https://phys.org/news/2018-08-nasa-debby-transitioned-tropical-storm.html

On Aug. 8 at 1:05 a.m. EDT (0605 UTC), the Moderate Resolution Imaging Spectroradiometer or MODIS instrument aboard NASA's Aqua satellite captured an infrared image of Debby. The image showed three areas of strongest thunderstorms. They were around the center of circulation, southeastern, southwestern quadrants and in a band of thunderstorms south of the center. MODIS infrared data showed that some of those storms had cloud top temperatures as cold as minus 50 degrees Fahrenheit (minus 45.5 degrees Celsius), indicating they are high in the troposphere. Cloud tops continued to warm indicating stronger uplift in the storm, and the transition into a tropical cyclone.

The National Hurricane Center (NHC) said of the transition, "Over the past several hours, deep convection with cloud tops of minus 55 to minus 60 degrees Celsius has developed in the southeastern semicircle, with some of the convective tops covering the previously exposed low-level circulation center. In addition, outer banding features have dissipated, and an elongated upper-level anticyclone has developed over the cyclone. These convective- and synoptic-scale features indicate that Debby has made the transition from a subtropical to a tropical cyclone."