Following a premature start and active first several weeks of 2020 Atlantic hurricane season, the tropics have been quiet for more than a week and should remain so through at least July 4 weekend. The lull in activity as we are heading toward the heart of what is expected to be a hyperactive, potentially destructive hurricane season largely has to do with the Saharan dust we have recently seen in Texas.
Every summer, large clouds of African dust from the Sahara are periodically lofted and blown across the tropical Atlantic by prevailing easterly winds. This Saharan Air Layer (SAL) contains bone-dry air in the mid-levels of the atmosphere, which inhibits tropical storm and hurricane formation.
In my graduate school research at the University of Miami (RSMAS), I found that high humidity in the middle atmosphere is a very important aspect of tropical storm formation and intensification, confirming that these Saharan dust storms squash hurricane development. Weaker storms have overall lower humidity, and stronger ones have higher humidity. Very high humidity in a storm also precedes rapid intensification.
Average humidity in the mid-levels of a storm
- Tropical Depression: 45%
- Major Hurricane (category 3-5): 55%
- Rapidly-intensifying storm (of any strength): 64%
Our research also found that there is a humidity threshold of 40% when it comes to new storm development. Storms with lower humidity than that are more likely to die out, while storms with higher than 40% humidity are more likely to develop and intensify.
All of this is to say that dry, dusty air lowers the storm’s average mid-level humidity and makes it much more difficult for the storm to survive.
The occasional exception to this rule is when a preexisting, well-developed tropical storm moves into an environment of dry, dusty air. The already-developed storm can develop a “marsupial pouch” of high moisture at its core, and stay insulated from the surrounding storm-killing dust.
While ongoing dust clouds are forecast to keep the tropical Atlantic quiet for the next five days, the dust will not last forever. Low hurricane activity in June and July is not correlated to a quiet hurricane season, as the large majority of major hurricanes occur after August 1.
Stay with the First Warning Weather team as Atlantic hurricane season continues through November 30.