(female narrator) September is peak Atlantic hurricane season, which extends from June through November, when we await the next big storm to see where it will make landfall and what they'll name it. While we wait, let's consider why even have a season for hurricanes in the first place. In order to understand why we have a hurricane season, let's build a hurricane. The perfect hurricane recipe calls for precursor storms, warm waters, and low wind shear. Let's start with precursor storms. Hurricanes build from smaller what are called precursor storms. Start with a small disturbance, or a thunderstorm, add warm water and wind, which we'll discuss, to build the storm bigger and bigger and bigger until it becomes a hurricane, a.k.a. tropical cyclone, with sustained winds of 74 miles per hour or more. That's like putting your head out the window on the highway. In the Atlantic area, these precursor storms sweep in from Africa in what are called easterly or tropic waves-- bands of thunderstorms that move west across the tropics. These are our precursor storms. Another ingredient in the hurricane recipe is a warm ocean. As air flows over the ocean, it transfers heat and water vapor up into the atmosphere. Through a cycle of evaporation and condensation, the warm, wet air builds clouds and creates updrafts. This warm, wet air acts as fuel. Add more fuel, and you build a stronger hurricane. Hurricanes gain strength by forming high into the atmosphere. In order to form high into the atmosphere, the atmosphere must be somewhat calm-- not too windy up high or down low. If one or the other changes, the hurricane topples over. So we need low wind shear, which means there's little difference between the wind in the high versus low atmosphere. Our hurricane stays standing, and its circulation is not disrupted by strong winds. Now let's look at why hurricanes only form in the Atlantic during the summer to fall months. To do that, let's look again at our basic hurricane ingredients. Remember those precursor storms blowing in from Africa? Those easterly waves form around summer and fall and peak in September. But due to factors involving temperatures and other atmosphere science we won't discuss now, in our winter, all is quiet on that eastern front, and there are no real disturbances waiting in the wings. And winter in the northern hemisphere means cooler ocean waters. Cool waters means no fuel, means no hurricanes. Finally, in the winter months, the jet stream, which had been farther north during summer, shifts back down south, where it creates high wind shear in the prime area for hurricane formation. With all of these factors in play, the winter months of the Atlantic region are hurricane-free, and we should consider ourselves lucky. In other areas of the world where ocean water is warm year round and wind shear is weak, tropical cyclones can occur all year long.

(Describer) ...in the Western Pacific.

That begs another question. With the climate warming and all, will the Atlantic hurricane season expand? And scientists have determined that that's a real possibility. For now, we'll hold our breath until the hurricane season comes to a close and hope that this year we don't make it all the way to Wanda or Walter or Whitney.

(Describer) Seasonal Science by Melissa Salpietra. Accessibility provided by the US Department of Education.

Accessibility provided by the U.S. Department of Education.