Weather Watch

How Patricia became the strongest hurricane ever in the Western Hemisphere

A P-3 Orion Hurricane Hunter aircraft used by the National Oceanic and Atmospheric Administration.
A P-3 Orion Hurricane Hunter aircraft used by the National Oceanic and Atmospheric Administration. Courtesy of the Department of Commerce

A P-3 Orion is a large four-engine turbo-prop aircraft, weighing about 135,000 pounds when loaded. The National Oceanographic and Atmospheric Administration’s P-3, nicknamed “Miss Piggy,” purposely flew through the eye of Hurricane Patricia that rapidly intensified off the southwest coast of Mexico on Friday.

This highly instrumented aircraft took numerous 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.

Generally speaking, the lower the atmospheric pressure within the tempest, the stronger the storm. My barometer at home only goes down to 28 inHg. That’s approximately the same pressure you would find on top of a 4,000-foot mountain peak. For comparison, in 2005 Hurricane Katrina intensified to 920 millibars. The steep pressure gradients of Patricia produced sustained winds of 200 mph with gust estimated at over 225 mph!

So what exactly is air pressure and why did Patricia reach record strength?

The strong invisible force of gravity accelerates trillions of air molecules toward the Earth’s surface and produces weight. The weight of the air is what we feel as pressure. Imagine a 1-square-inch column of air measured from the top of the atmosphere down to sea level — it would weigh about 14.7 pounds. All the air surrounding the Earth would weigh about 5,600 trillion tons!

Normally, we don’t notice it because our bodies maintain an internal pressure that balances the external pressure. But swift changes in elevation can cause us to detect atmospheric pressure changes. One local example can be found driving over the Cuesta Grade, which is 1,522 feet above sea level. Maybe you have felt your ears “pop.” This is caused by your inner ear trying to equalize the pressure from the outside air.

Meteorologists often express air pressure in units called millibars. The standard atmospheric pressure is 1,013 millibars at sea level. Depending on the surrounding atmospheric conditions, levels above this may be considered areas of high pressure. Strong high pressure usually produces clear and dry weather.

The highest pressure ever recorded in the continental United States occurred in December 1983 in Miles City, Mont., which reached 1,064 millibars.

On the flip side, and once again depending on the surrounding atmospheric conditions, levels below standard atmosphere are areas of low pressure. Along the Central Coast, I consider anything less than 990 millibars to be a deep low-pressure system.

On Jan. 21, 2010, an intense low-pressure system moved down the California coastline. The barometer at the Cape San Martin marine buoy, 55 miles west-northwest of Morro Bay, reached 978.3 millibars, while the Diablo Canyon Ocean Lab barometer decreased to 978.7 millibars. These atmospheric pressure readings were some of the lowest I have seen for our area.

Later that day, according to Joe Sirard, a meteorologist at the National Weather Service in Oxnard, the pressure dropped to 980 millibars at Los Angeles International Airport, setting an all-time record for that location since readings began in 1931.

The lowest sea-level pressure ever recorded worldwide was 870 millibars when Typhoon Tip passed Guam and rapidly intensified as it moved toward Japan during October 1979. Eight ships were grounded or sank as a result of the typhoon. A Chinese freighter broke in half, but its crew of 46 was rescued. Tragically, 13 U.S. Marines lost their lives at Camo Fuji in Japan.

As was predicted earlier this year, the current strong El Niño has significantly reduced hurricane activity in the Atlantic but increased it in the Eastern Pacific.

Scientists believe that abnormally warm sea temperatures due to El Niño and further elevated by global warming is probably the reason Patricia intensified so quickly. These warmer waters produced greater amounts of evaporation. As the air rises, it condenses. This condensation process released latent heat and, like turning on the burner in a hot-air balloon, it warms the surrounding air, causing it to lift even further. In physics, we call this a positive feedback loop.

According to the Climate Prediction Center, a strong El Niño is still on track to produce stormy weather this winter. PG&E urges its customers to be prepared before storms arrive. Here are some useful tips:

  • Have battery-operated flashlights and radios with fresh batteries. Listen for updates on storm conditions and power outages.
  • If you have a cordless phone that requires electricity to work, have a standard phone or cell phone as backup.
  • Keep your cell phone charged, and have a portable charger handy.
  • Freeze plastic containers of water for ice that can keep food from spoiling in your refrigerator/freezer during an outage.
  • If you have a stand-by generator, notify PG&E and make sure it’s installed safely. Information on the safe installation of generators can be found at