In the days before the magic of green screens and Doppler weather forecasting, people with a view of the Baltimore skyline had another method for determining the next day’s weather – the color-changing letters on top of the Art Deco Maryland National Bank Building (now called the Bank of America Building)

Maryland National added the lowercase “mn” letters in 1971.  On the north and south faces the letters were permanently white, but the east and west-facing letters were lit with neon lights in three colors.

The bank once gave out cards that explained the system:

When the sign is red
warm weather’s ahead
When the sign is blue
cooler weather’s due
An amber light
means no change in sight
When a color moves in agitation
there’s going to be precipitation

The letters went dark for a few years in the 1970s to save energy, and the blinking stopped in the 1980s.  The famous “mn” letters were removed by helicopter in April 1994 when NationsBank bought the building, and they were donated to the Baltimore Museum of Industry.