These images generated from data retrieved by the sensors in the AIRS instrument suite show Hurricane Frances on Monday, August 30. With sustaining winds of 125 mph (195 km/hr) reaching out 85 miles from the eye, this powerful storm was a category 3 hurricane at the time this image was taken. Located in the Atlantic about 265 miles (425 km) east-northeast of the northern Leeward Islands, Frances on August 30 was moving westward north of Puerto Rico towards the Bahamas at 12 mph (19 km/hr).
The image above shows how the storm looks through an AIRS Infrared window channel. Window channels measure the temperature of the cloud tops or the surface of the Earth in cloud-free regions. The lowest temperatures are associated with high, cold cloud tops that make up the top of the hurricane. The infrared signal does not penetrate through clouds, so the purple color indicates the cool cloud tops of the storm. In cloud-free areas, the infrared signal is retrieved at the Earth's surface, revealing warmer temperatures. Cooler areas are pushing to purple and warmer areas are pushing to red.
The microwave image reveals where the heaviest precipitation in Frances is taking place - these are the blue areas surrounding the eye of the storm. Rain appears cold in the microwave, and blue in this image. (The extensive blue patches away from Frances indicate clear skies. A clear ocean surface looks very cold in the microwave; the warmer green patches are areas where clouds obscure the ocean surface.)
An interesting and unusual aspect of these images is cold microwave temperatures associated with warm infrared temperatures in an arc north of the eye of the storm. Normally, very cold microwave temperatures in cloudy regions indicate the presence of convection and precipitation. Corresponding infrared temperatures are then also low, indicating high, cold clouds. In this case, the infrared temperatures are warmer than the surrounding regions. The warm anomaly is about 15 degrees Kelvin (27 F), which is what one would observe if the cloud tops were 50-100 mb lower in the atmosphere (where it is that much warmer).
One explanation for such a lowering would be an upper level downdraft that lowers the cirrus shield that is always present over a hurricane. Another explanation would be that the latent heat released by the intense convection may cause the local air temperature to rise by that much. Finally, it is also possible that the area is almost clear. The microwave instrument would then see the ocean surface (which appears very cold and sometimes looks similar to the precipitation signature), and the infrared instrument would see a mix of cold cloud tops and warm ocean surface, which would appear warmer than surrounding areas that are completely covered by clouds. The AIRS retrieved quantities are silent on this issue since the system is unable to peer deeply into this region of wild winds, heavy rain and thick clouds.