Meaning of DEW line

DEW' line"

Pronunciation: (d, dy), [key]
  1. a 3000-mi. (4800–km)–long network of radar stations north of the Arctic Circle, maintained by the U.S. and Canada for providing advance warning of the approach of hostile planes, missiles, etc.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.