Week of May 6-13, 2020 Sahara Dust Episode in Puerto Rico

  • 12Noon EDT/AST update on Thursday, May 7th:
  • Visibility at San Juan Airport varying between 8 and 9 miles this morning in haze in all directions.
  • US Naval Research Laboratory’s NAAPS model predicts dust will move on (toward Florida) by Saturday.
  • For more updates, follow the links at the bottom of the post!

Visibility is dipping to less than 10 miles in Puerto Rico this week and may last through the weekend. The tropical skies in Puerto Rico are often crystal clear, at least when it’s not raining. You can see more than 10 miles most of the time in Puerto Rico, despite the high humidity commonly occurring in this tropical locale. But when the aviation weather observation occasional says “HZ ALQDS” (haze in all quadrants), dust from the Sahara Desert is likely the cause.

Below is a list of today’s observations since midnight local time at the San Juan International Airport on May 6th : Visibility was as low as 8 miles. Despite some nearby power plants, due to a relative lack of combustion emissions, and good dispersion in trade winds, there is not much sulfate that causes the white summer haze seen here in the eastern United States.

TJSJ 070056Z 09007KT 10SM CLR 27/23 A3002 RMK AO2 SLP164 T02720233
Example decoded, below:
TJSJ 062356Z 08007KT 10SM FEW033 27/23 A3000 RMK AO2 SLP158 T02720233 10333 20272 53013
TJSJ 062256Z 12007KT 9SM FEW034 FEW150 29/22 A2999 RMK AO2 SLP153 HZ ALQDS T02940217
TJSJ airport 06 May, 22:56Z is 6:56pm May 6th 9 miles visibility, few clouds at 3,400 feet, few clouds at 15,000 feet, temperature 29C, dew point 22C, altimeter setting 29.92 inches Remark about automated sensing, sea level pressure 1015.3mb, haze in all directions, temperature more precisely at 29.4, dew point at 21.7.
TJSJ 062156Z 14006KT 9SM FEW036 SCT150 31/21 A2997 RMK AO2 SLP148 HZ ALQDS T03060211
TJSJ 062056Z 15009KT 9SM FEW038 SCT130 32/21 A2996 RMK AO2 SLP145 HZ ALQDS T03170206 56007
TJSJ 061956Z 15012G17KT 9SM SCT040 SCT130 32/21 A2997 RMK AO2 SLP146 HZ ALQDS T03220206
TJSJ 061856Z 12009G16KT 9SM SCT044 SCT130 33/22 A2996 RMK AO2 SLP145 HZ ALQDS T03280217
TJSJ 061756Z 09016KT 9SM FEW042 SCT050 SCT130 31/23 A2998 RMK AO2 SLP152 HZ ALQDS 60001 T03110228 10333 20283 58019
TJSJ 061656Z 14009G17KT 9SM FEW045 BKN130 33/21 A3000 RMK AO2 SLP158 HZ ALQDS T03330206
TJSJ 061556Z 12010G19KT 9SM FEW037 SCT050 SCT090 32/22 A3003 RMK AO2 SLP167 HZ ALQDS T03170222
TJSJ 061456Z 09018G21KT 8SM FEW031 SCT041 SCT090 32/23 A3004 RMK AO2 SLP170 HZ ALQDS T03170228 50004
TJSJ 061356Z 09017KT 8SM FEW028 FEW130 31/23 A3004 RMK AO2 SLP171 HZ ALQDS T03110228
TJSJ 061256Z 09014KT 8SM FEW026 FEW120 30/23 A3003 RMK AO2 SLP168 HZ ALQDS T03000228
TJSJ 061156Z 10007KT 8SM FEW026 FEW120 28/23 A3002 RMK AO2 SLP166 HZ ALQDS T02830228 10283 20244 53015
TJSJ 061056Z 14003KT 8SM FEW024 26/22 A3001 RMK AO2 SLP159 HZ ALQDS T02610222
TJSJ 060956Z 12004KT 10SM FEW033 FEW085 26/22 A2999 RMK AO2 SLP153 T02560222
TJSJ 060856Z 00000KT 10SM FEW032 SCT080 26/22 A2998 RMK AO2 SLP151 T02560222 55001
TJSJ 060756Z 00000KT 10SM FEW030 SCT050 SCT075 26/23 A2997 RMK AO2 SLP149 T02560233
TJSJ 060656Z 00000KT 10SM SCT022 SCT075 26/23 A2998 RMK AO2 SLP149 T02610228
TJSJ 060556Z 09004KT 10SM FEW025 SCT075 26/23 A2999 RMK AO2 SLP153 T02560228 10272 20256 56008
TJSJ 060456Z 12004KT 10SM FEW024 FEW060 27/23 A2999 RMK AO2 SLP155 T02670228

The US Navy uses computer models to predict the transport of dust and other particles that can reduce visibility. It would seem to me the US Navy’s interest in haze and dust is the Navy doesn’t like the idea of not being able to see in all directions, in case they need to protect themselves from potential attackers. It’s good to know when low visibility might allow foes or even landmarks or other ships to be hidden. Thus, one of the many projects at the Naval Research Laboratory is the sensing and measurement of particles in the atmosphere, where they are and they the come from.

Here’s the NRL’s forecast for sulfates, dust, and smoke for 1200 UTC on May 7th.

From the US Naval Research Laboratory web site.

This map predicts dust concentrations of over 80 micrograms per cubic meter over Puerto Rico, denoted by the light blue color on the ‘dust’ map in the lower left quadrant. The health standard for particulate matter less than 10 microns is 150 micrograms per cubic meter. Particles of Saharan Dust are left after traveling across the Atlantic Ocean are typically between 2 and 5 microns in size.

The dust is lofted into the air in sandstorms in the Sahara Desert. Most of the strong winds come from the downdrafts of large thunderstorms or when large pressure gradients produce high wind speeds. This dust gets lofted into the middle of the troposphere and gets pushed out over the Atlantic Ocean by tropical trade winds. Because of the stable layer caused by the hot, dry air from the desert, over moist, cooler air at the ocean’s surface, this dust can travel thousands of miles, only slowly falling out of the sky. Over land, more mixing occurs and the dust is brought near the ground, creating the low visibility conditions.

Here’s some quick links to weather information, so you can track this Sahara Dust episode:

Latest San Juan Airport Weather observations: https://www.wrh.noaa.gov/mesowest/timeseries.php?sid=TJSJ&num=72&banner=gmap&raw=0&w=325

Latest satellite photo: https://www.star.nesdis.noaa.gov/GOES/sector_band.php?sat=G16&sector=car&band=GEOCOLOR&length=12

Local National Weather Service forecast discussion for Puerto Rico and the Virgin Islands: https://forecast.weather.gov/product.php?site=sju&product=AFD&issuedby=sju

More information is at my other blog post with some more sources of data at https://bkellysky.wordpress.com/2018/06/12/saharan-dust-arrives-in-puerto-rico/

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