Flying home from England, I looked out the window to see the sights. We were flying well above the cloud cover. It was a glorious, sunny day. Some distance away, I saw a small dark sphere that seemed to hover above the clouds. Upon closer inspection, it almost seemed to be bobbing and weaving, maintaining its position relative to the cloud deck.The object seemed distant, yet it was about the size of a pea as seen through the aircraft window. It was perfectly round and dark, with a dark reddish-brown cast to it. It was perfectly outlined against the brilliant white clouds, and did reflect sunlight back from its edges as if it were very reflective but part of it was in shadow. (?) There was only one object, and it remained relatively motionless, just sort of bobbing and weaving. On several ocassions, the object seemed to drift too much in a vertical or horizontal distance, then it quickly and smoothly returned to its original position. It stayed in full-view for as long as I could see as the airplane progressed. My seatmate was asleep, this being a flight from Birmingham, UK to Newark, NJ. So, there was no one else for me to confirm the sighting with. I don't know what it was, but it seemed strange. Do balloons of any kind ascend to that height? I seem to recall the pilot saying that we were at about 38,000 feet. And the little inflight video screens noted that we were over the Atlantic Ocean, somewhere near Iceland. I am a marketing professional with no prior sightings of this kind. I was returning from a major trade show in the UK. I am a 35-year-old male American.