NASA's Atmospheric Infrared Sounder (AIRS) captured Hurricane Hilary on the morning of Aug. 18, 2023, when it was a Category 4 storm roughly 470 miles (760 kilometers) south of Baja California. Hilary could be the first tropical storm to make landfall in California since 1939, according to the National Weather Service.
Hilary grew from a tropical storm into a Category 2 hurricane within 24 hours on Aug. 17. Another period of rapid intensification – an increase in maximum sustained wind speed of at least 30 knots (35 mph) within 24 hours – occurred Aug. 17-18. The animation (below) shows some of this rapid growth, with images taken by AIRS Aug. 15-18. This intensification was driven by very warm ocean surface waters and weak wind shear, a term for vertical changes in wind speed. Strong wind shear can keep hurricanes from forming, or can tear them apart.
AIRS measures cloud temperatures in infrared wavelengths, which can reveal information about the atmosphere not visible to the human eye. Hilary shows several indicators of a powerful hurricane: a well-defined eye surrounded by a ring of very cold clouds in purple, with warmer outer regions seen in yellows and oranges. Purple and violet areas are colder, between about minus 82 degrees Fahrenheit and minus 46 F (minus 63 degrees Celsius to minus 44 C). Blue and green regions are roughly minus 28 F to 26 F (minus 33 C to minus 3 C). The cooler parts of the clouds are associated with very heavy rainfall.
Most hurricanes in the Pacific Ocean off Mexico travel westward, following tropical trade winds. Occasionally, one of these storms will head northward. Hurricane Hilary is being steered by a weak low-pressure system off the coast of California, an area normally dominated by high pressure and an atmospheric circulation pattern that would deflect storms from the region.
The current forecast from the National Hurricane Center has Hilary closely following the western coastline of the Baja California peninsula, weakening as it moves north. Rainfall projections for Southern California range from 2 inches (5 centimeters) in coastal areas to 8 or more inches (20 or more centimeters) in local mountains. For comparison, San Diego and Los Angeles receive no rain in August most years, and the wettest parts of the local mountains receive about 1 inch (3 centimeters) of rain over a normal summer.
In conjunction with the Advanced Microwave Sounding Unit (AMSU), AIRS senses emitted infrared and microwave radiation from Earth to provide a 3D look at the planet's weather and climate. Working in tandem, the two instruments make simultaneous observations down to Earth's surface. With more than 2,000 channels sensing different regions of the atmosphere, the system creates a global, 3D map of atmospheric temperature and humidity, cloud amounts and heights, greenhouse gas concentrations, and many other atmospheric phenomena. Launched into Earth orbit in 2002 aboard NASA's Aqua spacecraft, the AIRS and AMSU instruments are managed by NASA's Jet Propulsion Laboratory in Southern California, under contract to NASA. JPL is a division of Caltech.
More information about AIRS can be found at https://airs.jpl.nasa.gov.
Original image and animation release in the JPL Photojournal: https://photojournal.jpl.nasa.gov/catalog/PIA25779