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Humidity indicator

You may remember seeing toy "weather forecasters" that were nothing more than decorated pieces of cardboard with a fuzzy pink or blue patch. Some of these, like the one pictured on this page, were Cracker Jack premiums. The fuzzy patch was supposed to change color according to the surrounding humidity levels. The chemical in the felt patch that changes color is cobalt chloride. Dissolve some cobalt chloride in enough water to make a strong solution. Cobalt chloride was once regularly available in pharmacies, but now must be purchased through chemical suppliers. Soak a piece of absorbent paper (coffee filter) or a piece of white felt in the solution. Allow to dry. The indicator will be sensitive to humidity. If you live somewhere very humid or very dry all of the time, you might not see much of a day to day change in colors. Ideally, it will be pink for a storm, purple for rain, blue for fair skies, and light blue for dry days. Of course, it is only affected by humidity and cannot truly predict storms. But it is an interesting novelty nonetheless.
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