MANY traditional methods of predicting the weather depend for their effectiveness on the reaction of certain substances to changes in humidity. Implicit in the use of such devices is the assumption that a rise in relative humidity is a portent of rain, and that low humidity means good weather - criteria that are often justified, but by no means totally infallible.
A classic example is the clump of seaweed. Many types of kelp contain large amounts of magnesium chloride, a substance which is highly hygroscopic, absorbing water vapour from the air. When the relative humidity rises, the kelp absorbs the moisture and feels damp to the touch some considerable time before any other signs of rain may be apparent.
In parts of central Europe, where seaweed is a scarce commodity, a branch of the fir tree serves the same purpose. All twigs except one are removed, and the branch itself is peeled and mounted on a wall. As fine weather approaches, a sprig allegedly stands perkily erect; when rain is on the way, it sags limply towards the floor. Pine cones, too, can be observed, closing up against wet and cold weather and opening out when dry weather is assured.
Other more sophisticated methods involve the use of chemicals that change colour with variations in humidity. A piece of paper impregnated with cobalt chloride, for example, will appear blue when the humidity is low, and pink or red when the moisture content of the air is high. Ornamental devices operating on this principle frequently incorporate a street lamp inscribed with helpful advice along the lines: "Blue light, weather, right; pink light, rain in sight."
Yet other devices depend on the fact that some materials, like human hair or even ordinary rope, shrink as the moisture content of the air increases. A strand of such moisture sensitive material, if fixed at one end and wound in a spiral, will rotate at the free end as the humidity goes up and down. This phenomenon is used to operate the traditional Dutch Weather House - the little chalet where the man comes out if it is about to rain, and the woman emerges when dry weather is expected.
All these devices, however, will only operate in the way intended if exposed to the external atmosphere. Displayed indoors, they merely reflect changes in the humidity of your living room - more closely related to heating and cooking arrangements than to any likely trends in future weather.