Look up. The clouds over your head right now might carry dust particles that a week ago, were swirling around in a sandstorm in the Sahara or the Gobi Desert. You’ve probably seen a sandstorm in Iraq on television — blowing clouds of stinging sand and dust.
Dust from that very storm might be in the clouds above you right now. During a sandstorm, the heavier sand particles stay pretty close to the ground. But the lighter particles can get lifted much higher — high enough, in fact, to get swept away by strong wind currents in the upper atmosphere. Until recently, scientists weren’t aware of just how far that dust travels. Ken Sassen is an atmospheric scientist at the University of Alaska in Fairbanks.
Ken Sassen: Back in the late ’60s, people were thinking that they were observing Asian dust in Hawaii … But it wasn’t until probably the last five years that we’ve recognized — using satellites and ground-based remote sensors and airplanes — that an Asian dust storm will make it all the way to the east coast of America.
So sandstorms can be truly global. And scientists believe that the frequency of sandstorms is increasing, in a way that might be linked to human population growth and activity. Sassen is now looking at the effect of dust on clouds.
And atmospheric dust might affect climate. It’s known that dust absorbs solar energy — which heats the atmosphere. But dust also reflects sunlight — so might cause a little cooling at Earth’s surface.
Using satellite images, scientists can watch a dust storm forming over the Mongolian or Sahara Desert and follow its path across the ocean.
Bruce Boe — a cloud physicist — writes, “I note that dust, not only from the Sahara but from all sources, definitely can affect our climate by affecting our clouds. Too much dust often leads to “continental” clouds, that is, clouds made up a large numbers of very small droplets. Such clouds have a more difficult time making precipitation because the collision-coalescence process is impaired. In this way, dust, at least the wrong kind, can contribute to drought.”
Excerpts from an interview with Ken Sassen:
Back in the late Sixties, people were thinking that they were observing Asian dust in Hawaii … But it wasn’t until probably the last five years that we’ve recognized using satellites and ground-based remote sensors and airplanes that an Asian dust storm will make it all the way to the east coast of America1.
You can follow it now with satellite images. you can see the dust storm forming over the Mongolian Desert or Sahara Desert and you can just follow it across the oceans.
We know that the sand that’s suspended in the atmosphere, the dust, will reflect sunlight, so it has a chance of cooling the surface of the Earth, it can fertilize the ocean, because there’s minerals in it. … It can glaciate clouds also, which means they can turn a cold-water cloud into ice, which is a significant thing to do.
We use ground-based laser radar systems, we use laser beams and radar beams to study whatever’s in the atmosphere. We normally look at high cirrus clouds, but in our data records, we found lots of examples where there was dust overhead. And it took us a while to figure out, but …. we traced it back to Asia.
We’d regularly looked at clouds over Salt Lake City, and the laser also detects not only the cloud particles, but dust particles suspended in the air
I was scratching my head for a number of years , and I knew it was dust that was coming from someplace far away because often we saw it up at 30 thousand feet, where the jets were flying, 6.32 … it takes a while to get to 30 thousand feet, so if it’s a big windstorm, I would have heard about it anywhere in the western US, so I knew that these were global clouds.
I think I feel pretty silly that I was looking at this stuff for a number of years, ten years without figuring out that it actually came from China.
The direct effect we can say — it reflects sunlight to outer space so it may cause a little cooling at the surface, but it also absorbs infrared and solar energies so it heats the atmosphere and the effects on clouds may be numerous and they’re very hard to disentangle.