Over a couple of days, the skies have been turning brown over the Caribbean. Dust that was raised from the Sahara Desert in North Africa has made steady progress, as a deep homogenous layer in the atmosphere, westwards across the Atlantic.
The normally deep blue skies are now brown. The sun a mere dull orange ball. Air quality has dropped at ground level too. The measurement of PM10 dust particles in Fort-de-France on Martinique was 192. This made Martinique the 8th most polluted country on Tuesday out of the 105 monitored by the World Air Quality Index project.
In Puerto Rico, the figure is similar, maybe slightly worse, and according to Dr Olga Mayol from the University of Puerto Rico this is the most dramatic Saharan sand event in more than 50 years. PM10, or Particle Matter of diameter 10 micrometres, is an irritant in the human respiratory tract.
The source of all this woe really is the great Saharan Desert, more than 5,000 kilometres (3,107 miles) to the east. This Saharan Air Layer (SAL), is a dry dust plume and is commonly repeated during the summer. Wind and convective currents lift this fine dust high into the atmosphere and the general wind flow carries it west.
While taking away the blue sky and warm sunshine, this dust layer will have the effect of reducing or even preventing rain showers and thunderstorms. It is likely to last for a few more days yet, fluctuating in intensity. On Tuesday, it reached Panama and before the weekend it may well be seen over the skies of Texas.