I thought I’d go over some hurricane basics.
“Tropical cyclones” is the generic term for an organized system of convective clouds that rotate around an area of low pressure over tropical or subtropical waters. For these storms to strengthen, the ocean temperatures must be at least 80 degrees Fahrenheit or warmer. The eye wall of a hurricane is its engine and warm seawater its fuel. The warmer the water, the faster it will intensify.
According to the Saffir-Simpson Hurricane Wind Scale, once these systems reach a sustained wind speed of 74 mph or greater, it’s classified as a hurricane, typhoon or cyclone. The term “hurricane” is used in the Atlantic and Northeast Pacific.
In the Northwest Pacific, it’s called a “typhoon,” and “cyclones” happen in the South Pacific and the Indian Ocean. Category 2 hurricanes have sustained winds of between 96 and 110 mph, while a category 3 hurricane ranges between 111 and 129 mph. A category 4 hurricane goes from 130 to 156 mph, while anything above 157 mph sustained wind is classified as a Category 5 hurricane. Category 3, 4 and 5 are considered major hurricanes, and here’s why. Doubling the wind speed does not double its force upon an object, like a house, but quadruples it. A Category 5 hurricane is 500 times stronger than a Category 1 cyclone.
Which leads to the question what is the maximum sustained winds that a hurricane can produce? MIT atmospheric scientist Kerry Emanuel theorized about 200 mph using the variables of seawater temperature, the temperature at the cloud-top level and how fast heat moves from the ocean to the atmosphere.
However, in October 2015, Hurricane Patricia that tracked along Mexico’s western coast reached sustained winds of 213 mph with gust estimated well over 225 mph! A National Oceanographic and Atmospheric Administration’s P-3 Orion, nicknamed “Miss Piggy,” flew through the eye of Hurricane Patricia. This highly instrumented aircraft took various atmospheric measurements as it made two passes through the storm and confirmed that Patricia reached the lowest atmospheric pressure ever recorded in the Western Hemisphere; an awe-inspiring 879 millibars or 25.96 inches of mercury (inHg) at the ocean’s surface. That’s low, real low. My barometer at home only goes down to 28 inHg. So far, Patricia is the strongest hurricane in recorded history.
Hurricane Irma reached 185 mph over the Caribbean and maintained these wind speeds for more than 37 hours. No storm on record, anywhere on the globe, has maintained winds 185 mph or above for such an extended period. Current research is attempting to understand why these storms reach more than 200 mph and last for so long.
Not only can tropical cyclones produce nearly unimaginable winds and high storm surge, but they are also extremely efficient rain producers as they draw large amounts of water vapor into the atmosphere from seawater and liberate vast quantities of latent heat as water vapor condensed into clouds and rain. Hurricane Harvey unloaded more than 50 inches of rain east of Houston, the most ever recorded in the continental United States from a single storm.
So why are all these records being broken? A warmer atmosphere can hold more water vapor. For every 1 degree Fahrenheit increase in temperature, the air can hold around four percent more water vapor. This condition can lead to more intense storms and rainfall events. As the atmosphere and oceans continue to warm, storms are expected to become more intense.
By the way, the first person to name tropical cyclones was Clement Lindley Wragge of Queensland, Australia, in the late 1800s. Wragge had an inclement temper, and he took to naming cyclones after politicians whom he disliked. A forecaster could publicly describe a storm named after a politician as “wandering about the ocean with no aim or purpose.”
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John Lindsey’s column is special to The Tribune. He is PG&E’s Diablo Canyon marine meteorologist and a media relations representative. Email him at firstname.lastname@example.org or follow him on Twitter @PGE_John.