National Geographic : 1938 Aug
Photograph bySirHubert Wilkins ONE CLOUD BANK CASTS ITS SHADOW ON ANOTHER AS THE PLANE PLUNGES INTO THE OPEN JAWS OF MERGING FOG LAYERS At higher altitudes the aerial explorers several times met this spectacular phenomenon. The upper layer mightbeahalf mile thick and reach above 20,000 feet. The top of the lower layer would rise to about 10,000 feet above the hidden Arctic Ocean. "The upper and lower layers were like the lips ofanopen mouth into which we would fly only to find that they joined in a hundred miles or so, forming an impenetrable barrier."