How does such a strong storm form? What can we do to prepare for the next one?
Last week Mexico braced for the worst: the first Category 5 hurricane to strike the western coast in over 50 years and the strongest hurricane ever recorded in the entirety of the western hemisphere. Luckily, Hurricane Patricia dissipated even faster than it intensified, leaving the death toll at zero and sparing any major cities. However, remnants of the storm continue to cover much of Texas in heavy rain and add to the flood threats already plaguing the southeastern United States.
Why was there so little notice before the hurricane made landfall? What can we do to prepare for the next one?
What Is a Hurricane?
Hurricanes, or tropical cyclones, are fast rotating storm systems that typically form over the tropics near the equator and sustain high winds with speeds over 74 miles per hour. A few key ingredients are necessary for the formation of a hurricane.
Hurricanes are fueled by warm, moist air. Ocean temperatures of at least 80 degrees Fahrenheit and down to depths of at least 50 meters (or 165 feet) are needed in combination with winds that aid in the evaporation of this warm water off the ocean’s surface. As this newly-evaporated warmer air rises, a low pressure area is left in its wake at the surface. The surrounding air will then push into this low pressure area but will in turn also get heated and begin to rise forming a column of humidity. Once this column grows tall enough, water vapor at the very top will begin to cool again and form clouds, all the while rotating around the center of the column or what has become the eye of the storm.
Once wind speeds of 39 miles per hour are reached, a low pressure system officially becomes a tropical storm. After winds exceed 74 miles per hour, we have a tropical cyclone. The lower the pressure in the low pressure system, the more intense the storm and the higher the potential for destruction (except for rainfall which is independent of pressure and wind speeds).
Tropical cyclones that form in the Atlantic Ocean basin or in the Pacific Ocean basin from the coastal U.S. out to a longitude of 180 degrees West (the International Date Line which is past Hawaii) are called hurricanes and are monitored by either the National Hurricane Center or the Central Pacific Hurricane Center. If the same storm were to form instead further west between the longitude of 180 degrees West and the coast of Asia, it would be called a typhoon and monitored by the Joint Typhoon Warning Center and the Japan Meteorological Agency. If wind speeds exceed 150 miles per hour, the cyclone is called a super typhoon. Cyclones that move west will change names, like Hurricane Genevieve in 2014 that became Typhoon Genevieve and eventually Super Typhoon Genevieve as it passed the International Date Line.
The Strongest Hurricane Ever Recorded in the Western Hemisphere
Less than 24 hours before the storm reached the coast of Mexico, sustained winds of 200 miles per hour were measured for Hurricane Patricia which was quickly classified as a Category 5 Hurricane: those with the highest potential for destruction. Hurricane Katrina in 2005 was a Category 4. Such strong winds have never been recorded for a hurricane in the Western hemisphere. In fact, the maximum winds on record at all are only slightly higher at 215 miles per hour during Typhoon Nancy in 1961 according to the World Meteorological Organization.
Hurricane Patricia may also be both the lowest pressure system on record and the storm with the largest recorded pressure drop. Roughly twelve hours before making landfall, Hurricane Patricia weighed in at 880 millibars of pressure, a drop of roughly 100 millibars in pressure in only 24 hours. For reference, Hurricane Wilma in 2005 holds the current record for the lowest measured pressure of 882 millibars.
Since we are still in the immediate aftermath of the storm, the precision of these measurements is still being verified by the National Oceanic and Atmospheric Administration. It is clear, however, that the destruction from Hurricane Patricia could have been far worse.