October 26, 2015

Hurricane Patricia, seen on Friday, October 23, 2015, was the strongest storm ever recorded in the Western Hemisphere. (NOAA)

On October 21, a small tropical storm formed from a system of thunderstorms in the Pacific Ocean off the coast of Mexico. Just two days later, that storm became became Hurricane Patricia, the strongest storm ever recorded in the Western Hemisphere, with winds of 200 miles (320 kilometers) per hour. Soon after, the storm hit coastal villages in Jalisco in western Mexico. Then, less than a day later, Hurricane Patricia fizzled out and was gone. The storm’s rapid development and disappearance caused scientists to ask new questions about extreme weather’s connection to worldwide climate cycles.

Hurricanes are powerful, swirling storms that begin over warm seas. The storms then move westward, often toward the poles. When a hurricane hits land, it can cause great damage through fierce winds, torrential rain, flooding, and huge waves crashing ashore.

Hurricanes require a special set of conditions, including ample heat and moisture, that exist primarily over warm, tropical oceans. For a hurricane to form, there must be a sufficiently warm layer of water at the top of the sea. This warm seawater evaporates into the air. The moisture then condenses (changes into liquid), forming clouds. As the moisture condenses, it releases heat that warms the air, causing the air to rise. The warm, rising air creates a region of relatively low atmospheric pressure. (Atmospheric pressure is the weight of the air pressing down on a given area.)

Air tends to move from areas of high atmospheric pressure to areas of low pressure, creating wind. Earth’s rotation can cause the wind to swirl as it blows into a low-pressure area. As the swirling winds increase in speed, more ocean water evaporates and condenses. The moisture releases more heat, further warming the storm’s core. The warm air rises faster, increasing surface wind speeds, and so on. This cycle, called a positive-feedback loop, continues to strengthen the hurricane. Only when friction between the air and the water surface becomes great enough will a hurricane weaken.

Scientists still aren’t sure why such hurricanes as Patricia intensify so quickly. In Patricia’s case, some of the energy may have come from exceptionally warm surface waters in the region, caused by this year’s developing El Niño event. El Niño is part of the interaction between Earth’s atmosphere and the tropical waters of the Pacific Ocean. An El Niño occurs about every two to seven years, and it can affect climate throughout the world. The El Niño of 2015 is shaping up to be one of the strongest events on record. Studying Hurricane Patricia’s development will help meteorologists make better predictions concerning extreme weather in regions affected by El Niño.

Despite Patricia’s incredible intensity, it caused relatively little damage to Mexico. It struck an area of the country where few people live. Evacuation efforts were also effective. The hurricane’s enormous strength quickly waned over land, as tall coastal mountains helped beak up the storm. Overall, fewer than 10 people died and damages were not expected to exceed $200 million.