History of the Saharan Dust

And its effects on our air quality

courtesy NASA

Blog updates are in bold: A lot of people ask me how long we’ve known about Saharan dust. One of the pioneers of dust study, Dr. Joseph Prospero, saw this blog and sent me an email. He’s been studying African dust since the mid 1960s and sent me a link to a paper he co-wrote about the history of dust studies. One interesting fact is that the early dust discoveries occurred while three separate scientists were looking for other atmospheric phenomena (like cosmic dust). In fact, at first no one quite understood the impacts of African dust on our oceans, ecosystems and air. That has changed, of course, and our understanding continues to increase. Thanks to Dr. Prospero for some great information! I encourage you to have a look at his paper.

In addition, Dr. Prospero recently published a study along with Texas A&M professor Shankararaman Chellam et al on the effects of African dust on air quality in Houston and Galveston. You can find that study right here. It’s scientific with the bottom line being that during a big dust outbreak, that dust accounts for 32-39% of aerosols--more than vehicle emissions!

Believe it or not, half of Saharan dust that comes our way begins in one place and you see it in today’s cover shot: the Bodélé Depression (pronounced Bah-del). Just what and where is this dust source in the Sahara?

First the what. Over thousands of years, the once huge Lake Chad (about the size of Lake Erie and now only 5% of that) dried up leaving silt and sediment in the hot desert sun which baked into a fine dust. This bowl, or depression, of dust is northeast of the current Lake Chad and is 310 miles long, 93 miles wide and about 500 feet deep. In fact, it only takes up .2% of the whole Sahara Desert and yet is THE primary source of all this dust--hundreds of thousands of tons of it every year! You can see on the map below the exact location of the Bodélé Depression. Other countries to the west--Algeria, Mali, Mauritania and Algiers supply airborne dust due to the very hot 100+ temperatures causing rising air which takes the dust upward where winds transport it westward.

The Bodele Depression is in the southern Sahara Desert just northeast of Lake Chad

Imagine, this relatively small geographical spot produces tons of dust that travels all the way across the Atlantic to Texas!

I've drawn the path from the Sahara dust source to Texas

Of course, the dust affects much of the rest of the southeast United States, Caribbean, South America, and even Europe. To be sure, the dust is full of iron rich minerals and phosphorus and without this transport of rich nutrients to the Amazon forests in South American, those forests would likely die. And the dust inhibits hurricanes by producing a dry atmosphere and downward motion of air. We’ve seen that this year. On the downside, the dust causes serious nasal congestion and even deadly breathing problems for some.

So how does such a small space produce so much dust? To the north of the Bodélé Depression are two mountain ranges, the Tibesti and the Ennedi, and between the two a wind tunnel is created. Those strong winds lift the dust into the atmosphere and then upper level easterly trade winds move it across Africa and the Atlantic. Here’s a map illustrating the geography and I’ve drawn in the wind tunnel arrows:

Photo from NASA

In addition, storms to the south of the Sahara Desert in the Sahel region, move across the continent creating winds that also carry dust. You can see the forecast there for more storms this week and they are pretty constant this time of year:

courtesy tropicaltidbits.com

But notice the storms, which can be seeds for hurricanes in the Atlantic, wither away over the ocean--a victim of their own dusty transport? Here is a look at the upper level winds and it’s easy to pick out the easterly flow--those are the blue colors and arrows in the bottom part of the graphic:

The winds in the lower part of the screen are the easterly trade winds. Courtesy tropicaltidbits.com

Back to the original question: Why has there been so much dust this year? I’m not honestly sure we can know. An active upper level wind pattern, more storms crossing the continent, perhaps a layer of finer silt and sediment in the Bodélé Depression. Certainly a warmer atmosphere creates stronger winds (a matter of steeper pressure gradients which is a more meteorological answer), and a warmer atmosphere may create more rising dusty air from the desert. This one small area of the Sahara is not easy to study, but it IS being studied. As for how long these dusty days last, they usually wane by mid-August.

In the meantime, be glad for the inhibited tropical storm season as new forecasts have just come out and they haven’t gone down but by one storm! An active hurricane season is still forecast--just over the weekend, we had an African wave emerge and it has a 40% chance to develop this week.

Frank

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About the Authors:

KPRC 2's chief meteorologist with three decades of experience forecasting Houston's weather.