Question: I see online how there’s a cloud of dust from the Sahara Desert making its way across the Atlantic Ocean. How can that happen? I mean, we have dust storms here in Arizona, but they rarely extend beyond our state. — SM, Casa Grande, AZ
Answer: The sandstorms you’re used to in Arizona (where I also live) are most often caused by haboobs. They are fairly localized events, but can decrease visibility and air quality to hazardous levels. What happens over the Sahara is way bigger. The Sahara covers most of North Africa, with an area of 9,200,000 sq km (3,600,000 sq mi). It is the largest hot desert in the world and the third-largest desert overall. The cold deserts of Antarctica and the northern Arctic are larger.
During Northern Hemisphere spring and summer, there are strong westerly winds (“trade winds”) that cross the Sahara from east to west. There are also haboobs that contribute additional dust. The combined effect is to launch large quantities of sand into the atmosphere, some of which can reach the stratosphere. That allows it to be swept up by the jet stream, which can transport the dust thousands of miles — even across the Atlantic!
Saharan dust particles that cross the Atlantic can range in size from 100 nanometers to 50 micrometers (1 micrometer is equal to 1000 nanometers). Larger particles tend to settle out closer to the source, while smaller particles can travel greater distances. Here’s a graphic using decimal inches that shows sand grain sizes as recognized by geologists:
I had to use an inset in this graphic, otherwise the stuff we normally think of as “sand” would be to large to fit. Either that, or the Saharan dust cloud sand be too small to see. Even with the inset, those smallest dust particles are only 3 pixels across. Suffice it to say, when particles that small get lofted into the air they can be carried for great distances — especially if they reach the stratosphere.
People in the Caribbean, Florida, and other parts of the Southeast, may wake up to find a visible coating of dust on their cars, but that’s a minor nuisance. Typical dust clouds are around 2.5 miles thick, and the actual dust content that makes it across the Atlantic is on the order of millions of tons, but it’s spread over a very large area.
The bigger problem is what it does to the air quality index (AQI). As I write this, the AQI in Florida is “moderate” (one step below “good”). Still, people with respiratory issues can be affected. The accompanying haze also reduces visibility for drivers. The silicate particles reflect light even better than fog.
This has been going on for centuries and will likely continue. But it’s not all bad news. When these dust clouds arrive during hurricane season, they suppress hurricane formation with their dry air and their cooling effect on the ocean. They also deliver more than silicates — the dust cloud contains biological and mineral compounds that act as fertilizer when they fall into the ocean. That sparks algae blooms, the first step in the ocean food chain. The dust also provides some nutrients when it falls on soil, but the effect in the ocean is more dramatic.
Next Week in Sky Lights ⇒ Evolution of Stealth