By Jonny Lupsha, Wondrium Staff Writer
Tropical cyclones are weather events born in the tropic areas of Earth. They encompass other phenomena like hurricanes and typhoons and constitute a major component of extreme weather. What defines a tropical cyclone?
Tropical cyclones are rapidly rotating storms that tend to originate over tropical oceans. They don’t have “fronts,” nor do they form in places with horizontal temperature gradients, which are temperature changes across the surface. They are low pressure systems and can wreak havoc on whatever land over which they pass.
Recently, Australia was devastated by its most impactful tropical cyclone in eight years—Cyclone Ilsa—bringing winds nearing 200 miles per hour. What makes Ilsa a tropical cyclone? In his video series Meteorology: An Introduction to Wonders of the Weather, Dr. Robert G. Fovell, Professor of Atmospheric and Oceanic Sciences at the University of California, Los Angeles, explains what a tropical cyclone is and why it matters.
What Is a Tropical Cyclone?
Generally speaking, “tropical cyclone” is the name given to hurricanes and typhoons.
“Like their name suggests, tropical cyclones are born in the tropical areas of the Earth, but no tropical regions are favorable to tropical cyclones at all times, and some never see tropical cyclone development at all,” Dr. Fovell said. “And once formed, many tropical cyclones don’t see fit to stay in the tropics. That usually doesn’t do them any good—or us either.”
According to Dr. Fovell, tropical cyclones are roughly circular regions of low pressure that feature powerful counterclockwise-turning winds in the Northern Hemisphere. These traits are shared with extratropical cyclones, but this is where their similarities end.
“Unlike extratropical cyclones, the strongest winds in a tropical cyclone are near the ground, although not right at the ground, owing to friction,” he said. “In other words, wind speeds decrease with height in the fully-formed tropical cyclone, the opposite of what we typically find in mid-latitude cyclones.”
Another thing that separates tropical cyclones apart from extratropical cyclones is that tropical cyclones need the ocean. They form over water and need sea surface temperatures to be warm enough to accommodate them. Extratropical cyclones merely use the ocean’s ability to increase the horizontal temperature contrast.
When and Where Do Hurricanes Form?
“Hurricanes are a phenomenon of the summer and fall seasons,” Dr. Fovell said. “It extends as far as it does into fall only because of the high heat capacity of liquid water. Why do hurricanes need such warm water? One reason is that very warm ocean water, extending over a wide area, helps make the tropical atmosphere moist and unstable.
“The hotter the water is, the more moisture it can provide to the air above it.”
According to Dr. Fovell, hurricanes only constitute one-fifth of all tropical cyclones that make landfall in the United States, but they cause more than 80% of the damage. Meteorologists look at tropical cyclone tracks for an entire year to determine their paths of destruction.
“The starting and ending points of each track are not identified, but we should realize that they start closer to the equator and then move away,” he said. “In the Northern Hemisphere, we see track clusters in the Atlantic, the East Pacific, and the West Pacific as well. The Atlantic and West Pacific tracks share a similar shape; they’re generally moving northwestward at first, away from the equator, and then curve eastward.”
Meteorology: An Introduction to the Wonders of the Weather is now available to stream on Wondrium.