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Weather Stick Science

D. H. Ross ~ February 2, 2002

(The Search for the Why and How of Weather-sticks)

What is a weather-stick?

Weather-sticks (WS) have been sold for many generations from shops in Maine, New Hampshire and Vermont, and they definitely do "work". The Northeast's Abenaki Indians are said to have invented the weather stick. A stick consists of a tapered debarked branch some claim from a balsam pine tree, that is typically less than 0.2 inches in diameter at the base and perhaps less than 0.1 inches at the tip. From base to tip is typically 16 inches. The tip will have a deflection of approximately plus or minus 7 to 9 inches between wet weather and very dry. A good weather stick, when deflected, follows pretty much the arc of a circle of varying radius. The stick seems to deflect downward before rain or snow arrive and up before clearing weather.

Does it forecast the weather?

The large movement of the tip is quite amazing and appears to track the relative humidity.  Once installed in a sheltered outdoor location, it performs for many years.  If the tip is restrained, it produces a substantial force, measured at 4 or 5 oz. to return a 7" deflection to zero.  Coming-weather may or may not be predicted by the WS, but data does show that the deflection tracks with the relative humidity and most often the relative humidity increases before rain commences, and decreases as good weather approaches.

How does it do it?

The relative humidity is defined as the ratio of the amount of water actually present in the air to the greater amount possible at the same temperature, expressed as a percentage.  The textbooks all say that the equilibrium-water-content of lumber (wood) is proportional to the relative humidity, and that the wood will change its dimensions with a change in water content.

Various explanations for the large deflection performance have been suggested.  The most likely involve a differential length change between top and bottom fibers of the branch possibly caused by:

    1.  Adding a moisture resistant sealing coat to either the upper or lower surface of the branch so that the water from the atmosphere affects only one side of the branch (Indians might have used pine tree resins).

    2.  Selectively heating the top (or bottom) of the branch with a hot object to spoil the local ability to expand with increasing absorbed water or contract with decreasing water content.

    3.  Natural differences between the top and bottom of the branch structure that cause them to expand or contract at different rates in response to relative humidity changes.

Close examination of a brand new WS purchased at a store fails to disclose any clues such as a discolorization of one side of the branch.  It appears that in preparing the WS, the bark is removed with a knife and not too carefully. Microscopic examination of thin cross-sections of the stick suggests that the wood structure does tend to be different between the top and the bottom of the branch, thus calling our attention to studies concerning "reaction-wood".

 Brief Wood-cell Physics

Reaction Wood – The experts tell us that the reaction-wood subject relates to the fact that the tiny cells (~20 nanometers) that make up the wood are influenced by the stress that they endure during their formation.  For example, trees of the conifer family (or other soft-woods) that are regularly stressed by a strong prevailing wind "react" and develop compression-wood on the downwind side of the trunk.  A tree that is leaning (due to avalanche or other upset) will do the same, and all branches at some large angle to the trunk will be expected to exhibit similar characteristics because gravity forces are stressing the branch as they grow.  Bending due to the weight of the branch tends to pull on the fibers on the top of the branch and squeeze-together (compress) the cell-fibers on the bottom.  Further we are told that the compression-cells expand or contract more than "normal" cells with a change in their moisture content.

It has been reported that during a long dry spell the living balsam fir tucks in its branches to "save water", and holds out its branches during a very wet season.  This undoubtedly has to do with the changing supply of fluids from the root system.  But once the WS is prepared (no longer living), it reacts to humidity changes in the air irrespective of its installed orientation.  The installed position of a WS might be horizontal or vertical or any other position.  In other words, its performance does tend to confirm that the reaction-cells are created during the growth of the living branch, so that subsequent exposure to gravity-forces is not important to the WS performance. The WS is quite stiff at any deflection angle, but changes its deflection with changes in relative humidity that affect the quantity of fluid in its cellular structure.  So we can safely assume that there is a difference between the cells on the top of the branch and the cells on the bottom that were grown under a compressive stress due to a gravity influence. With much of its fluids removed the WS has its maximum "good weather" deflection.  Add water from the atmosphere and it uncurls, becomes straight, and then if more water is available curls in the other direction. 

There have been extensive efforts to characterize the nature of the cells that make up wood (ref 1, 2, 3). After debarking a candidate conifer WS, some of the internal fluids appear to be able to escape to the outside world after the bark is removed.  Perhaps the cells when pulled-on maintain their strength due to the crystalline fibers (called fibrils) that probably are good at supporting the tension force (like pulling on a rope or cable).  However, on the compression side it is possible that with some of the fluids removed from within the cell and with the fibrils at a significant angle to the stick axis, the cells may buckle a little allowing the cell to partially collapse (we all know what happens when we push on a rope).  When the WS encounters an increased humidity more water is absorbed apparently reinflating the cell.

So the search for a conceptual model is almost over.  The literature tells us that leaning coniferous trees or branches form compression-wood cells on the compression side of the living limb, which helps to control the direction that the tree or limb grows (It helps to straighten a leaning tree).  Experiments conducted on the Space Shuttle, Columbia have run some theory-confirming-tests in a micro-gravity situation that show that a compressive stress will cause compression-wood to form, without a gravity influence.  The young plants were bent mechanically and the compression-cells formed where the compression stresses existed (ref 4).

When thin WS cross-sections are prepared, the normal wood transmits light quite well, but the compression-wood is denser and appears much darker.   A picture made under the microscope, with the light source underneath the sample, shows that indeed the denser light scattering material is found where the gravity-compression existed on the underside of the branch (see the figure below).  In addition, a scanning electron microscope shows that the cell structure in conifers, is different at top and bottom (ref 5).  As stated previously, we are told that the compression-cells on the bottom of the branch expand or contract more than the normal-cells at the top when an external humidity change influences the water content of the cells.

The dark arc on the right side of each section is compression-wood.  The right side of each section is the bottom of the branch as grown.

Hysterisis

According to the Merriam-Webster Dictionary, "hysteresis" is defined as "a retardation of an effect when forces acting upon a body are changed".  In other words for a particular value of relative humidity there will be a small difference in the WS tip position dependent on whether the relative humidity had increased or decreased to reach that specific value.  Weather-stick deflection as plotted against relative humidity does demonstrates such an effect.  This hysteresis effect may be due to the energy exchange required to evaporate or condense the cell-water, but this complication is only significant should we wish to attempt to calibrate a weather-stick to measure relative humidity.

Conclusions

I have attempted to present the rationale for the WS behavior.  It turns out that living softwood trees do have a reaction to stress in their branches that cause the growing cells to grow differently when the stress is compressive.  For example, the downside or bottom of a branch experiences a compressive stress just because of the weight of the branch.  When the moisture content of these "compression-cells" change, they do shrink or expand excessively with changes in moisture content compared to "normal" cells.  After the bark is removed, the cells can communicate with all outdoors and they will lose water relatively quickly if the surrounding air becomes drier, or regain water as the humidity increases.  Small diameter sticks react more rapidly to a humidity change than larger ones, as might be expected.

And here is a final zinger that will determine whether the reader has been paying close attention.  As grown on the tree, the compression cells form on the bottom of the branch, and with increased water content (increasing relative humidity) those cells will expand more than those at the top, and the branch will bend upward.  So be aware that when the weather stick is prepared for prediction-duty and nailed to the side of your house it is traditionally installed horizontally BUT UPSIDE DOWN, since the Abanakis people may have felt that good weather was better indicated by an upward deflection, and bad weather by downward.

There is a whole other world of fascinating science involved in growing things.  The weather-stick "mystery" is only one microcosm of nature at work.  Your hair expands and contracts with changes in humidity (this may explain bad-hair days), but why does it do it?   Stay tuned!

  DHR
March 2002

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