Frank P. Smith

The massive clouds, the vivid lightning and pealing thunder, the heavy rain and gusty wind accompanying the summer thunder storm, are manifestations of nature's forces which produce in our minds feelings of wonder, and to the timid are a cause of uneasiness and fear. A brief presentation at this time of the meteorological causes and effects of such storms will be of interest to readers of this magazine.

It is the summer thunder storm only which will receive our attention, many storms with lightning and thunder being the result of rather different conditions from the heavy summer storm. The latter is generally a comparatively local affair, travelling but a few hundred miles, or less, from the point of its origin. Many of the storms traversing New England States, originate in New York and have spent their force before or shortly after reaching the Atlantic ocean.

The formation of such storms begins generally in the morning of a hot day with a fairly high humidity. The heat of the sun causes an expansive upward movement of the moisture laden atmosphere which, upon reaching the cooler heights, is condensed into misty whisps of clouds. These nebulous beginnings increase in size until about noon, or soon after, they have reached a towering size and the cloud masses extend to a great height, and assume the so-called "an-vil " shape of the well developed thunder storm.

This anvil shape of the clouds is the result of air currents which have developed coincidently with the cloud masses. The illustration shows a cross section of a thunder storm, as far as present day knowledge enables us to graphically represent the action taking place. The long arrows show the air currents, and attention is directed to the low, projecting undercurrents which denote the approach of the rain bearing section. The preliminary wind squall is probably caused partly by the cooling of the atmosphere lying within the shadow of the clouds, which has been deprived of the heat of the sun and further, by the air movement which results from the upward motion of the central air currents.

As the heated and moisture laden air rises in the center of the cloud mass, it eventually reaches the cooler region of the upper air, and condensation of the moisture follows and continues until minute droplets of rain are formed. These droplets unite to form larger drops until they are of such a size that their weight causes them to fall to the "ground.

Should the intensity of the storm be sufficient to cause the air currents to rise to a great height, where low temperature prevails, the condensing moisture may form into snow flakes instead of rain drops, and the flakes, in falling to the warmer, moist air, will collect moisture on their surface, which will freeze in successive layers until the weight eventually brings them to the ground in the form of hail stones. It is thought that from the first condensation of the snow-flake to the final fall as a hail stone, the air movement has caused the condensing moisture to rise and fall several times, this theory being supported by the formation of hail stones, which are known to be of several distinct layers, the number varying somewhat according to size.

So far, no mention has been made of the causes of the thunder and lightning, the latter being, it is almost needless to say, of an electrical nature. The atmosphere is known to be at nearly all times, of a differing potential from that of the earth, the latter being negative and the atmosphere of a positive charge. This difference is greater as the height increases, but is comparatively small in quantity and variation during pleasant weathei. During thunder storms, however, the potential varies widely, and the charge may fluctuate from positive to negative and the reverse, but is generally of somewhat higher than normal potential and positive in character.

The moisture in the air is carried upward with the ascending air currents, and upon reaching the higher and cooler altitudes, condenses into raindrops. These raindrops undoubtedly become electrically charged upon their surfaces, and the potential of the charge increases as the smaller drops unite to form larger ones. When the magnitude of the cloud is considered, it will be apparent that these myriads of minute charges in the movement of falling, serve to give to the adjacent atmosphere a tremendous electrical charge, which, breaking down the resistance of the air, discharges to earth, for which it has an affinity by reason of its being of opposite polarity.

Hence the lightning flash or rather flashes, as what appears to the eye as one irregular flash is frequently several flashes or surges. Two or more flashes frequently unite near the earth to form one intense flash, the intensity and disruptive effects of which are familiar to all. The greater number of flashes do not reach the earth, but exhaust their energy in breaking down the air gaps between adjacent sections of clouds. The form of lightning having the appearance of flaming balls with comparatively slow movement, is not as yet fully understood, but may result from certain air currents which form a path for the electrical discharge.

The thunder results from the violent vibrations of the air caused by the lightning flashes, which in breaking down the resistance of the air, create a vacuum of an extent depending upon the intensity of the flash.

The air rushes to fill the voids thus caused, creating violent vibrations, which travel long distances, and do not differ in character from other sound waves, but are of greater amplitude than ordinary because of the greater forces causing them. The velocity at which sound travels is about 1100 feet a second, a rough approximation being five seconds to the mile; from which one may readily calculate the distance of the flashes. What has been stated above is of a general character, and based upon the present knowledge of meteorological conditions. Skilled observers in many sections of this and other countries have devoted much attention to the study of this interesting subject, and additional knowledge will undoubtedly be forthcoming in due time. The study of how to protect buildings from the disastrous effects of lightning is also being followed by scientific observers, and a more exact statement of how this may be accomplished will undoubtedly be available in the near future.

Summer Thunder Storms 219

It has already been found that lightning rods, to properly serve their purpose, should have numerous sharp projecting points above the building; that these points should be well cross-connected with copper rods, making a sort of net-work, and that this network should have a number of conductors to earth. The earth end of the conductors should also be buried deep enough to be in permanent contact with moist earth, the essentials being to supply an ample path to the earth for heavy flashes. Single points on the roof, with rods direct to the earth and without cross connections, provide capacity for only the smallest discharges, and large ones, in seeking a path, leave the rod, with consequent liability to damage the building.

Should a person receive a shock from lightning, it frequently happens that respiration has stopped because of temporary paralysis. If assistance is at hand the usual methods for restoring respiration should be actively continued for at least an hour, the services of a physician being obtained as soon as possible.