Puerto Rico meets the Saharan Desert

IWhen you think of Puerto Rico, you probably think of tropical beaches, bright blue water, and warm humid weather.  So what could the Sahara Desert possibly have to do with a Caribbean island?

Believe it or not, Saharan dust can travel west across the Atlantic to as far as the Gulf of Mexico.  It's called the Saharan Air Layer (SAL), and it affects the daily lives of people in the Caribbean and Central America.

It just so happens that two of my former roommates (and current best friends) are from Puerto Rico. My friend Janice let me pick her brain on the topic, and her first reaction when I asked what it was like when the SAL blew in was, "Oh no! You can't go outside on those days!"

She says the dust is a killer for people with allergies and asthma.  With the dust comes pollen, microorganisms, bacteria and even insects.  She says it also means, "Don't wash your car!  With all the dust, cars and everything outside can be covered."

It sounds like living through a low-level dust-storm... in the middle of the tropics!  Most daily activities continue as normal, but visibility can be low.  So much for that clear view of the beach.

It's something most visitors and tourists have never heard of, yet it's a regular occurrence.  There are a plenty of published papers and web articles on the SAL, but Janice also happens to be a meteorologist for the National Weather Service... so I asked her for a basic explanation.

You've probably heard of the trade winds?  Well, in northern Africa, air movement is predominantly from east to west.  In other words, winds blow over the Saharan Desert and into/across the Atlantic.  The dust is light and easily carried by the winds across the ocean.  You can even see the SAL on satellite.

It's strongest from May to September and can cover 2000-5000 km.  It comes in waves (NOAA says every 3-5 days), but Janice tells me the worst month in Puerto Rico tends to be July.

Although the dusty days keep some Puerto Ricans inside, there are a few pros.  Janice says it helps fertilize the soil and provides nutrients for phytoplankton in the ocean.  But maybe the biggest pro?  She says, "Sometimes this phenomenon can be beneficial for the island, because it helps weaken tropical systems that are a threat to Puerto Rico."  Hurricanes and tropical depressions don't like dry air, so the dust sometimes weakens them before they make landfall.