NASA's Infrared Image of Tropical Depression 21W (image) NASA/Goddard Space Flight Center Share Print E-Mail Caption Infrared imagery from the AIRS instrument aboard NASA's Aqua satellite captured this image of Tropical Depression 21W as a developing low pressure area on Sept. 30. The purple areas indicate the most powerful thunderstorms with coldest cloud top temperatures. Credit NASA/JPL, Ed Olsen Usage Restrictions None Share Print E-Mail Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.