How does a vintage weather predictor work?

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Dear Tom,

I just found an old weather predictor from my great grandmother’s house. It changes color to predict the weather. How does it work?

Thanks,
Cindy Spudich

Dear Cindy,

Weather predictors, like the one you describe, were quite common back in the 1950s and 60s and were usually made with a piece of paper or cloth treated with the chemical cobalt chloride. Cobalt chloride is affected by changes in humidity and undergoes a color change from blue in low humidity to pink in high humidity. It works best in summer in areas with no air conditioning. When the air is dry, the strip is blue. As moisture in the air increases and the air become more humid, the strip gradually turns pink indicating that rain is likely on the way. Cobalt chloride paper is also used to test for the presence of water leaks.

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