The Mentor: The Weather Part 2

The tornado destroyed a house and barn, but left a path in the center with practically no harm done]

_West India hurricanes_ occasionally visit the United States, especially in the late summer and early autumn. These storms begin as violent cyclones of small extent (300 to 600 miles in diameter), usually somewhere east of the West Indies, sweep in a long curve across the Caribbean Sea, and then turn north, either pa.s.sing up along the Atlantic Coast or crossing the Gulf of Mexico into the southern United States. Soon after entering the temperate zone they increase in size and diminish in violence, but are still vigorous enough on reaching the Gulf or South Atlantic Coast to cause great devastation. Low-lying sh.o.r.es are often inundated by the immense waves they generate.

_Cold waves_ are the rapid and severe falls in temperature that sometimes occur in winter, especially at the front of an anticyclone.

Warnings of these occurrences, issued by the Weather Bureau twenty-four to thirty-six hours in advance, often result in the saving of millions of dollars' worth of merchandise susceptible to damage by freezing.

_Frosts_ in the spring and autumn are also predicted with great success, to the immense advantage of farmers, market-gardeners, and horticulturists. The practice of smudging or heating orchards, now so widespread, is usually carried on under the advice of the Weather Bureau, which gives prompt notice to the orchardist when such precautions are in order. The bureau publishes charts showing the average and extreme dates of the last frost in spring and the first frost in autumn for all parts of the country.

[Ill.u.s.tration: LOOKING DOWN ON A SEA OF FOG FROM MT. TAMALPAIS, CALIFORNIA]

A _fog_ is a cloud resting on the surface of the earth. In the United States fog is commonest along the northern and middle parts of the Atlantic and Pacific Coasts. In the interior of the country, especially the western part, it is of rare occurrence, the average number of days a year with fog being less than ten.

Lastly--weather fallacies are rife. _Indian summer_ is merely a type of mild, hazy, heavenly weather that prevails intermittently during our long American autumns. The _equinoctial storm_ is a myth; the climate has not "changed" anywhere within the span of a human lifetime (one year differs from another, but there is no progressive or permanent change); and the _moon_ has nothing whatever to do with THE WEATHER.

SUPPLEMENTARY READING

CLIMATE AND WEATHER _By H. N. d.i.c.kson_

AMERICAN WEATHER _By A. W. Greely_

WEATHER SCIENCE _By R. G. K. Lempfert_

SOME FACTS ABOUT THE WEATHER _By W. Marriott_ Second edition.

METEOROLOGY _By W. I. Milham_ The latest general textbook on the subject in English.

FORECASTING WEATHER _By W. N. Shaw_

ELEMENTARY METEOROLOGY _By F. Waldo_

Consult also the numerous publications of the United States Weather Bureau, which will be found in most public libraries.

*** Information concerning the above books and articles may be had on application to the Editor of The Mentor.

THE OPEN LETTER

"What is lightning and what causes it?" The question came to us a few days after we had made announcement of a "Weather" number of The Mentor.

It was a natural question, for lightning is the most sensational of all weather phenomena. It has always had a fearful sort of fascination for humanity. To the ancients it came as a bolt of wrath from the hand of Jove. To the fire-worshipers it was a warning message. To parched travelers it was a bright promise, for it heralded the coming of rain.

To the superst.i.tious it was a signal flash from the spirit world. And to those of nervous temperament it was a highly disturbing phenomenon producing emotions varying from uneasiness and alarm to hysteria. The question then, "What is lightning and what causes it?" has an interest for all. I referred it to Mr. Talman, the author of the Mentor article on "The Weather." His reply follows.

"Not so many generations ago 'natural philosophers' thought that inflammable gases, exhaled from the earth, took fire spontaneously in the air, and that this was lightning. The idea also prevailed--and it is not yet quite extinct--that a stroke of lightning involved the hurling down from the sky of a ma.s.s of rock, called a 'thunderbolt.' In the eighteenth century people became quite familiar with the process of generating, by friction, a mysterious something called 'electricity,'

which, when it pa.s.sed from one body to another through a small layer of intervening air, produced sparks. Several philosophers noticed the resemblance between these sparks and lightning. It remained, however, for Benjamin Franklin to prove that lightning was really an electrical discharge on a large scale. The experiments by which he proposed to demonstrate this were successfully performed, first by others, in France, and then, by Franklin himself, at Philadelphia. With the aid of his famous kite he drew down from a thundercloud a little of the 'electrical fluid' (as it was then called), and produced tiny sparks from an iron key at the lower end of the wet kite-string.

"We do not even yet know what electricity is, but we know a great deal about the way it behaves and the effects it produces. There are two kinds of electricity, which we call _positive_ and _negative_. A body is said to be _charged_ when it has an excess of either kind, and the two kinds have a tendency to unite and neutralize each other's effects.

Thunderclouds become heavily charged with electricity. We are not quite sure how this happens, but it is now commonly believed that the strong uprising currents of air that occur in the storm, in the process of breaking up the water-drops in the cloud also separate positive from negative electricity; leaving the former in excess in the part of the cloud next to the earth, and carrying the latter far aloft.

"By a process called 'induction' the positive charge in the cloud draws an excess of negative electricity to the surface of the ground underneath. The stronger the contrast between these opposite charges, the harder they try to break through the interposing barrier of the air (which is a poor conductor of electricity) and to neutralize each other.

At length they succeed in doing so. A powerful stream of electricity flows for an instant between cloud and earth. Its pa.s.sage heats the air and makes it luminous--just as the pa.s.sage of an electric current heats the filament of an electric lamp and makes it luminous. This is lightning.

"These discharges occur not only between the clouds and the earth, but also, and probably more often, between clouds charged with opposite kinds of electricity.

"The sudden expansion of the heated air along the path of the discharge affects our ears just as does the sudden expansion of the air at the mouth of a gun when it is fired. In each case a wave is sent through the air in all directions from the place of disturbance, and our ear-drums are set in vibration. That is thunder."

Take courage then, you timid ones, who wince in the lightning's flash and tremble under the thunder's roll. Thunder is simply a vibration of your ear drums--and, when you hear the thunder, be a.s.sured, all danger is over.

[Signature: W. D. Moffat]

EDITOR

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