“Generally it is safer to be indoors than out during a thunderstorm, and greatly so if the house has a well-grounded metallic roof or properly installed system of lightning rods. If outdoors it is far better to be in a valley than on the ridge of a hill, and it is always dangerous to take shelter under an isolated tree—the taller the tree, other things being equal, the greater the danger. An exceptionally tall tree is dangerous even in a forest. Some varieties of trees appear to be more frequently struck, in proportion to their numbers and exposure, than others, but no tree is immune. In general, however, the trees most likely to be struck are those that have either an extensive root system, like the locust, or deep tap roots, like the pine, for the very obvious reason that they are the best grounded and therefore offer, on the whole, the least electrical resistance.
“If one has to be outdoors and exposed to a violent thunderstorm, it is advisable, so far as danger from lightning is concerned, to get soaking wet, because wet clothes are much better conductors, and dry ones poorer, than the human body. In extreme cases it might even be advisable to lie flat on the wet ground. In case of severe shock, resuscitation should be attempted through persistent artificial respiration and prevention from chill.
“The contour of the land is an important factor in determining the relative danger from lightning because the chance of a discharge between cloud and earth, the only kind that is dangerous, varies somewhat inversely as the distance between them. Hence thunderstorms are more dangerous in mountainous regions, at least in the higher portions, than over a level country. Clearly, too, for any given region the lower the cloud the greater the danger. Hence a high degree of humidity is favorable to a dangerous storm, partly because the clouds will form at a lower level and partly because the precipitation, and probably therefore the electricity generated, will be abundant. Hence, too, a winter thunderstorm, because of its generally lower clouds, is likely to be more dangerous than an equally heavy summer one.”
It is estimated that the total property loss due to lightning in the United States is about $8,000,000 a year, and the number of persons struck about 1,500, of whom one-third are killed. Nine-tenths of these accidents occur in rural localities.
Lightning rods neither prevent lightning stroke nor do they, as is sometimes alleged, attract lightning to buildings. They merely provide good conductors along which a stroke of lightning may reach the earth without doing damage, and, within very moderate limits, determine the path of discharge. While there are many unsettled points regarding the theory of lightning rods and details of construction, their general utility is strikingly indicated by statistics showing the comparative amount of damage done by lightning to rodded and unrodded buildings. According to the United States Bureau of Standards, information gathered in this country shows that “taking rods as they come in the general run of installations, they reduce the fire hazard from lightning by 80 to 90 per cent in the case of houses, and by as much as 99 per cent in the case of barns.” The same bureau, in its valuable publication, “Protection of Life and Property Against Lightning” (Washington, 1915), supplies the answers to a multitude of questions that are constantly asked about lightning rods.
Buildings with metal roofs and frames connected with the ground are generally well protected from lightning (except as to nonmetallic chimneys) without rods.
During actual thunderstorms, and also at other times when there are high potential gradients in the atmosphere, luminous electric discharges of a more or less continuous character are sometimes observed to occur in the shape of small jets and flames, chiefly from pointed objects, including lightning rods, the masts and spars of vessels, the angles of roofs, etc. These are identical in character with the “brush” discharges, or incomplete sparks, produced by electric machines. They are accompanied by a hissing or crackling sound. Their luminosity is comparatively feeble, and for this reason they are much more often observed by night than by day. They are especially common during snowstorms.
This phenomenon is known as St. Elmo’s fire or corposants (not to mention a score of other names, ancient and modern). As seen at sea, corposants sometimes take the form of one or two starlike objects at the trucks of the masts or the ends of yard arms, but occasionally the spars, rigging and other parts of the ship are lighted up with a great number of stationary or moving flames, producing a weird spectacle. The finest examples of corposants are, however, observed on high mountains, and the phenomenon has been carefully studied at certain mountain observatories, such as those on Ben Nevis and the Sonnblick.
Of its occurrence on Ben Nevis, Angus Rankin writes: “The most frequent manner in which it makes its appearance is as caps of light on the tips of the lightning rod, but occasionally it appears as jets of flame projecting from all objects on the top of the tower and from the cowl of the kitchen chimney, which rises from the roof at some distance from the tower. These jets are at times from 4 to 6 inches in length, and make a peculiar hissing sound. During a very brilliant display, the observer’s hair, hat, pencil, etc., are aglow with the ‘fire,’ but, except for a slight tingling sensation in the head and hands, he suffers no inconvenience from it. On such occasions, if a stick be raised above the head, jets of electric light will be seen at its upper end. The only drawback to observing it with advantage is the unpleasant character of the weather in which it appears, namely blinding showers of snow and hail, and squally winds, causing a good deal of snowdrift.” Rankin records that it was sometimes heard in the daytime, when its light was invisible. On the Sonnblick a display of St. Elmo’s fire has been observed to last as long as eight hours.
No luminous electrical phenomenon is more beautiful or, at first sight, more mysterious than the aurora, popularly known, in the northern hemisphere, as the “northern lights.” This phenomenon is due to the passage of electrical discharges through the rarefied gases of the upper atmosphere, and it now appears to be settled beyond controversy that the discharges are caused by corpuscles or radiations of some kind emitted from the sun.