"And a butterfly can flutter its wings over a flower in China and cause a hurricane in the Caribbean. I believe it. They can even calculate the odds. It just isn't likely and it takes...so long." -- RobertRedford as card shark Jack Weil in Havana.
Can a butterfly in Brazil really cause a tornado in Texas? How can a dust storm in Africa affect weather in the United States?
These are some of the questions surrounding "chaos theory" which is generally credited to the late MIT mathematician and meteorologist Edward Lorenz.
The idea is simple enough. Tiny disruptions or changes in "initial conditions" can magnify over time and space. What might seem like an inconsequential change in wind, sunlight, or temperature can ultimately change the course, direction and intensity of weather systems thousands of miles away.
In meteorology we try and constantly measure and model the atmosphere based on incomplete measurement, and imperfect physical models. It should come as no surprise that forecast models often lose their grip within a few days.
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Tracking an African Dust Cloud
It's hard to believe, but the dust storms that sweep the massive African Sahara are sometimes lofted miles up into the atmosphere and travel thousands of miles. The belt of tropical easterly winds can carry these massive dust clouds thousands of miles. Sometimes, African dust is found in soils as far away as Florida and the southeast United States.
The fine airborne dust particles have another effect. They reflect some of the incoming sunlight and reduce the heating effect on the atmosphere and oceans below. The dust layer is also warm and dry, and effectively creates an inversion which can suppress thunderstorm formation -- and put the brakes on any would be tropical systems.
Here's an amazing animation form NASA which models the path of the dust plume over the next few days.
Hurricane Suppression in early August?
The dust cloud may serve to put the kibosh on Atlantic hurricane activity formation for the next week or so. Most of NOAA's hurricane models look awfully quiet. I had the pleasure to speak with Weather Underground and Weather Channel hurricane expert Jeff Masters last fall at the Science Museum of Minnesota about extreme weather and hurricanes. Jeff has an interesting take on how the African dust cloud may affect the Atlantic hurricane season in early August.
A massive dust storm that formed over the Sahara Desert early this week has now pushed out over the tropical Atlantic, and will sharply reduce the odds of tropical storm formation during the first week of August. The dust is accompanied by a large amount of dry air, which is making the Saharan Air Layer (SAL) much drier than usual this week. June and July are the peak months for dust storms in the Southwest Sahara, and this week's dust storm is a typical one for this time of year. Due in large part to all the dry and dusty air predicted to dominate the tropical Atlantic over the next seven days, none of the reliable computer models is predicting Atlantic tropical cyclone formation during the first week of August.
How dust affects hurricanes
Saharan dust can affect hurricane activity in several ways:
1) Dust acts as a shield which keeps sunlight from reaching the surface. Thus, large amounts of dust can keep the sea surface temperatures up to 1°C cooler than average in the hurricane Main Development Region (MDR) from the coast of Africa to the Caribbean, providing hurricanes with less energy to form and grow. Ocean temperatures in the MDR are currently 0.7°F above average, and this anomaly should cool this week as the dust blocks sunlight.
2) The Saharan Air Layer (SAL) is a layer of dry, dusty Saharan air that rides up over the low-level moist air over the tropical Atlantic. At the boundary between the SAL and low-level moist air where the trade winds blow is the trade wind inversion--a region of the atmosphere where the temperature increases with height. Since atmospheric temperature normally decreases with height, this "inversion" acts to but the brakes on any thunderstorms that try to punch through it. This happens because the air in a thunderstorm's updraft suddenly encounters a region where the updraft air is cooler and less buoyant than the surrounding air, and thus will not be able to keep moving upward. The dust in the SAL absorbs solar radiation, which heats the air in the trade wind inversion. This makes the inversion stronger, which inhibits the thunderstorms that power a hurricane.
3) Dust may also act to produce more clouds, but this effect needs much more study. If the dust particles are of the right size to serve as "condensation nuclei"--centers around which raindrops can form and grow--the dust can act to make more clouds. Thus, dust could potentially aid in the formation and intensification of hurricanes. However, if the dust acts to make more low-level clouds over the tropical Atlantic, this will reduce the amount of sunlight reaching the ocean, cooling the sea surface temperatures and discouraging hurricane formation (Kaufman et al., 2005.)
It remains to be seen how the dust cloud will dissipate over time, and what the forecast holds for later in August and into the "climatological" peak of hurricane season, which occurs around Sept. 10.