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From: Dan Richter (1:317/3)
To: All
Date: Thu, 13.05.21 13:00
MODIS Pic of the Day 13 May 2021
May 13, 2021 - Dust and Vortices Behind the Canary Islands


Cloud vortices, Saharan Dust

Strong winds raised dust from the Sahara Desert and sculpted
paisley-like patterns in the clouds in mid-May 2021. The Moderate
Resolution Imaging Spectroradiometer (MODIS) on board NASA’s Aqua
satellite acquired this stunning true-color image of dust and vortices
swirling in the atmosphere off of western Africa on May 12.

Theodore von Ka'rma'n, a Hungarian-American physicist, was the first to
describe the physical processes that create long chains of spiral
eddies like the ones shown trailing in the wake caused by wind striking
the Canary Islands. Known as von Ka'rma'n vortices, the patterns can form
nearly anywhere that fluid flow is disturbed by an object. In this
case, the unique flow occurs as winds rush past the tall peaks on the
islands. As winds are diverted around these high areas, the disturbance
in the flow propagates downstream in the form of vortices that
alternate their direction of rotation.

On this particular day, wind also carried dust from the Sahara Desert
offshore, tinting the air south of the Canary Islands a camel-colored
tan. Some of the dust appears to be caught in the vortices, swirling
along with the cloud. The slice of silver seen at the top of the image
and crossing into the vortices is not dust. This is an optical
phenomenon known as “sunglint”. It occurs when sunlight reflects off
the surface of the ocean at the same angle that the satellite sensor is
viewing the surface. The result is a mirror-like specular reflection
off the water and back to the sensor, which, on smooth water gleams
with a silver tint.

Image Facts
Satellite: Aqua
Date Acquired: 5/12/2021
Resolutions: 1km (2.5 MB), 500m (7 MB), 250m (15.4 MB)
Bands Used: 1,4,3
Image Credit: MODIS Land Rapid Response Team, NASA GSFC

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