On September 6, 2017, Hurricane Irma slammed into the Leeward Islands on its way toward Puerto Rico, Cuba, and the U.S. mainland. As the category 5 storm approaches the Bahamas and Florida in the coming days, it will be passing over waters that are warmer than 30 degrees Celsius (86 degrees Fahrenheit)—hot enough to sustain a category 5 storm. Warm oceans, along with low wind shear, are two key ingredients that fuel and sustain hurricanes.
The map above shows sea surface temperatures in the Atlantic Ocean, Caribbean Sea, and Gulf of Mexico on September 5, 2017. The data were compiled by Coral Reef Watch, which blends observations from the Suomi NPP, MTSAT, Meteosat, and GOES satellites and computer models. The mid-point of the color scale is 27.8°C, a threshold that scientists generally believe to be warm enough to fuel a hurricane. The yellow-to-red line on the map represents Irma’s track from September 3–6.
By definition, category 5 storms deliver maximum sustained winds of at least 157 miles (252 kilometers) per hour. When it hit the Leeward Islands, Irma’s winds surpassed 185 miles (295 kilometers) per hour, making it the strongest storm to ever hit the islands and one of the strongest storms ever measured in the Atlantic basin.
The Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi NPP satellite captured a nighttime view of the storm at 1:35 a.m. local time (05:35 Universal Time) on September 6 as the eye was over the island of Barbuda. The image was acquired by the VIIRS “day-night band,” which detects light in a range of wavelengths from green to near-infrared and uses filtering techniques to observe signals such as city lights, auroras, wildfires, and reflected moonlight. In this case, the clouds were lit by the full Moon. The image is a composite, showing storm imagery combined with VIIRS imagery of city lights.
The Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite acquired the third image at 10:35 a.m. local time (14:35 Universal Time) on September 6, 2017. By then, the storm had also hit Anguilla and was poised to strike the Virgin Islands.