The eye of the butterfly

The difference between the tropical cyclone and mid-latitude cyclone

- JIM AUSTIN

June’s arrival marked two significant weather periods, the beginning of “summer” and the June-November hurricane season, both raising the possibility of stormy weather and the more important question: What kind of storm? Nightly TV predictions of fronts, winds and rain — weather typical of the mid-latitudes — are increasingly appended by “what the tropics are doing,” with an eye kept on disturbances and depressions in the Atlantic, especially the belt between 5° N and 20° N that can include tropical misbehavior. So, let’s a look at both, starting with the tropics.

Senegal’s Cape Verde (Cap-Vert) is a peninsula at Dakar jutting into the North Atlantic at about 14.7° N by 17.5° W, claiming priority as the westernmost point of the mainland African continent.

(Note that “Cape Verde” also applies to the independent island nation about 370 nm slightly northwest of its continental namesake.) Historically, Cape Verde has been associated with about 80 to 85 percent of the more destructive hurricanes (Category 3 to 5) that strike westward to the Antilles, the Gulf, Caribbean and U.S. East Coast, with September to October being statistically the most active period. But why would this African cape have such an outsized influence?

Imagine a rectangle, its west flank running from Florida southeast to the Lesser Antilles, crossing the North Atlantic, with its eastern flank not at Cape Verde but 2,500 miles further east across Africa to the Darfur Mountains and Ethiopian Highlands, their meridional ends linked by latitudes of about 5° N to 20° N. The trade winds flow west, prompted by Saharan hot, dry air to the north and warm, moist air from the Gulf of Guinea to the south; the African easterly jet results from this north vs. south contrast of temperature and density.