The weather signal flag is a familiar object to our St. Paul citizens, and yet there are a great many who do not understand the signals. In the bureau two classes of signals are used – the weather signals and the wind velocity signals. Their colors and white, blue, black and red. A square all-white flag indicates clear or fair weather, and a square all-blue one, rain or snow. The temperature signal is a triangular black flag, and when placed above the white or blue flag indicates warmer weather; when placed below it means colder weather, and when it does not fly, stationary temperature. The square white flag with a small black square in the middle, such as we often see flying from the top of the chamber of commerce building at this season of the year, signifies a cold wave is near at hand. The red flag with a black center is a storm flag, indicating that a storm of unusual violence is brewing.
Source:
The Saint Paul Daily Globe
Sunday Morning, December 30, 1888
Volume X, Number 365, Page 4