Glowing UFO caught from an airplane in USA

On January 2, 2017, a witness was inside an airplane in USA. While, he was somewhere between New York and Fort Lauderdale, he has seen a mysterious bright UFO in the sky.

Witness statement:

While on a Delta Airlines flight from NY to Fort Lauderdale flying over the Norther Atlantic Ocean, flying at approximately 29,000 feet, several passengers and I observed a silver oval object hovering over the clouds. The object changed from solid to translucent. I took several photographs of the object with my I-phone. The object was at an altitude of approximately 40,000 feet. The object was last observed moving in an unknown direction into clouds. No other aircraft were observed at the time of the sighting. At first sight I thought the object was a plane and then a weather balloon, but then the object was observed to appear to almost disappear/invesible or become translucent in quick intervals.