Dust Particles
Click on the image for Quicktime movie from 7/15-7/24
A continent-sized cloud of hot air and dust originating from the Sahara
Desert crossed the Atlantic Ocean and headed towards Florida and the
Caribbean. A Saharan Air Layer, or SAL, forms when dry air and dust rise
from Africa's west coast and ride the trade winds above the Atlantic
Ocean.
These dust clouds are not uncommon, especially during the months of July
and August. They start when weather patterns called tropical waves pick up
dust from the desert in North Africa, carry it a couple of miles into the
atmosphere and drift westward.
In a sequence of images created by data acquired by the Earth-orbiting
Atmospheric Infrared Sounder ranging from July 15 through July 24, we see
the distribution of the cloud in the atmosphere as it swirls off of Africa
and heads across the ocean to the west. Using the unique silicate spectral
signatures of dust in the thermal infrared, AIRS can detect the presence
of dust in the atmosphere day or night. This detection works best if there
are no clouds present on top of the dust; when clouds are present, they
can interfere with the signal, making it much harder to detect dust as in
the case of July 24, 2005.
In the Quicktime movie, the scale at the bottom of the images shows +1 for
dust definitely detected, and ranges down to -1 for no dust detected. The
plots are averaged over a number of AIRS observations falling within grid
boxes, and so it is possible to obtain fractional numbers.
Total Water Vapor in the Atmosphere Around the Dust Cloud
Click on the image for Quicktime movie
The dust cloud is contained within a dry adiabatic layer which originates
over the Sahara Desert. This Saharan Air Layer (SAL) advances Westward
over the Atlantic Ocean, overriding the cool, moist air nearer the
surface. This burst of very dry air is visible in the AIRS retrieved total
water vapor product as a region of depressed water vapor (brown in the
images) migrating slowly Westward toward the Caribbean. The SAL phenomenon
inhibits the formation of tropical cyclones and thus has given the West
Indies and the East Coast of the US a respite from hurricanes.
The Atmospheric Infrared Sounder Experiment, with its visible, infrared,
and microwave detectors, provides a three-dimensional look at Earth's
weather. Working in tandem, the three instruments can make simultaneous
observations all the way down to the Earth's surface, even in the presence
of heavy clouds. With more than 2,000 channels sensing different regions
of the atmosphere, the system creates a global, 3-D map of atmospheric
temperature and humidity and provides information on clouds, greenhouse
gases, and many other atmospheric phenomena. The AIRS Infrared Sounder
Experiment flies onboard NASA's Aqua spacecraft and is managed by NASA's
Jet Propulsion Laboratory, Pasadena, Calif., under contract to NASA. JPL
is a division of the California Institute of Technology in Pasadena.
