365 Days of Climate Awareness 41: Hurricanes 2: Maturity to Dissipation

A large fully-formed hurricane can span nearly a thousand miles across, though not many reach this size. The size of the storm is not an indicator of intensity, however--there is such a scale, the Saffir-Simpson scale, which divides hurricanes into five categories based on wind speed:

    1: 74-96 mph: very destructive winds which will do some damage

    2: 97-110 mph: extremely dangerous winds which will cause extensive damage

    3: 111-129 mph: devastating damage: likely major damage to homes.

    4: 130-156 mph: catastrophic damage severely damaging homes.

    5: 157+ mph: catastrophic damage destroying most or all homes.

In the North Atlantic, hurricanes typically track toward the Gulf of Mexico, then turn north either skirting the east coast, or coming ashore along the US gulf coast and then moving northeast, roughly following the general wind circulation of the Hadley (easterly trade wind) and Ferrel (westerly) winds. A similar pattern holds in the Pacific.

Structure of a mature hurricane.

Hurricanes bring three principal threats. First is the wind. In the northern hemisphere, when to the east of the eye of a northward-bound hurricane, the wind speed is added to the storm speed, amplifying the wind strength. On the west of the eye, storm speed is subtracted from wind speed. But the direction of the wind changes as the storm passes, which puts a tremendous amount of strain on trees and buildings. Another is rain, which can reach 3 in/hr and cause major flooding. In 2017, Houston recieved 60 inches of rain from Hurricane Harvey. Along the coast the biggest threat might be the storm surge, a tsunami-type wave created by high wind and the low air pressure of the storm, which allows the sea to rise. Katrina brought a storm surge of close to 20 feet in some locations.

2020 North Atlantic hurricane tracks.

When the hurricane is either over land, or over sufficiently cool water, it no longer takes up warm water vapor capable of sustaining the central convection, and the storm loses strength. The winds slow down, the rain loses intensity and the system gradually dissipates, though it remains a dangerous storm long after ceasing to be a hurricane. Typically the storm continues to weaken, but recent years have had two examples of storms retaining their intensity as they traveled north: Sandy in 2012, and this year's Ida, each of which dropped nearly 10 inches of rain on New York. Very warm water and air temperatures contributed to the prolonged strength of both storms, well beyond historical norms.

2020 global hurricane tracks.

Tomorrow: hurricanes 3: energetics.

Be well!