The Saffir-Simpson Hurricane Wind Scale Explained

Just like you might plant seeds in your garden, the way tropical storms and hurricanes form can come from different types of meteorological seeds.

Here's five of them, starting with the most common pathway, followed by four others we sometimes see.

1. Tropical Waves

• Big Picture: Tropical waves are one of the most common seeds watched for tropical development as they move westward from Africa across the Atlantic Ocean toward the Caribbean Sea and Gulf. According to the National Hurricane Center, 60 tropical waves track across the Atlantic Ocean each year. Roughly one in five of these tropical waves becomes an Atlantic basin tropical depression, storm or hurricane.
• How It Happens: These are not waves in the ocean in the sense that you can play in them at the beach, but rather areas of spin, moisture and energy that hurricanes can form from under the right mix of atmospheric and oceanic conditions. Sometimes these tropical waves can form into storms immediately after exiting the African coast. Other times they can wait longer to develop in order to find proper conditions, perhaps not until reaching the Caribbean or near the Bahamas, for example.

(IN DEPTH: Tropical Waves 101)

2. Dying Fronts

• Big Picture: Tropical storm development can sometimes occur when cold fronts stall out and eventually fizzle in the Gulf or the western Atlantic Ocean. That's especially the case both early and late in the hurricane season.
• How It Happens: The fronts are an immediate source of lift and spin in the atmosphere, which can give birth to a consolidated area of low pressure. If the low gains persistent thunderstorm activity amid favorable upper-level winds and warm water temperatures, then a tropical depression or storm can form as the front dies away.

3. Thunderstorm Complexes

• Big Picture: Thunderstorm complexes, called mesoscale convective systems by meteorologists, often cross through the southern tier of the United States during the early part of hurricane season and usually lose steam as they bubble eastward. On rare occasions, they can remain intact and move into the Gulf or off the Southeast coast and help generate the formation of a tropical storm or hurricane.
• How it Happens: These complexes of storms often contain a small-scale area of low-pressure spin. That spin festering over warm enough waters amid favorable upper-level winds can trigger persistent thunderstorm development which slowly consolidates into a tropical depression or storm. The best recent example was in July 2019, when an area of spin was formed by a complex of thunderstorms located over Kansas on July 2. That spin moved east, then south and eventually gave rise to Tropical Storm (later a hurricane) Barry several days later over the northern Gulf.

4. A Gyre Can Form Storms Early And Late In The Season

• Big Picture: The term "gyre" refers to a broad area of low pressure that sometimes sets up, as its name implies, over Central America. Tropical development from this pathway is most likely to happen early and late in the hurricane season.
• How It Happens: Sometimes a smaller area of low pressure will consolidate and break away from the parent larger area of low pressure, or gyre. When that happens, a tropical depression or storm can develop if the low remains over water. What's often in question in the longer-range forecast is whether the storm will develop on the Caribbean or Eastern Pacific side of Central America.

(IN DEPTH: Central American Gyre)

5. Non-Tropical Low Becomes Tropical

• Big Picture: Another rarer form of tropical development that most often happens at higher latitudes can come from non-tropical low-pressure systems in the upper atmosphere located in the northern Atlantic Ocean. These lows can sometimes first transition into a hybrid subtropical depression or storm. From there, some of them become full-fledged tropical depressions, storms or even hurricanes.
• How It Happens: Bear with us, this is complicated to explain. Colder non-tropical storms thousands of feet above the Earth first generate an area of low pressure closer to the ocean's surface. This can happen when a strong vertical temperature gradient develops. This gradient typically comes about when a cold air mass forms over or moves over lukewarm waters. When this gradient is big enough, thunderstorms can grow, kicking off the transition. Once that happens, the low often goes through the subtropical phase by gaining energy from just warm enough oceans, as tropical storms or hurricanes do, but also have colder air aloft from the original non-tropical low-pressure system. If thunderstorms are persistent, waters are warm enough, and upper-level winds are cooperative, then they can make the full transition into a tropical storm.

Chris Dolce has been a senior digital meteorologist with weather.com for nearly 15 years after beginning his career with The Weather Channel in the early 2000s.