From wildfires to flooding to weeks of triple digit temperatures, nearly every part of the country experienced some form of extreme weather over the last few months. Though hurricanes are one form of extreme weather that frequently makes headlines, this year didn’t see any on the scale of some previous storms, such as Katrina or Maria. While overall trends are still being studied, knowledge of what causes hurricanes as well as other weather events is key to a greater understanding of how wind, water, air, and our effect on them are all connected.
Both ocean temperatures and atmospheric conditions play a role in the formation of hurricanes. The National Oceanic and Atmospheric Administration (NOAA) states that the official definition of a hurricane includes rotating circular winds with an “eye” surrounded by an “eyewall” where winds are the strongest. In the northern hemisphere, winds rotate counterclockwise and in the southern hemisphere, they rotate clockwise. Jessica Storm, a meteorologist with the National Weather Service in Burlington, VT, said that the reason for the difference between hemispheres was due to the Earth’s rotation. Philippe Papin, a hurricane specialist with the National Hurricane Center, which is part of the NOAA, said that ocean temperatures generally need to be around 79-80 degrees Fahrenheit, though he noted this was not a “hard and fast rule” and that hurricanes could form in lower temperatures. He said that unstable air conditions, which allowed thunderstorms to grow and intensify, were also an important factor in the formation of hurricanes. Storm concurred stating that a cooling atmosphere creates moist air which can contribute to hurricane formation. She also noted that hurricanes tend to form within 300 miles of the equator because that is where warmer ocean temperatures tend to be. The official “season” starts June 1 and runs until November 30, but hurricanes can happen at other times.
Hurricanes are divided into 5 categories based on wind speed and the Saffir-Simpson scale is as follows:
Category 1: 74-95 mph
Category: 2 96-110 mph
Category 3: 111-129 mph
Category 4: 130-156 mph
Category 5: 157 or higher
Storm said that the vertical wind shear, which is the change of wind speeds in different parts of the atmosphere can influence the strength of a hurricane. Papin said in addition to wind shear, a lot of moisture can also play a role in a hurricane’s strength. According to the NOAA National Hurricane Center’s data by decade on hurricanes to hit the US from 1851-2022, the decade with the most hurricanes was from 1941-1950 with 24 total storms. It was also the decade with the most category 3-5 storms with 5 category 4 storms and 5 category 5 storms. From 1851 to 1935 there were 161 total hurricanes to hit the US and 46 storms that were category 3-5. From 1936 to 2020 there 141 total storms with 48 that were category 3-5. The above statistics represent the 17 full decades covered by the data. However, from 2021-2022, there were 3 storms to hit the US and 2 that were category 3-5. (Both of those were category 4). Papin noted however, that earlier records are “spotty” and satellite images as well as surface temperature observations have greatly improved over the last 50 years.
El Nino years affect hurricane activity. El Ninos result from an increase in ocean temperatures of 0.9 degrees for at least 5 successive overlapping 3 month seasons. Trade winds along the equator weaken and low pressure over Australia and Indonesia rises. This prevents normal upwellings of nutrients and causes fish to migrate. El Nino years have strong upper level winds, which Storm said can “topple” hurricane winds, which are more likely to be in the lower part of the atmosphere. Papin said that in the Atlantic El Nino years generally result in less hurricane activity due to more change in wind speeds. However, he said that in the Pacific, they tend to have the opposite effect because of warmer ocean temperatures.
Papin said that there is “some evidence” that hurricane activity is increasing in the Atlantic, but in the Pacific, hurricanes have been staying about the same or decreasing, but the evidence is “not conclusive.” He also noted that a warmer atmosphere holds more moisture which thus leads to more rainfall. As mentioned in the beginning of this article, the year 2023 saw quite a number of weather-related events. Water temperatures, wind, and atmospheric conditions all influence both the formation and strength of hurricanes. The climate is a complex topic, with many forces that affect it and sometimes those forces can seem to contradict each other. Nevertheless, an understanding of how wind, water, and air are connected gives us a greater understanding of how our climate works and our effect on it.
(For other climate related articles on this blog, see two on wildfires from Oct. and Dec. 2019 and one on climate averages from Aug. 2021).
The Nature Journal 