The little storm that could: Watch a tenacious tropical depression race ahead of a huge blob of Saharan dust

The GOES-16 weather satellite eyed Tropical Depression Four in the central Atlantic Ocean on Thursday afternoon, July 6, 2017, as it raced ahead of a huge blob of brownish dust streaming off the Sahara in Africa. (Source: CIRA/RAMMB/NOAA) It may have a humdrum name, but since Tropical Depression Four formed in the central Atlantic Ocean on Wednesday evening, it has certainly distinguished itself. It is a "small, tenacious depression" that "has continued to hold its own," the National Hurricane Center said in its update this morning. The spectacular animation above, from the GOES-16 weather satellite, suggests why that word, tenacious, is appropriate. Tropical Depression Four has managed to stay alive despite dry and dusty air streaming west from the Sahara in Africa. In the video, you can see the swirling depression racing ahead of a large mass of dusty, brown-tinged air. For a tropical depression to rev up and grow into a full-blown hurricane, it needs to be fueled by warm, moist air. So dry air has the opposite effect — it saps the strength of depressions and hurricanes. Tropical Depression Four has held its own while struggling against the dry air. Yet ultimately, the Saharan Air Layer, or SAL, as scientists call it, is winning. And that means in all likelihood the depression will degenerate by tonight.