FIVE THINGS ABOUT ‘WHOPPER’ DUST CLOUD

A 6,000-kilometre-long plume of thick dust from the Sahara Desert has arrived in the Caribbean, moved into the Gulf of Mexico, and is forecast to move inland before reaching as far as Washington this weekend. The Saharan Air Layer (SAL) develops every year off the coast of Africa, but this one is setting records.

1

AN ANNUAL TRIP

The SAL typically forms over the Sahara in summer, and moves into the tropical North Atlantic every few days. This plume started out at 20,000 feet above the desert before slipping out to sea on June 14. On satellite imagery, the dust resembles a long appendage extending from the western edge of northern Africa, across the tropical Atlantic Ocean, and divided into batches connected by thinner wisps.

2

SINKS TO OUR LEVEL Dust associated with SAL plumes typically stay high in the atmosphere, but not this cloud. In Antigua on Sunday, it caused air quality levels to tank into the unhealthy range. Visibility dropped to three miles in the U.S. Virgin Islands Monday and five miles in Puerto Rico. By Tuesday, it had spread to Cuba and the Yucatán. By Thursday or Friday, a second wave of dust will drift westward across the Atlantic — and behind that will be still more dust.

3

SURE, LEAVE THE SMALL

IRRITANTS FOR US

By the time it hits North America, much of the larger sand will have fallen into the ocean, with only infinitesimal granules left in the sky. But studies have

found that microbial life can hitch a ride on the SAL and land on the shores and waters of the Gulf, potentially creating harmful algal blooms, as the dust deposits iron into the water.

4

TEMPERATURE FORECASTS DIFFICULT Aerosol optical depths measure how much light is scattered by the dust; an optical depth of 1.5 reduces the direct sunlight reaching the surface to almost 20 per cent of normal. This plume raised the aerosol optical depth to about 1.5 in Barbados, a monthly record for the island. Guadeloupe hit 2. In Puerto Rico, its level 2 has not been seen there during June in 15 years of record keeping. Miami will be measured next.

5

JUST ADD IT TO 2020’S LIST OF BIG ’UNS One expert on wind interactions with the landscape says “This one is a whopper.”