The 2017 hurricane season has already been an active one in North America – Hurricanes Harvey, Irma, Katia and Jose have all caused great damage – but the season isn’t over yet.
Hurricane season runs from June 1 to November 30.
The peak of the season is typically on Sept. 10, according to the National Oceanic and Atmospheric Association.
From mid-August through mid-October, the activity spikes, accounting for 78 percent of the tropical storm days, 87 percent of the category 1 and 2 hurricane days (Saffir-Simpson Hurricane Wind Scale), and a whopping 96 percent of the major (category 3, 4 and 5) hurricane days.
Tropical waves form off the coast of Africa “roughly every three days,” so why is this time of the hurricane season usually so active?
According to NOAA, dynamics (wind factors) and thermodynamics (temperature and moisture) are both factors:
Wind shear, which can tear disturbances apart before they strengthen, is strong in May, but gradually fades through June and July, reaching a minimum by mid to late August.
This minimum in the shear combines with favorable thermodynamics – ocean temperatures in the deep tropics that increase with each day of summer sun, warmer air temperatures, and increasing atmospheric moisture.
When the dynamics and thermodynamics are in sync, as they often are from mid-August through early October, disturbances like African tropical waves can easily strengthen.