In the late 19th and early 20th centuries, signal flags were used to alert U.S. citizens to weather conditions. Postcard of the U.S. Weather Bureau Building at Charles City, Iowa. c. 1900.

Red flags are for danger; do not enter, and stay far away from whatever the red flag signals. Green means Go!, it's all good, and the course ahead is clear. Yellow flags are trickier. Yellow flags ...