A Closer Look at Dust

                    A wide plume of dust blowing off the Saharan Desert toward the Canary Islands.  Credit: NASA Earth Observatory

Each summer, sandstorms lift millions of tons of dust from the Sahara, carrying plumes of it off the West Coast of Africa and over the Atlantic Ocean. Eric Wilcox, a researcher at NASA’s Goddard Space Flight Center, has been using data from NASA satellites to examine the impact such storms can have on rainfall patterns. Wilcox has a new paper about his findings in Geophysical Research Letters; Nature Geoscience also highlighted it in a recent issue. As a result, we sat down with Wilcox to discuss his new findings and some little-known details about dust.

WhatOnEarth: How does dust affect the atmosphere?

Wilcox: Well, we know that dust isn’t just a passive particle floating around. We know it can either absorb or scatter sunlight. Dust outbreaks surely cool the planet by reducing the amount of sunlight that reaches the surface. Likewise, we know dust can warm the atmosphere, although the magnitude is still uncertain.

WhatOnEarth: What causes a particle to absorb rather than scatter light?

Wilcox: It has to do with the color of the particle and, to some degree, the shape. We call this property the single scattering albedo, which is the probability that a photon of light will scatter versus getting absorbed. If the scatter is very high—say 0.99—then we’re confident that 99 out of 100 photons will scatter. If it’s lower, say 0.85, that means there’s a 15 percent chance that the proton will be absorbed.

WhatOnEarth: Where does Saharan dust fit in?

Wilcox: Some Saharan dust is quite bright and some is much darker. It depends on where the sand is coming from and what its mineralogy is like.

WhatOnEarth: Why does it matter if dust is absorbing light?

Wilcox: We’ve found that dust outbreaks, along with other factors, seem to be shifting tropical precipitation (which typically occurs in a narrow band where winds from the northern and southern hemispheres come together) northward by about four or five degrees, which is about 240 to 280 miles at the equator.

WhatOnEarth: Really? What does dust have to do with precipitation?

Wilcox: The main pathway for dust off the Sahara is usually well north of the band of tropical Atlantic rain storms. However, dust storms coincide  with a strong warming of the lower atmosphere, so the atmospheric circulation over the ocean responds to that warming by shifting wind and rainfall patterns northward during the summer. The rainfall responds to the passage of a dust storm even if the dust does not mix with the rain. 

WhatOnEarth: Will the upcoming Glory mission help you study this phenomenon? I know it has an instrument that will measure aerosols such as  dust? 

Wilcox: Definitely. Over bright reflective surfaces such as deserts — where it has been nearly impossible to distinguish aerosols from the surface — we’re at the point that any new information will be helpful. 

WhatOnEarth: What’s the significance of a northward migration of rainfall during dusty periods?

Wilcox: Certainly people local to the area have an interest in understanding how dust affects their rainfall patterns. The finding also lends support to an idea from one of my colleagues—Bill Lau—who studies the elevated heat pump.The idea is that aerosols from dust storms and air pollution actually affect monsoons. Space Flight Center.

Image Details: The lead image was acquired by the MODIS Land Rapid Response Team at NASA’s Goddard Space Flight Center in 2004.

— Adam Voiland, NASA’s Earth Science News Team

14 thoughts on “A Closer Look at Dust”

  1. I live in Florida, sometimes we have this red “dust” and/or yellow “dust” (? from Africa ?) Does this affect our weather?

  2. Hello.
    I read about the dust blowed off from Saharan Desert to the Atlantic Ocean. How the perspective of all the dust of that desert be blowed to the sea. Someday that sand will desappear from there?
    I am from Brasil.
    Thanks for your attention.

  3. Thanks for the question! Indeed dust storms from the Sahara do periodically reach the Caribbean and South Florida during the summer months. This can lead to a very hazy day in Miami. Saharan dust can even be identified as a constituent of soils in the Caribbean. As discussed above, dust can affect the temperature of the atmosphere which we have found to have an effect on weather over the Tropical Atlantic Ocean. So far, it is unclear whether enough dust remains in the atmosphere during the long trip across the ocean to have a meaningful impact on weather in Florida. There is some evidence that dust near Florida may be mixing with clouds, which could influence how much rain is produced by the cloud. This is a challenging problem that the scientific community is still working on. It is important to keep in mind that the effects of dust we are studying may be important over a season or years, but are relatively subtle shifts in rainfall compared to typical large variations in day-to-day weather that we experience in our daily lives.

    -Eric Wilcox, NASA’s Goddard Space Flight Center

  4. Thanks for your question, Maria. Indeed a very large amount of dust is blowing away from the Sahara Desert and out over the Atlantic Ocean. A recent NASA study estimated that about 230 teragrams of dust is blowing out over the ocean from the Sahara (1 teragram is one trillion grams). Being from Brazil, it may be interesting to you to know that those same scientists estimated that as much as 45 teragrams of that Saharan dust falls upon the Amazon basin every year. It is possible that if climate conditions remain as they are for a very long time, the supply of Saharan dust may begin to dwindle. But I suspect that if that is the case, it will well beyond the end of our lifetimes before it occurs.

    Eric Wilcox, NASA’s Goddard Space Flight Center

  5. Hello,
    I study many times about the dust bellowed off to atlantic ocean from sharan desert. Will it effect on atlantic ocean and the size of land will increase or not . if increase what will be the effect on earth.
    vivek rathi

  6. Sir one more question, that sand blowed towards western coast of africa.will this sand change western cost of africa into a desert. and will all sand blow towards the western coast in future?
    vivek rathi

  7. Vivek, thanks for your questions The Sahara desert stretches to the Atlantic coast of Northern Africa. To the south lies the Sahal region of savannah supported by seasonal rainfall. Persistent and recurring droughts are a feature of African climate, and when drought combines with stresses from widespread grazing and forest burning in the Sahal region, the expansion of desert regions is a possible consequence. If indeed this process of desertification is occurring, it may impact the amount of dust transported out to sea. Once transported over the ocean, much of the dust settles out and is deposited on the ocean surface. There are several potential impacts of dust on sea life. The growth of some species of algae may respond to minerals added to the sea water from the dust. Furthermore, as I noted below, the presence of Saharan dust has been found in Caribbean soils, and dust has even been suggested to degrade coral reefs.

    -Eric Wilcox, NASA Goddard Space Flight Center

  8. hello sir,
    i wanna to about what’s lenght b/w sharan and western coast of africa & what’s the speed of winds.

  9. I live in Trinidad, which is the most Southern island in the Caribbean. The air quality in our capital city Port of Spain is very poor at the moment. Consistent forest fires and the burning of a landfill situated next to the capital city are definitely the contributors. However I was wondering whether there may be a dust cloud over the Caribbean at the moment?

  10. I live in Curacao and our skies have been hazy and dusty for 2 months now. It only seems to be getting worse. On flying here from Miami the other day, I noticed that the haze and degraded visibility seemed to actually start aways north of here. Is this due to the Sahara Sands and, if so, is it exceptionally bad this year

  11. sir, if the direction wind will change in future what will be effect for that

  12. i wanna to know about difference between sharan desert and western cost of affrica. i followed the atlas before ……..but i didn’t come to know the actual distance…….
    vivek rathi

  13. Lovely pictures of dust. Gee you must have a really big zoom lens

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