Imágenes de páginas

August there were only 12 days when the temperature rose to 90° or above, but there were many days when it nearly reached that height so that the average of the highests for the month was 87.4°, just the same as for July. August, however, should normally have been one degree cooler. For September the records are not yet complete; but during the first 19 days there were 13 on which the temperature went to 90° or above, and 4 on which it went above one hundred degrees. So, while the past July and August averaged about like those of other years, September 1925, was a record-breaker for heat. Sometimes

"Ere noon as brass the heavens turned,

The cruel sun smote with an unerring aim,

The sight and touch of all things baked and burned,

And hot brick walls seemed shimmering into flame."

To account for such hot, dry weather, we could only fall back on the ancient Greek explanation that Phaeton, son of Apollo, had teased his father into letting him drive the chariot of the sun. As the temperature rose above 100°, it was not hard to fancy that the horses were running away with the chariot and carrying the sun altogether too near the earth. After a restless night, it was perhaps a bit of relief to see the Pleiades, the Hyades, Orion and Sirius in the eastern sky just before dawn suggesting the cold weather of midwinter when they are familiar to us in the evening sky. But with returning day the mercury went up again. Then on September 21st, as the sun crossed the equator, rain came, cooler weather followed, and we are glad to live in St. Louis again.

Does It Ever Get Too Dry to Rain?

(Weather Bureau Office, Nashville, Tenn., September 26, 1925) During severe droughts, when the sun blazes day after day, running the surface temperature up to 90 or 100 degrees, the layman is apt to think that moisture is so scarce that it would be impossible for any to be precipitated from the overlying air. Vegetation may be dried up and the ground surface show no sign of moisture. The relative humidity may be as low as 20 per cent, indicating a very dry atmosphere. But the relative humidity alone does not indicate the actual amount of moisture in the air. The actual amount of moisture, or absolute humidity, is expressed in grains per cubic foot of air, or as pressure exerted by the water vapor, the latter method being used by the Weather Bureau. From these records it is easily seen that when the temperature is high there is always present a considerable quantity of water vapor, even when the air is at a maximum of dryness.

On the occasion of the lowest relative humidity recorded at Nashville in many years, which was at noon of September 7, 1925, when the relative humidity was 18 per cent, the vapor pressure was at the same moment .347 inch. Now a vapor pressure of .347 inch shows a large amount of moisture in the air, although small as compared with the possible amount at the prevailing temperature (which was 101° at the observation just mentioned).

As the actual amount of moisture in the air, with a certain relative

humidity, depends upon the temperature, the "capacity" of the air for moisture being many times as great at very high temperatures as at low temperatures, it is true that during the driest summer months there is more moisture in the air than during the wettest winter and spring months. In the arid regions of the United States records show that there is always considerable moisture in the air. In Arizona, for example, the vapor pressure in the driest months ranges near .200 inch, which is as high as the vapor pressure at Nashville in one of the wettest months (January). At Fresno, California, during the months of June, July, and August, 1923, there was no rainfall, yet the vapor pressure averaged .302 inch.

Some of the heaviest rains in the United States fall over southwestern Texas, notwithstanding the general aridity of that country. This happens when certain contrasts of atmospheric pressure exist, that produce conflicting currents and under-running of masses of warm air by cold air and the elevation of warm air to such heights as to cause rapid condensation and heavy precipitation. It is not prevented by the relative lack of moisture in the air.

Of course, if only the moisture contained in the body of air directly above a given ground area were extracted in the process of precipitation, the amount would not be large, probably never more than one to two inches; but in any local rain-producing disturbance, neighboring air may be drawn in and it may furnish sufficient additional moisture to cause a heavy downpour. The amount of rainfall will depend upon the extent and intensity of development of the local convectional disturbance. What is needed to produce rain, as all meteorologists know, is a process whereby large masses of warm air are elevated and cooled thereby to the point of precipitation.

It may be said, therefore, that while the more humid atmosphere gives the more favorable condition for showers, there is never a time, anywhere, in the warm season when temperatures are well above the freez ing point, when the air is too dry to yield precipitation, provided it be cooled sufficiently. Hence it need not be surprising that heavy local showers fall even in the severest drought periods, when the face of the earth is parched, and moisture, from appearances, is almost entirely lacking. Good illustrations of this occurred in Tennessee in the summer of 1925.

Summer Showers That Perplex Meteorologists


(Weather Bureau Office, Nashville, Tenn., September 23, 1925) While it is characteristic of summer rainfall in general, but especially in the southern states, that it comes largely in the form of local showers, which hit promiscuously, but sooner or later cover the entire area, the summer of 1925 in Tennessee showed unusually capricious behavior in respect to rainfall. There were wet and dry spots in early and midsummer of peculiar persistency, as if habitually opposite conditions pre


vailed for the time in different localities. Then, the state as a whole, except the extreme western counties, was held in the grip of a drought, but with favored and unfavored spots still apparent, until, finally, the drought became firmly established over the entire state and was probably the most severe since records began.

Charts were made of the daily and monthly rainfall for the 70 stations in Tennessee during June, July, and August, also a chart showing total rainfall for the three months. Efforts to assign causes for the abrupt, local variations in the amounts were rather fruitless. A station within one hundred miles of Nashville, for example, had copious showers for several days, while only traces fell at Nashville. Similar contrasts occurred in different parts of the state. Nashville, however, remained rather consistently dry, while other stations, on nearly every side, received more than twice as much rainfall as Nashville for the three months. What might be termed a case of double "persistency” occurred at Carthage, a station that was blessed during June and July with good rains, but received the least of all stations in August, when only traces fell. Other such peculiarities could be pointed out in the Tennessee records for the summer of 1925.

Meteorologists are often asked to "explain" such occurrences as are mentioned above. They cannot do it satisfactorily; they may theorize. One can take the daily weather maps and point out, with a degree of satisfaction, the reasons for hot spells and lack of general rains over a large district, such as the southern states, but he cannot explain satisfactorily the capricious local distribution of rainfall. Yet, one often cogitates upon these meteorological mysteries.

From superficial observation, the distribution of such showers seems to be purely accidental. Yet there are reasons for every occurrence, and one recalls ideas and explanations that have already been expressed concerning them, and one may conceive of possible explanations. Scarcely anything can be stated with certainty. Why is vertical convection more pronounced over this few square miles than over that? Is it the lay of the land? Is it the character of the earth surface strata and soil? Is it the vegetation, or lack of vegetation? Does the condition of the soil with respect to moisture have much effect? That is to say, does an area of wet ground contribute considerably to the inception of clouds and the generation of showers in the vicinity? And does a drought-stricken area have a covering of air sometimes so thirsty that it absorbs or dissolves the clouds as fast as they approach it?

One was often reminded this season of the idea of "persistency," or "inertia"-slowness to change from the existing regime. At times there seemed to be good examples of this in Tennessee. There were instances of orographic rains, too, and these are to be expected in Tennessee, as the topography is favorable for them. Then, that idea that local storms follow paths of least resistance, or are drawn in a direction of favoring conditions, came to the front.

It does seem, from instances observed, that an area once having

received a heavy soaking rain, while nearby places remain dry, stands a better chance for further showers than the dry places. And that an established drought becomes harder and harder to break until some overwhelming general storm movement forces an end to it.*

Generally the rainfall, in these capricious spells, is more variable in amounts than in numbers of occurrences. Very small amounts may fall at one station while a nearby station receives heavy falls. Evidently, the dry places are away from the main area of activity, or out of the path of the storm, while the wet places happen to be in just the right position to catch the rainfall-quite obvious remarks—but why this spot is favored and that not is a mystery, and no one can see the reasons for such discriminations. Such occurrences may be repeated several times in a month at stations not far apart. And they occur notwithstanding the fact that the conditions as shown by the weather maps may seem equally favorable for rain at the different stations involved. In this connection it should be remembered that the point of actual origin of a rain-producing disturbance need not necessarily receive rain; the disturbance probably starts to drift immediately, and by the time rain begins, the center of the disturbance may have passed some miles from its point of initiation, so that, although a locality may have the necessary conditions of heat and moisture to start convection and produce clouds, it fails to reap the harvest of rain and only contributes to the welfare of some neighboring area.

In many instances the drift of the rain clouds must be governed largely by such air currents-not very high above the earth-as prevail at the moment. The meteorologists have no way of discovering these currents except by kite and balloon flights, and these are so few and far between that little assistance locally can be expected from them except at points where upper-air observations are being made.

Evidently, there are times when very slight elements of advantage turn the wheel of fortune. We see a threatening cloud right at hand and expect a downpour, but actually little or no precipitation occurs here. The supply of moisture was not quite sufficient, or the upward currents were too weak to elevate masses of air in sufficient quantity to the required height, bring about sufficient cooling, and produce substantial rainfall, or, unobserved currents steered the rain clouds to one side or the other. But, in the vicinity, unknown, unseeable, and undiscoverable by any means that meteorologists have at their command, there are con ditions that are just right-possibly just a little more strength of vertical currents, or a little more lifting power of a current underrunning a mass of warm, moist air, or a slightly less arid upper stratum, or a current that brings the rain clouds the right way—and the neighboring farm receives a good rain.

May we hope ever to be able to detect with certainty the inception of local storms, follow their development, and name the area where rain

* See remarks by Professor Cox, page 802, "Weather Forecasting in the United States."

will fall and the hour of its beginning? If one could only see in reality the activities that go on in the air within the range of ordinary vision! The changing temperatures, the changing densities, the varying currents, the varying moisture content—if these could only be actually seen! In fancy, one attempts to devise a scheme, whereby all these forces and motions could be indicated by colors or shadings in the atmosphere and the panorama become visible and intelligible. It is impossible, of course. At present we can only accept the conclusion, long ago drawn, that, however hopeful meteorologists may be of adding to their store of knowledge of the general laws of the atmosphere, and its large movements, there seems little possibility of their being able to foresee definitely the time and place of summer showers, for these are, in the language of Professor Henry, "the bête noire of the forecaster.”**


Lunar rainbows, infrequently seen and rarely reported, were described by Mrs. L. E. Wallace, of Larned, Kansas, in a letter to the Weather Bureau. After a heavy shower July 14, 1925, "the nearly full moon in the southeast shone through a disappearing mist after the rain and a rainbow showed clearly across the northwest sky. . . Only a quarter bow showed, but its coloring was distinct. It was a rosy pink, though the other bands of color could be faintly distinguished, the blue best of all." Another on August 3, "the entire half circle of the rainbow was visible the conditions of rather misty atmosphere were similar to those observed on July 14."


The frequency of storm damage done transmission lines with its attendant loss of revenue from interruption to industry, leads one to doubt that all has been done to avoid much of this loss.

Standardization is a wonderful thing and should be closely adhered to at the top of the pole or tower, but the pole or tower, constructed in itself should be governed wholly by the conditions it must meet. These conditions are storms, the usual and unusual direction of gales and ordinary winds, the character of the ground over which the lines are built, whether firm earth or clay, peat bogs or quicksand, practically level, or rolling; the direction of the line relative to storm paths, whether it traverses or parallels them. Railroads present a striking example of standardization in their bridges, rolling stock gauge, subgrade, rails, ties, and general maintenance; yet none attempt high speed where curves are indispensable and not super-elevated or where the roadbed is known to be located on soft or marshy ground until, after years of work, the fills give promise that the sag and settle has been reduced to the minimum.

The reconnaisance survey for a line should include a minute examination and report of all available weather reports for the vicinity, noting ** See page 280, "Weather Forecasting in the United States."

« AnteriorContinuar »