As the single-engine plane dipped over the Greenland ice cap, Dr. Yaron Rado could see only a sliver of daylight separating the sky from the vast expanse of ice. “The picture that touches me the most is just a picture of white on the top, white on the bottom, and a sliver of light blue in the middle,” he said, describing the challenge of passing through terrain where distinguishing between land and sky required constant vigilance. Flying over ice and snow requires constant vigilance.