Reading the Weather

by Thomas Morris Longstreth

A close, sultry morning is the best indication of a thunder-gust. The large piles of cumulus clouds are called thunderheads for the very reason that they almost always precede a thunderstorm. The heaviest electrical disturbances have cirrus clouds a few hours in advance of them very much as their winter relatives. A thunderstorm that does not cause the barometer to fall considerably will not amount to a great deal.

At night the different kinds of lightning furnish a running commentary to the storm. On calm evenings the sky will be cloudless, with perhaps the exception of a low rim on the northern horizon. Yet flashes of lightning, of course without thunder, may be seen illuminating that entire quadrant of the sky. This is called heat lightning and is popularly supposed to be the result of the heat only. As a matter of fact it is caused by a normal thunderstorm that is operating below the horizon. Reflections from this storm are shown on the rim of clouds, or if no clouds are visible, on the bowl of the sky. If you see lightning be sure that there is a storm somewhere.

If this disembodied sort of lightning continues to flash from the western sky it is quite possible that the storm will reach you. If it shows on the northwest or north of you the chances are that the storm will be carried around. If the wind is from the southwest and the lightning appears there only the progress of the clouds will show whether the storm is pursuing the normal track from the west and around you or whether it is edging up toward you. One cannot be very well surprised by a thunderstorm of any energy in camp as the lightning shows as much as two hours before the storm breaks and the thunder gives fifteen minutes’ notice on most occasions.

The sort of lightning that spends itself illuminating the clouds in serpents and willowy branches confines itself to the altitudes and is very beautiful and harmless. It is accompanied by thunder that sounds hollow, that rumbles over the sky, and usually does not end with the crash and thud of the more vigorous variety. Such lightning and such thunder are more often connected with the sort of storm that comes up very swiftly on a western wind. It gives shorter warning than any other sort of thunderstorm and is not connected with the cyclonic area. I have known such a storm to manifest itself low in the west, approach, and break within twenty minutes. Much wind results and not much rain, although the temperature falls. Lightning with storms of this impromptu kind rarely does any damage.

But if the storm rises slowly against the wind, requiring an hour or two or three to approach and break, the lightning will grow almost continuously, some of the flashes being broad streamers cleaving the western sky. It is this sort of lightning that does the damage. The thunder, instead of rolling like an empty barrel, hits into a series of concussions. If the lightning strikes an object nearby the crash is rather appalling. There are several freak sorts of lightning such as the ball form, which are rare.

The approach of the center of disturbance may be gauged by the length of time that elapses between flash and crash. In reality the thunder occurs immediately after the discharge of electricity, but sound travels so slowly, compared to light, that a minute may intervene between stroke and clap. You may count the seconds, noticing the regular decrease, signifying the nearing of the crisis. Soon a flash in front and a simultaneous peal will show you that you are in the thick of things. The next bolt or two may hit very close and you can appreciate what it means to be on the firing line. Then the next river of fire with its detonation streams behind you and you are saved.

In a severe thunderstorm there are several centers, several nuclei that shed destruction like great batteries and their progress over and beyond you has its thrills. You may find the exact number of feet away that the bolt hit by multiplying the number of seconds elapsing between the lightning and thunder by 1120. But an easier way is to allow a mile for every five seconds on the watch. One or two seconds, and you are pretty near the center of the fray.

Lightning compresses the air, leaving a partial vacuum. The other air rushing in to fill this partial vacuum forms the wave motion that produces the noise. That is the whole why of thunder. The reason thunder rolls is that the lightning is a series of discharges each of which gives rise to a particular detonation. If lightning were but one discharge, the thunder would be but one stupefying crash. Reflections from the clouds and from layers of air of different densities and from the ground are agencies that prolong the sound.

Our atmosphere is never lacking in electricity. This electricity is always positive in clear weather and sometimes negative in cloudy. Science concludes, then, that negative electricity invariably indicates rain, hail, or snow within a radius of forty miles.

Moist air is a good conductor. Our powerful motors can now produce a spark of electricity several feet long. But some of the flashes that shoot across the sky in a big storm extend over five miles. The duration of the flash varies from 1-300th of a second to a second. The reason that lightning does not always pass imperially along a straight line is that some air, either moister or warmer than the air around it, offers less resistance. The lightning takes this line of least resistance along the pathway of warmer or less dense air.