Popular Science Monthly/Volume 49/September 1896/Dust and Sand Storms in the West
|DUST AND SAND STORMS IN THE WEST.|
PROFESSOR OF GEOLOGY AND NATURAL HISTORY, AUGUSTANA COLLEGE, ROCK ISLAND, ILL.
FOR some years the writer has been gathering data on the transportation of sand and dust by the atmosphere, with a view of studying the geological significance of these phenomena. Among other sources of information the newspapers have been drawn upon, and it is to the facts gathered from these, and by personal correspondence that it is at present desired to direct attention. The newspaper man may not always state facts with such exactness and precision as would be desirable, but his ubiquity no less than the very conservatism of the scientist, who seeks the broadest possible foundation for all generalizations, combine to give him a function in the investigation of the laws of Nature. Of course, it can be only a humble function—that of an observer who is not always to be trusted. For the lack of training or by reason of other shortcomings his accounts of natural phenomena must sometimes be taken cum grano salis. Dust storms occur chiefly over arid lands, and they develop their greatest force mostly only in regions which are but sparsely inhabited, if at all. They are not often witnessed by geologists. As a consequence, they have been but little studied, and it is desirable to collect information from all sources with regard to their nature and occurrence.
While dust storms are sometimes to be seen east of the Mississippi River, they are much more frequent in the arid and semiarid regions of the western part of the United States, where the rainfall is small. Of the thirty-eight storms found recorded during 1894 and 1895, only one occurred east of the Mississippi. The distribution over the Western States and Territories was as follows:
While the number of reported storms is too small to warrant a discussion of their geographical distribution in the United States, the above table so far, no doubt, correctly represents the facts that it shows an increase from the Western plains to the Pacific coast, and comes to a maximum in the Southwest. On the plains most places are reported to experience dust storms two or three times in a year, while on the Pacific coast estimates and records for different places range considerably above this number. At Los Angeles, Cal., one observer says that there are "two or three in the course of a year, possibly a few more." From Yuma, Ariz., the statement comes that "any high wind, without rain, generally blows clouds of dust," and six sand storms were recorded by observer A. Ashenberger in the Weather Bureau station at that place during 1893. At Ontario, Cal., it is estimated that there are from twelve to forty dust storms in a year. It is said that these storms are "most common and strongest in passes in the mountain ranges in California," that they are "very severe on the east side of the Coast Range," and that "nearly every part of California is afflicted [by them] at times." From the reports which are at hand an estimate of the minimum frequency of these disturbances in places in the West, where topographical and climatal conditions do not forbid them, is two in a year for the territory east of the Rocky Mountains and five in a year for the Great Basin and the western slope. A maximum estimate would be four annually for the former region and twenty for the latter.
Data on the areal extent of each separate storm are meager, as reports of simultaneous observations have been secured in but few instances. Where such observations have been reported they represent, as is evident, the minimum extent of the storm in one direction, since it may have extended beyond the points from which the reports have come. The few instances of reported areal extent of dust storms may be tabulated as follows:
|From||Milton, Ore., to Colfax, Wash.||80||miles.|
|"||Fresno, Cal., to Santa Maria, Cal.||120||"|
|"||Mojave, Cal., to Oceanside, Cal||140||"|
|"||Salem, S. Dak., to Sanborn, N. Dak||216||"|
|"||Santa Anna, Cal., to San Diego, Cal||270||"|
|"Over the greater part of Nevada"||300||"|
|Over most of northern Iowa and Illinois||400||"|
This gives an average diameter of two hundred and sixteen miles.
Information has also been sought with regard to the length of time that these storms continue to blow at any one place. In several instances the reports state this time, and in these cases it varies from one to forty-two hours, and averages a little over thirteen hours, as may be seen below:
Duration of Dust Storms.
|2||storms||lasted||for||. . . . . . . . . . . . . . . . . . . . . . .||1||hour.|
|2||"||"||"||. . . . . . . . . . . . . . . . . . . . . . .||2||hours.|
|1||storm||"||"||. . . . . . . . . . . . . . . . . . . . . . .||4||"|
|2||storms||"||"||. . . . . . . . . . . . . . . . . . . . . . .||6||"|
|2||"||"||"||. . . . . . . . . . . . . . . . . . . . . . .||8||"|
|2||"||"||"||. . . . . . . . . . . . . . . . . . . . . . .||12||"|
|1||storm||"||"||. . . . . . . . . . . . . . . . . . . . . . .||16||"|
|1||"||"||"||. . . . . . . . . . . . . . . . . . . . . . .||18||"|
|2||storms||"||"||. . . . . . . . . . . . . . . . . . . . . . .||24||"|
|2||"||"||"||. . . . . . . . . . . . . . . . . . . . . . .||30||"|
|1||storm||"||"||. . . . . . . . . . . . . . . . . . . . . . .||42||"|
Supplementing these direct observations, estimates have been obtained from a few parties, and these make the time considerably longer. These estimates are as follows: "Sometimes an hour, sometimes three days, coming with great violence at intervals." (One hour, seventy-two hours.)—"We have known one that continued a week, with one day for a recess."—"The sand storms last from one day to three days, but sometimes only a few hours." (Four hours, twenty-four hours, seventy-two hours.)—"The sand storms last about one day." (Twenty-four hours.)
Leaving out the storm that lasted six days, thirty-two hours would appear to be a fair average for the other estimates. But these estimates perhaps apply more particularly to the greatest storms, while the previous table no doubt includes a rather large proportion of short,winds. A mean of the averages is not far from twenty-four hours, and this is perhaps a safer estimate of the average time of a single dust storm. These atmospheric disturbances occur on the steeper gradients of areas of low barometric pressure, and they partake of the progressive eastward motion of these. Their duration at any particular point will hence depend upon their areal extent and upon the velocity of the low area. If the diameter of the dust storm be divided by the time, we will have this velocity, which in this case will be less than ten miles an hour. This is less than half the usual rate of eastward progress of a low area in the eastern part of the United States, but it corresponds more nearly with the rate observed in the Great Basin, where most of these storms occurred.
In nearly every instance where these disturbances have been described some mention is made of the quantity of the material transported by the atmosphere. Nevertheless, it is exceedingly difficult to make any definite estimate in this direction, as no actual measurements have been made by any of the observers. But a number of the accounts are such that comparisons can be made between the phenomena described and some other instances of dust transportation which have come to the writer's notice, and which have furnished some quantitative estimates. The results of such comparisons are here given for what they are worth. In several cases the effect of the dust on the transparency of the atmosphere is noted. It is thus stated that—"It gives the sun a sickly color."—"It is dense enough to obscure the mountain ranges from view" (at a distance of from five to ten miles),—"It is sufficient to allow the sun being viewed with the naked eye."—"Immense quantities of sand and dust filled the air, until the sun became so obscure that it could only be seen as a round ball, at which one could gaze with impunity."
During a high wind on the 25th of March, 1895, following a dry season, the atmosphere over the northern part of Illinois and over a part of Iowa had an appearance which corresponded to the instances here described. The storm lasted about three hours, and during that time an apparatus for collecting dust from the atmosphere was suspended at an elevation of about a hundred feet above the ground back of the bluffs of the Mississippi River at Rock Island, Ill. This apparatus was so arranged that dust could not be taken from a current of air more than a tenth of a square foot in cross-section. It is possible that the actual current was not more than a tenth as large as this. The quantity of dust collected was about two ounces. The wind velocity for the three hours was thirty miles per hour. This indicates that the atmosphere on that day carried a load of one hundred and sixty, or possibly sixteen hundred, tons of dust to the cubic mile of air.
Some of the notes refer to the accumulation of dust and sand in dwelling houses and other buildings, viz.: "Merchants closed their doors to protect their goods" (from the dust).—"The quantity of sand swept from houses (by housekeepers after a storm) showed the severity of the storm the two previous days. The sand penetrated every nook and corner."—"The pattern of the carpet may be obliterated; drifts have been formed on the floor from one to two inches in depth."—"The dust filled every residence completely [!] covering up everything while it lasted."—"The wind hurled a few quarter sections [!] of rich loam into the residences and business houses."
Blown dust is a general and familiar nuisance to housekeepers over the entire West. A minimum estimate, verified by direct observation, for the quantity of dust settling on floors during such storms is about a fourteenth of an ounce of dust on a surface of a square yard in half a day. A maximum estimate made on the basis of the above newspaper accounts would be at least five pounds to a square yard of surface for a storm lasting twenty-four hours. If we then suppose that a house that is twenty-four feet wide and thirty-two feet long has open crevices, which average a sixteenth of an inch in width and have a running length in windows and doors of one hundred and fifty feet, the wind may be supposed to enter half of these crevices with a velocity of five miles per hour for the time the storm lasts, or for twenty-four hours. The dust may be supposed to settle on not less than eighty-five square yards of surface, including floor space and horizontal surfaces of furniture. The minimum estimate, based on these figures, gives us two hundred and twenty-five tons of dust to the cubic mile of air. The maximum estimate would be one hundred and twenty-six thousand tons.
In the following citations the optical aspects of the dust-laden air are again characterized in a definite way: "The air was so full of sand that it resembled a fog."—"A wind storm struck us, bringing a dense cloud of dust."—"The sky assumed a deep, tawny hue, and fifty yards was the limit of clear vision."—"The dust was so thick and heavy that a person could not see more than a block through it."—"The dust was so thick that it was impossible to see halfway across the street."—"I have seen the dust so fill the air [in a Western dust storm] as to make it difficult to see more than a few rods."—"At times it was impossible to see across the street on account of the flying sand."—"A strong wind was made thick and yellow by flying real estate."—"The wind filled the air with dust as far as the eye could see. It immediately became dark, and lamps had to be lighted."—"During the sand storm it was dark as night, and people ran into each other in their flight through the streets."—"The wind was accompanied by dense clouds of dust that obscured the sky until all was dark as midnight."
From the phrases used it is evident that the transparency of the atmosphere must have been considerably less than when the sun could be viewed through it, or when objects might be seen dimly at a distance of one or two miles, as in some instances previously mentioned. This difference in the two cases is, of course, due to the increased quantity of dust carried by the air. Such conditions as are described here may readily be produced experimentally on a small scale by throwing dust into the air on a windy day. If the quantity of the dust be known, it is necessary only to estimate the degree of opacity produced and the bulk of the air in which the material is dispersed. From a number of experiments, it appears that two ounces of dust suspended in about four thousand cubic feet of air render it as thick as it must have been, at the least, in the storms described. This would make about two thousand tons to a cubic mile. It should be added that these experiments were made in a wind moving about eight miles an hour, and with dust quite fine enough to be suspended in such a wind. A considerably larger quantity would no doubt be required if the material were to be coarser, such as would be carried by a strong wind. The estimate is therefore believed to represent a minimum for such storms as are described above.
Another perhaps less reliable estimate may be made from accounts which describe the drifting sand, thus: "The sand drifts, as snow does, and has attained such a depth as to cause a fear that vegetation in the simoom's path will be greatly damaged."—"Drifts of sand one foot high were piled up in thirty minutes on a railroad track."—"Cuts [along a railroad] were filled with immense drifts, which averaged about two thirds sand and one third snow."—"At Cheyenne Wells, Colorado, thirteen cars of sand were taken from the depot platform" (after a storm).—"Tracks were obliterated [by drifting sand] and the whole landscape was changed."
Under such conditions it may be surmised that a drift of twenty-five tons of sand might be deposited during six hours from a current of air forty feet wide from the lowest ten feet, in the lee of some intercepting obstacle, as in a railroad cut. In fact, such instances are on record. The velocity so near the ground would not exceed fifty miles an hour. Twenty-five tons may therefore be carried by 633,600,000 cubic feet of air, which makes nearly six thousand tons to a cubic mile. It is by no means likely that all, or even the greater part, of the sand carried by the lowest ten feet of the atmosphere can be left in the drift, and the estimate may again be much too low.
Still another approximation can be made by experimenting on the effects of dust in the atmosphere on the respiratory mechanism of the human body. Such effects are referred to in the following paragraphs:
"The wind sweeps down from the deserts and brings with it sand in such quantities as to almost make breathing impossible."—"The sand was blown in stifling clouds about them."—"During the sand storm the air is so full of dust that it feels as if it were impossible to breathe."
By some simple experiments it has been ascertained that less than two grains of mineral dust suspended in a cubic foot of air interferes with inhalation in a normal way. This would make about twenty thousand tons of dust to the cubic mile. It is possible that the stifling sensations felt in a dust-laden atmosphere are the results of a cumulative stimulus on the nervous system, and a smaller quantity might produce similar effects. No doubt there is also a subjective factor which will modify estimates on this line.
Lastly, an estimate, or rather a measurement, has been made on the quantity of sand carried in such storms as are recounted in the following items: "The sand is blown about in such quantities that it is not possible to keep one's eyes open."—"A man venturing into a whirlwind of sand invariably returned in a few minutes with his face bleeding with hundreds of cuts."—"Clouds of sand were driven through the air by a high wind, obscuring all objects and rendering existence almost impossible (sic) for man and beast."
In 1886 the writer had the opportunity to be in the midst of such flying sand on the Western plains, and to make some observations on the quantity borne by the air. The storm was not as severe as those described in the above paragraphs, nor did it carry as much drift, for neither was the author's facial integument punctured nor did he experience any apprehensions as to the possibility of continued existence. But there was enough of sand in the air to deposit one ninth of an ounce during fifteen minutes in a vial with an aperture measuring one tenth of a square inch turned to the windward. A velocity of twenty miles per hour was probably not exceeded where the receptacle was placed, and this would make the load carried equal to nearly thirty thousand tons per cubic mile.
To sum up: The estimated loads of sand and dust that may be carried by the atmosphere range from 150 to 126,000 tons per cubic mile of air, or from 0·0009 to 0·77 grammes per cubic foot.
|Estimate on a thick haze||160||0·0009||0·031|
|Lowest estimate from accumulation in dwellings||225||0·0013||0·048|
|Estimate on opaque dust clouds||2,000||0·012||0·434|
|Estimate on forming sand drifts||6,000||0·037||1·30|
|Estimate from effect of dust in the air on breathing||20,000||0·123||4·34|
|Estimate from a quantity of sand collected in a storm||30,000||0·184||6·49|
|Highest estimate from a quantity of sand in dwellings||126,000||0·77||27·29|
With these figures and the data on the duration and on the frequency of sand storms in the western part of the United States, it seems possible to form some idea of the total amount of work performed by such storms over this territory. It is not believed that the data presented justify any great claims for exactness for a general estimate, which is made here in the absence of a better one.
If we say that this western country experiences a dust storm twice a year, we do not rate this work too high. During two such days the velocity of the wind for the lowest mile in the atmosphere will average, at least, thirty miles an hour, and the total wind movement will be 1,440 miles. Of course, there are many sheltered places where such winds will not be felt. The territory including the west two thirds of the Dakotas, of Nebraska, Kansas, Oklahoma, and Texas, and extending west to the Pacific Ocean, contains about 1,000,000 square miles of open land, allowing one third the area for mountains. For the time of the storm the atmosphere over this area may be regarded as a current of air 1,000 miles long, 1,000 miles wide, and a mile high, containing 1,000,000 cubic miles of air. If the lowest 200 feet in this current carry a load of 20,000 tons to the cubic mile, and if the remaining 5,080 feet carry 100 tons per cubic mile, there will be 853 tons of dust to each square mile of the whole area of the current, making a total of 853,700,000 tons transported a distance of 1,440 miles, or, if the expression be permitted, 1,229,342,400,000 mile-tons of transportation will be performed.
Comparing this with the quantity of work performed by the water of the Mississippi drainage system, we find that the latter is three hundred and thirty times as great. The Mississippi carries annually 406,250,000,000 tons (estimate by Humphrey and Abbott) of sediments a distance of, say, 1,000 miles, performing 406,250,000,000,000 mile-tons of transportation. The ratio of the atmospheric transportation in the West and the aqueous transportation in the Mississippi basin is then 1:330.
If the above estimates have any significance at all, it is to the effect that in this country the work of the atmosphere is less than the work performed by meteoric waters. So far the inference is in full accord with the well-grounded general consensus among geologists.
Care has been used to not overrate any of the factors entering into the estimates. In sandy regions a considerable amount of transportation is effected by pushing by the wind on the loose surface material. This part of the work of the wind is here entirely neglected. It is known, too, that even on calm days the atmosphere carries an appreciable load of dust, and this has not been taken into account, though it operates for the remaining three hundred and sixty-three days of the year. It is, therefore, possible that the general estimate is too low. But it is not necessary to have recourse to such a supposition alone to show that, locally, work by the atmosphere may exceed the work performed by the water. The observed average wind velocities in these storms appear to be about twenty-five miles an hour. Velocities two and a half times as high as this are not unknown, and the efficiency of such a current is two hundred and five times that of one having a velocity of twenty-five miles an hour. If the velocity is doubled it increases the transporting power sixty-four times. Should such storms then occur on twenty days of the year instead of on two, the total work would be six hundred and forty times more effective than in the first instance. Still another increase would result from the greater vertical dispersion of the lower greater load in a wind with high velocity. A predominance of aerial transportation may, of course, also be due to local inefficiency of the aqueous work. But this digression is not made for the purpose of proving the possibility of such a predominance in any place. A proof of this would be superfluous, since the topography of sand-hill regions is conclusive evidence on this point. It is merely desired to emphasize the fact that atmospheric work is subject to very great range in its effectiveness, and that as a consequence the great range of the estimates made above does not impeach their trustworthiness.
Some other stray items of information, culled from the newspaper accounts, may be briefly stated in closing. The only statement from which the distance over which dust has been transported can be definitely estimated is in a notice coming from the southern part of California. From the notice made it is certain that dust must have been carried twenty miles. Of course, this does not speak against the possibility of a transportation over twenty times that distance. In one case a slow settling of fine dust from the air is reported as actually observed, and this was also in California. The maximum wind velocities reported in connection with dust storms are few, running from 36 to 90 miles per hour, viz.: 36, 40, 45, 50-60, 90. Some instances of the erosive effects of blown sand are described. It is stated that winds will raise dust on sparsely covered, not cultivated land; that orange trees in California are sometimes girdled near the ground by sand blasts; that the glass in the windows of railroad coaches is etched by impinging sand; and that paint on the coaches is worn off in the same way. The softer wood in telegraph poles is sometimes worn away by sand so much faster than the harder wood in the knots that the latter are left protruding far out. Finally, there are some accounts of the coarseness of the transported material. It is generally fine enough to be called dust, but sand is often mingled with the dust, and occasionally there is fine gravel. Two reports mention pebbles, and in one instance these are said to have been large enough to "knock a man senseless."
To the writer the facts here presented appear most interesting in their incompleteness. While there are tens of thousands of square miles in the West where the wind has covered the land with monuments of its undisputed reign, the ever-changing sand hills, very little is known as to the quantity of the work performed by the wind in such regions, or as to the relation between this work and prevailing wind velocities. It is evident, also, that a considerable quantity of fine déhris is removed by the wind in such places, but observations are wanting as to what becomes of this product of trituration after it is raised from the creeping sand, how far it is carried, or where it is deposited. The atmosphere is known to carry appreciable quantities of dust, as already stated, even in low winds, but on this work the study of the dust storms and sand storms has no bearing. The study of the geological work of the atmosphere evidently requires a wider basis of facts.