The Atlantic hurricane season begins on June 1. While last year we experienced a very active hurricane season, the expectations for this year are (thankfully) somewhat tempered. However, the main factors still indicate an above-average active season. The seawater is still slightly warmer than the long-term average.
How Do Hurricanes Form?
Four ingredients are needed for the formation of hurricanes:
- Hurricanes form from severe thunderstorms. These thunderstorms often develop over Africa and then move westward over the Atlantic Ocean (the so-called tropical waves).
- The seawater over which the thunderstorms travel must be warmer than 27 ℃ for the thunderstorms to develop into a hurricane. Above 27 ℃, the seawater contains enough energy to feed the thunderstorms and make them stronger. The seawater evaporates, allowing the thunderstorms to grow into hurricane strength.
- The rotation of the Earth is crucial for the formation of hurricanes. Due to this rotation, air in a low-pressure area does not flow straight to the center but moves around it. The forming hurricane thus begins to rotate around its axis: counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. This creates a clearly visible center, also known as the eye of the hurricane. At the equator, the effect of the Earths rotation is not noticeable, which is why no hurricanes form close to the equator. This is the reason why significantly fewer hurricanes pass by Bonaire than by Saba and Sint Eustatius. Bonaire is much closer to the equator.
- Hurricanes only form if the wind speed and direction at sea level are approximately equal to those at higher altitudes at the top of the storm. In other words, the vertical wind shear must be small. The forming hurricane prefers to remain intact vertically; this way, it draws the most energy from the warm seawater. If the wind at altitude blows harder or in a different direction than at sea level, the hurricane gets blown apart.
Hurricane Season of 2024 in the North Atlantic
The hurricane season of 2024 was above average active. 18 tropical storms formed, 11 of which developed into hurricanes. Of these 11 hurricanes, 5 were even very strong hurricanes.
The hurricane season of 2024 was also a very special season: never before had a Category 5 hurricane been seen so early in the season (on July 2), hurricane Beryl. Additionally, there was a very remarkable silence. Between mid-August and mid-September, typically the peak of the hurricane season, no hurricanes formed.
Expectations for the Hurricane Season of 2025
The expectations for the upcoming hurricane season are, compared to last years expectations, more favorable. First, the (weak) La Niña conditions officially ended in April. A La Niña leads to more hurricane activity in the Atlantic Ocean, while its counterpart El Niño leads to less activity. For this season, the expectations are that we will not experience either, something we call neutral conditions.
Additionally, the seawater is currently less warm than it was last year, meaning that hurricanes generally have less energy at their disposal. However, the seawater is still warmer than the long-term average. Therefore, it is expected that we will still experience a more active hurricane season than average, but less active than last year. Colorado State University, a leading institution in hurricane forecasting, expects 17 storms of at least tropical storm strength (wind speeds of at least 18 m/s), of which 9 hurricanes and 4 major hurricanes. NOAA, the American meteorological institute, published a similar expectation yesterday: 13-19 storms of at least tropical storm strength, of which 6-10 hurricanes and 3-5 major hurricanes. The average for the period 1991-2020 is 14 storms, 7 hurricanes, and 3 major hurricanes.