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hygroscopicity
wood
Before us is a Weatherstick, a folk weather indicator that in popular circulation is often called a “barometer.” In reality it functions as a hygroscopic actuator: a dried branch changes its curvature as the equilibrium moisture content of the wood changes. Because atmospheric humidity often shifts as cyclones and precipitation systems approach, the device provides a simple indication: when the air is dry the stick bends upward, signalling fair weather, while in humid conditions it bends downward, suggesting rain or unsettled weather. This is therefore an indication of humidity conditions, not a measurement of atmospheric pressure.
The device consists of a branch attached to a small piece of trunk. To produce it, a live branch roughly 30–45 cm long is cut and then dried. The bark is usually removed, and the branch is cut from the trunk “with a heel” (a short section of the trunk attached). The assembly is mounted to a wall using a single nail driven through this heel. The stick is then installed outdoors, protected from direct rainfall, and allowed to acclimatize for several days, after which its curvature stabilizes relative to the local microclimate.
Commercial instructions often recommend mounting the stick on a wall or fence exposed to outdoor air. The orientation of the branch relative to “up” and “down” influences the direction of the signal, so some manufacturers specify a correct mounting orientation (for example by drilling the mounting hole so that it must face upward). The physics of the process depends primarily on water vapor in the air, but direct wetting by rain can produce abrupt bending and may slow the recovery when the wood dries again. For this reason practical experiments often place the weatherstick under a small overhang, allowing it to respond to humidity without the “hydraulic shock” of raindrops.
Film-forming coatings such as varnishes or paints reduce the vapor permeability of the wood and slow the process by which it reaches equilibrium moisture content. As a result many commercial weathersticks are left natural and minimally finished.
Historically the weatherstick should be understood as a non-instrumental descendant of a broader tradition of observing the movements and deformations of wood and other plant materials as they absorb or release moisture.
Modern popular descriptions often attribute early use of the weatherstick to Indigenous peoples of northeastern North America. In printed sources of the mass market the device is reliably documented at least from the 1970s, typically described as a simple combination of “branch + piece of trunk.” By the late twentieth and early twenty-first centuries it had become a stable souvenir and conversation piece in New England and Canada and was later exported to Europe.
From there the weatherstick became established as a regional souvenir and “home weather predictor”, particularly in New England (especially Maine). Media and retail outlets describe it as a tradition, while gardening and household notes present it as a way to turn spruce or fir cuttings — including Christmas tree branches — into a functioning weather indicator.
By the late twentieth century several commercial product lines existed simultaneously. “Maine” versions were commonly made from balsam fir, while “Canadian” versions (sometimes made from birch) also appeared. Eventually export packaging emerged with printed instruction cards emphasizing “Canadian” origin as a mark of authenticity.
However, the history of the weatherstick is considerably older than the souvenir industry of the 1970s. Its use as a demonstration and advertising weather indicator can already be documented in the early twentieth century. One example is the Baromètre Fabre, designed as an advertising display for FABRE graines METZ, a seed and agricultural goods company from the French city of Metz. From the perspective of advertisers of the late nineteenth and early twentieth centuries, this was nearly an ideal object: unlike a simple poster, it literally came alive, as the branch moved with changing weather conditions. Such advertising devices were inexpensive to manufacture yet could remain on a wall for many years, even decades. A newspaper advertisement the company offered everyone the possibility of obtaining a free barometer as follows: “A barometer approved by the Ministries of Commerce and Agriculture is offered FREE with every order of vegetable and flower seeds worth 30 francs. Request the illustrated catalogue (140 pages) from FABRE, Seeds, METZ (Moselle), 213, rue Mazelle”.
Another early example of a “branch-type” barometer is represented by the popular “Baromètre des Montagnards” (“mountaineers’ barometer”), manufactured by the craftsman A. Lavier, described on the instrument as Inventeur-Fabricant (inventor and manufacturer) in the town of Raon-l’Étape in the mountainous Vosges region of eastern France. Such instruments were widespread in the mountain regions of France in the late nineteenth and early twentieth centuries and were intended for simple observation of changes in the weather.
Wood itself is a hygroscopic composite. It absorbs and releases water vapor, striving toward an equilibrium moisture content (EMC) determined mainly by relative humidity and temperature. Atmospheric pressure by itself — or the mere presence of a nearby cyclone — does not directly deform wood. Instead, deformation results from changes in the amount of bound water in the cell walls, causing swelling or shrinkage below the fiber-saturation point. Classical wood science states that above the fiber saturation point (around 30% moisture content) dimensional change is small, while below it wood changes dimension significantly as bound water is gained or lost.
At the level of the cell wall, water binds to hydrophilic groups of polysaccharides (cellulose and hemicelluloses), altering distances within the microfibrillar structure and producing anisotropic swelling and shrinkage. Experimental studies emphasize that the “activity” of wood as a dead biological tissue — and its actuator-like behavior — depends strongly on the orientation and architecture of cellulose microfibrils.
The key factor for the weatherstick is not simply shrinkage but asymmetry of the hygroscopic response across the branch cross-section. In conifer branches there is often reaction wood (specifically compression wood) on the lower side of the branch — the side that resisted compression under the weight of the branch while it was growing. Reaction wood is known to produce unusually large longitudinal deformations compared with normal wood, and engineering references note that it is particularly prone to excessive shrinkage and distortion.
Modern studies show that compression wood in conifers may have a different microfibril angle and a distinct pattern of hygroscopic expansion. When humidity changes, these differences create bending — essentially a biomimetic “bilayer” actuator. This provides a physically plausible explanation for the apparent “magic” of the weatherstick.
The direction of bending depends on which side of the branch contains the active layer (for example compression wood) and how the branch is oriented when mounted. For this reason commercial instructions often specify a correct orientation. The same physical element can be mounted so that it bends upward in dry conditions or upward in humid conditions. This explains the variability of legends and explanations printed on instruction cards and websites.
In practice the weatherstick “predicts” weather because humidity and evaporation conditions genuinely change as atmospheric fronts approach. At the same time local factors — shade, sunlight, rainwater wetting, or icing — may produce delayed or misleading signals.
In modern commerce two material traditions dominate. Balsam fir is typical for the “Maine Weather Stick” and for British and European sellers emphasizing North American origin of the material. Birch (often meaning paper birch in a North American context) appears in many “Canadian” versions.
More broadly, many species can function in this role because hygroscopicity is intrinsic to wood itself. However, the strength and repeatability of bending depend on a combination of species, branch structure, presence of reaction wood, and geometry. For a pronounced effect a gradient of deformation across the cross-section is required, which in conifer branches is often produced by reaction wood. For this reason balsam fir has become dominant in the souvenir market as a reliable source of the effect, while birch appears frequently in Canadian versions as a more widely available material.
The weatherstick is therefore a genuinely functioning indicator of humidity conditions that culturally became known as a “weather predictor” because increases in humidity often accompany approaching fronts and precipitation. The scientific basis rests on well-studied properties of wood: equilibrium moisture content, sorption and desorption of water, anisotropic swelling, and the role of reaction wood in asymmetric bending.
Remarkably, the weatherstick is one of the few meteorological devices that contains no mechanism at all. It operates solely through the anatomy of wood itself, making it one of the simplest weather indicators in the history of meteorology.
