Also known as a Maine weather stick, the weather stick is a balsam fir rod, generally about 40cm (16") long which when mounted outside twists upward with the onset of good weather and downward as inclement weather approaches. These sticks were first used by the aboriginals of the American northeast and the Canadian east and southeast, who noted the behavior of dry branches prior to the arrival of weather changes. The weather stick is a rare example of a weather prediction tool which predates the mercury barometer.

Left: dry air, Center: increasing humidity, Right: very humid

Experimentation shows that this device is a generally accurate predictor of weather change. An experiment was done in which a weatherstick was placed beneath an overhang to protect it from direct precipitation, while allowing it to be affected by all other atmospheric conditions. Measurements of the amount of light, the air temperature, and the relative humidity were taken over a two-week period, and of the factors considered, the only one of significance in the bending of the stick appeared to be relative humidity. Don Ross, a New England researcher, has found that, in a controlled environment test, weather sticks show a strong response to relative humidity. Ross also constructed a weather stick over seven feet (210cm) long, which, when mounted horizontally, has a travel of its own length between dry and extremely wet weather.

Dampness causes wood to twist or warp. The wood used to construct a weather stick is from a tree whose branches differ in density between the top and bottom of the branch. Wood, as the product of a living organism, is made of cells. In green wood, these cells are filled with liquid. In dry wood, however, the cells have lost their moisture, shrunk, and left air pockets between them, the reason why a dry piece of wood is much lighter than a green or a waterlogged piece. Differential swelling is what causes the branch to bend - under certain conditions, such as a change in relative humidity, the top of the branch will swell to a different degree than the bottom, forcing the branch to bend in an arc with the less swollen part of the branch on the inside of the arc.

Weather sticks may be constructed from any of a number of varieties of fir, though willow sticks are common in Russia and there are reports of birch being used in North America. Whatever material is used, weather sticks appear durable, with usage times reported in the range of decades. Users also report that, as well as being good predictors of inclement weather, they are excellent conversation pieces.

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