurements of the direction of the polarised light emitted by the dust, which in turn indicates the orientation of the magnetic field.
This image shows the intricate link between the magnetic field and the structure of the interstellar medium along the plane of the Milky Way. In particular, the arrangement of the magnetic field is more ordered along the Galactic plane, where it follows the spiral structure of the Milky Way. Small clouds are seen just above and below the plane, where the magnetic field structure becomes less regular.
From these and other similar observations, Planck scientists found that filamentary interstellar clouds are preferentially aligned with the direction of the ambient magnetic field, highlighting the strong role played by magnetism in galaxy evolution.
The emission from dust is computed from a combination of Planck observations at 353, 545 and 857 GHz, whereas the direction of the magnetic field is based on Planck polarisation data at 353 GHz.