Suppose the cyclone stood still for a week, then the observer would see a watery sky for a week, without any rain following. Suppose the cyclone came on so far as to bring him under a watery sky, and then died out or moved in another direction, then, after seeing a watery sky, no rain would fall, but the sky would clear. The prognostic would then be said to fail, but the word is only partially applicable. The watery sky was formed and seen by the observer, because he was in the appropriate portion of the cyclone, and so far the prognostic told its story correctly—viz., that the observer was in the front of the rainy area of a cyclone. The prognostic failed in its ordinary indication because the cyclone did not move on as usual, but died out, and therefore never brought its rainy portion over the observer. This is the commonest source of the so-called failure of a rain-prognostic in Great Britain. The reason why all rain is not preceded by a watery sky is because there are other sources of rain besides a cyclone, which are preceded by a different set of weather-signs. Such is the whole theory of prognostics.
The same reasoning which applies to a watery sky holds good for every other cyclone-prognostic. We shall have explained why any prognostic portends rain when we have shown that the kind of sky or other appearance which forms the prognostic belongs to the front of the rainy portion of a cyclone. Conversely we shall have explained why any prognostic indicates finer weather when we have shown that the kind of sky belongs to the rear of a cyclone. It will be convenient, therefore, to describe the weather in different parts of a cyclone, and the appropriate prognostics together.
First, to take those prognostics which depend on qualities common to the whole front of the cyclone, viz., a falling barometer, increased warmth and damp, with a muggy, uncomfortable feel of the air, and a dirty sky.
From the increasing damp in this part of a cyclone, while the sky generally is pretty clear, cloud forms round and "caps" the tops of hills, which has given rise to numerous local sayings. The reason is that a hill always deflects the air upward. Usually the cold caused by ascension and consequent expansion is not sufficient to lower the temperature of the air below the dew-point; but when very damp, the same amount of cooling will bring the air below the dew-point, and so produce condensation.
From the same excessive damp the following may be explained: "When walls are more than usually damp, rain is expected." The Zuñi Indians in New Mexico say that "when the locks of the Navajos grow damp in the scalp-house, surely it will rain." From this we may assume that scalps are slightly hygroscopic, probably from the salt which they contain. Also, owing to excessive moisture, clouds appear soft and lowering, and reflect the glare of iron-works and the lights of large towns.