Sahara dust brings hazy summer days
Plume out of Africa tweaks twilights, suppresses showers.
A dusty tourist from overseas has made for hazy days in South Florida this week, giving sunrises a twilight feel and stealing rain from the sky.
Saharan dust, carried on summertime easterly breezes from Africa over low latitudes, drifted over the Sunshine State in the largest plume so far this season. The arid air sucks humidity from upper levels of the atmosphere, with the National Weather Service. “It suppresses thunderstorm development so rain chances, even though they aren’t zero, are really limited.”
Jason Dunion, a meteorologist at the University of Miami, said he’s been tracking the plume for days, watching it travel more than 3,000 miles across the Atlantic.
“It was the size of the continental U.S. as it came across,” said Dunion, who is also a research-
er with the National Oceanic and Atmospheric Administration’s Hurricane Research Division. “We finally began seeing it in Miami (Thursday).”
On Thursday, for the second time in a week, Palm Beach County was under a heat advisory — a rare warning that prior to July 27 had only been issued once since 2009. The “feels like” temperature at Palm Beach International Airport soared to 105 degrees, with a dew point of 77. Dew point is a measure of water vapor in the air.
That meant while the upper atmosphere was dried out by Saharan dust, the surface layer of air was nearly saturated. Dunion said the air between 1 and 3 miles up Friday had about half the typical amount of humidity for that level.
On Friday, the heat index rose to 103 degrees with a 77-degree dew point, which wasn’t enough to trigger a heat advisory. The daytime high was 91, about normal.
The Saharan dust coincides with the peak of the hurricane season as the steering winds start pushing tropical waves off Africa into the warming waters of the Atlantic. The dust acts to squash budding tropical cyclones by drying out the atmosphere and increasing storm-shredding wind shear.
But storms can grow between dust plumes, or sneak through beneath them.
“The one we are tracking now is also in the Caribbean so we’ve been watching how they interact,” Dunion said.
A tropical wave over the eastern Caribbean was given a 60 percent chance of development Friday by the National Hurricane Center.
In Palm Beach County, the Saharan dust increased the amount of particulates in the air, pushing the air quality to “moderate,” meaning unusually sensitive people should consider reducing prolonged or heavy exertion, according to the Florida Health Department.
Joseph Prospero, an expert on Saharan dust and professor emeritus at University of Miami’s Rosenstiel School of Marine and Atmospheric Science, said Saharan dust particles are large, absorbing more solar radiation and changing the appearance of sunrise.
“If you are looking east, the sunrises will be rather muted,” Prospero said in an interview earlier this year. “It’s like looking through a gray filter.”
Fisher said the dust should begin to dissipate during the weekend.
While Saturday is forecast to be dry, Sunday’s rain chances increase to 30 percent. Highs both days will be in the low 90s.