What is mud rain?

Mud rain is rain that contains a noticeable concentration of particles of sand or dust.

Saharan dust collected in a rain gauge in the Caribbean.

The soil can be of local origin or it can originate from very distant regions. As rain falls through the dust layer, the raindrops collect the soil particles. When the rain droplets hit objects on the ground, the water evaporates and leaves behind dry mud spots.

Another consequence can be health hazards to those with respiratory issues because, as the fine particles of dust settle out, they can reduce the quality of the air we breathe.

At the end of June, a large dust cloud arrived in North America from the Sahara Desert. Micrometer-sized soil particles from the desert were lifted by the wind to high altitudes and transported over thousands of miles. The dust was carried by atmospheric circulation patterns across the Atlantic Ocean. Satellite observations tracked the movement of this dust, or aerosol, from Africa, across the Atlantic Ocean and into the Upper Midwest. 

 Aerosol Optical Depth (AOD) measured by satellites outlined dust flowing from Africa over the Atlantic Ocean to North America in June 2020. Credit: NASA

Desert soil is carried across the ocean every year, but this particular June dust plume spanned thousands of miles. Iron and other nutrients in dust can lead to phytoplankton blooms as the dust settles into nutrient-limited waters. As the dust covered multiple regions of the southern U.S., it turned daytime skies very hazy and sunsets red and orange.

Dust can be transported across the globe. Desert dust from the Sahara and Gobi deserts has been observed on the ice sheet of Greenland. Ice cores in Greenland provide a history of the dust deposition as they appear as layers in the ice. The mineralogy of the dust in the ice allows scientists to trace the desert of origin. The source also allows scientists to determine global wind patterns from thousands of years ago.

Steve Ackerman and Jonathan Martin, professors in the UW-Madison Department of Atmospheric and Oceanic Sciences (AOS), are guests on WHA radio (970 AM) at 11:45 a.m. the last Monday of each month.

Category: Meteorology, Phenomena

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