Introduction

Hurricane Edouard had just passed by Cape Cod when I wrote these lines, after giving us a good scare, and keeping meteorologists of local TV stations out of bed all night. Approaching on a track along the East Coast, Edouard remained a category 3 hurricane, with 180 km/h winds, from the tropics to latitude 38°N. This is where it left the warm waters of the Gulf Stream behind, quickly to lose its punch over the much cooler Mid Atlantic Bight, and to be degraded to category 1, with 130 km/h winds, still enough to uproot a few trees on the Cape.

Edouard's fury came from water vapor, as it ascends the “eye-walls” (Figure 1.1) that surround a hurricane's core, condensing and releasing its latent heat of evaporation. The heat makes the moist air buoyant, turning the eyewalls into a giant chimney with an incredibly strong draft. The draft sucks in sea-level air, causing it to spiral toward the core in destructive winds and to drive waters against nearby coasts in storm surges. The fast air flow over warm water also ensures intense heat and vapor transfer to the air, sustaining the hurricane's strength. Over colder water, where not enough water evaporates, the hurricane dies: The lifeblood of a hurricane is intense sea to air transfer of heat and water vapor. On the other hand, as hurricane winds whip the waters along, they transfer some of their momentum downward.