A weather stick is a traditional means of weather prediction.
It is a piece of balsam fir mounted outdoors. It twists upwards in low humidity and downwards in high-humidity environments.
They’re incredibly simple – just a stick with the bark removed and a little piece of the tree’s trunk left attached so that you can easily nail it to a wall, fencepost, or a pillar on your porch. They might seem like a bit of folklore, but there is actually quite a lot of science behind them. These sticks bend based on the relative humidity. Higher humidity – which is often a signifier of bad weather – makes them curl downward. When they dry out, they straighten out or curve upwards. The interesting thing is that there aren’t many kinds of wood that can be used this way – you can’t simply cut a twig from any old tree and expect it to predict the weather. Most trees have what is known as reaction wood. In other words, if a tree is always exposed to winds coming out of the west, that tree will develop stronger wood fibers on its eastern side to help brace it against high winds. Balsam fir, however, develops its reaction wood a bit differently. Live balsam fir trees develop reaction wood on the undersides of their branches so that in dry weather, the reaction wood fibers shrink to conserve water, thus bending them towards the ground. Then, when rains come along and the fir trees start drawing more water, those same fibers expand and the tree’s branches unfurl. Dried balsam fir sticks retain these properties, but since they’re no longer attached to a root system, they rely on the amount of humidity in the air to curl and uncurl. This is why weather sticks have the bark removed. The wood can soak up more water and dry out more quickly, which makes them bend a lot more than if they were protected by a layer of bark. Traditionally, weather sticks point up for good weather and down for bad weather. Who would have thought that you could predict the weather with something as simple as a stick?