Something different for Christmas!
Posted February 27, 2012
When you're a meteorologist, you're sure to get the occasional fun weather-related birthday, Christmas or other holiday gift, and this year my wife Sandy made sure there was no exception, after she stumbled upon a "weather stick" on the web while she was looking for something else. This was something new to me,and I got a big kick out of it, although I found later that it is apparently fairly well known across northeastern parts of the country, New England in particular.
The instructions simply have you mount the weather stick outdoors, and the stick will point up for fair weather, and down for foul! I found a nice spot to put it where we can see it through our kitchen sink window, and sure enough, it works! While it is really more of an indicator than a predictor, it's been fun to have around, so I thought I'd snap a couple of photos and post them here.
The first image above shows the stick on a cool day with sunshine and very dry air in place - the stick curves up to reflect this, but it is a very different appearance you'll see in the second photo, taken on a mild, humid and somewhat foggy day. The stick responds pretty strongly, pointing way down!
Of course, the "secret" of the stick is that it acts as a form of hygrometer or psychrometer, instruments that respond to the amount of water vapor in the atmosphere or to the relative humidity. Many objects or materials undergo some change of physical characteristic that occurs as water vapor molecules are absorbed (when the air is moist) or released (when the air is dry), and these changes can be taken advantage of to measure humidity. One well-known example is human hair - in addition to the hair-style issues that can be associated with humid days, the fact that hair lengthens and shortens in a repeatable manner as humidity changes has led to it being used as the basis of rather accurate scientific instruments, at least for a moderate range of humidity values not too close to the absolutely dry or saturated ends of the scale.
The weather stick seems to be based on this kind of behavior, and is made from wood that has cells and fibers that expand and contract as vapor is absorbed and released. Of course, there is one additional requirement, and that is for the response to humidity to be "differential," such that the wood along the top of the stick expands and contracts more for a given change in humidity than the wood along the bottom of the stick. That is the property of these balsam or birch sticks that causes them to bend instead of simply becoming a little longer or shorter when the humidity changes.
A similar principle of differential expansion is used in some thermometers that you may be familiar with. Something called a bi-metallic strip is used in many thermostats or in inexpensive wall mount thermometers. These instruments frequently employ a coiled metal strip that is actually made of two different metals bonded together. The metals are selected such that they have different thermal expansion coefficients, meaning that as temperature changes one side of the strip expands or contracts more than the other, which makes the spiral tighten or relax a bit. The coil can then be linked to an indicator needle and calibrated so that it shows what the current temperature is with reasonable accuracy, or in the case of a thermostat , the movement of the coil can cause heat or A/C to trip on and off at a desired temperature.
It's looking like the stick be my back yard will be pointing down today, back up tomorrow, down Wednesday and then back up on Thursday, so it'll get a nice workout for the week ahead!