This section is from the book "Scientific American Reference Book. A Manual for the Office, Household and Shop", by Albert A. Hopkins, A. Russell Bond. Also available from Amazon: Scientific American Reference Book.
No. 1. White Flag.

No. 2. Blue Flag.

No. 3.
White and Blue
Flag.

No. 4.
Black Triangular
Flag.

No. 5.
White Flag with black square in center.

When number 4 is placed above number 1, 2 or 3 it indicates warmer; when below, colder; when not displayed, the temperature is expected to remain about stationary. During the late spring and early fall the cold-wave flag is also used to indicate anticipated frosts.
 
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