Article

09 July 2004

Sahara dust brings citizen science opportunities

Solar aureole caused by Sahara dust over South Texas on 19 June 2004. A similar display occurred on 4 July 2004. The solar disk is blocked by a dark sphere having diameter slightly greater than the camera lens. Photograph by Forrest M. Mims III. Click image to enlarge.

The 18 June 2004 issue of The Citizen Scientist presented "Citizen Science Challenge 3: Capturing storm images using cameras, weather radars and satellites."

The 25 June 2004 issue of The Citizen Scientist showed a spectacular example of this challenge, a massive Persian Gulf dust storm photographed by satellite and reported by NASA and AFCCC Strategic Weather Now (Air Force Combat Climatology Center, March 2004, Vol. 1, No. 3).

A very different kind of dust storm is now over the Atlantic Ocean, the Caribbean Sea and the Gulf of Mexico. This storm is an enormous cloud of dust from the Sahara desert that extends all the way from Africa to Texas.

Amateur scientists in the southeastern U.S. will have a good opportunity to study this event over the next week or so. Measurements of the optical thickness of the dust cloud can be made with a Sun and Sky Monitoring Station. because the light detectors in this instrument are very stable, you can calibrate it on a clear day next fall.

The Naval Research Laboratory aerosol forecast model for 1800 UTC on 12 July 2004 shows a giant river of Saharan dust extending from North Africa to Texas. Click image to enlarge.

You can also photograph the optical effects of the dust. Look for colorful twilight displays before sunrise and after sunset. If the horizon is clear of clouds, look for and try to photograph dust layers and colorful twilight glows.

The dust will cause a bright disk around the Sun known as a solar aureole. You can easily photograph the solar aureole by positioning your camera at the edge of the shade of a roof line, water tower, sign or a small sphere on a piano wire as shown nearby. (See F. M. Mims III, Solar aureoles caused by dust, smoke and haze, Applied Optics 42, 492-496, 2003. This paper is available at major university libraries and to members of the Optical Society of America at www.osa.org.)

Try to collect dust on an exposed microscope slides or clear adhesive tape left outdoors in an exposed place away from people and traffic for at least several hours. If you use tape, after the sampling period place it over clear plastic or, better a microscope slide. Then examine your sample through a microscope.

For the latest dust forecasts (see image above left), visit the excellent Naval Research Laboratory web site ar www.nrlmry.navy.mil/aerosol/.

Forrest M. Mims III