1902 Encyclopedia > United States > Climate

United States
(Part 19)




SECTION II: PHYSICAL GEOGRAPHY AND STATISTICS (cont.)

Part 19. Climate


From the Atlantic seaboard west to near the base of the Rocky Mountains the lines of equal mean temperature have a considerable degree of regularity, running approximately east and west. When, on the other hand, we reach the borders of the Cordilleran region we find the isothermal lines suddenly deflected from their normal course, and in passing across the mountain and plateau belt we find them irregular, often concentric over large areas and through great ranges of temperature, according as the altitude, width, and general trend of each separate range or system of ranges make their influence felt. Hence there are three distinct climatic divisions of the United States:—(1) the eastern region, from the Atlantic to the foot of the high plateaus at the base of the Rocky Mountains; (2) the plateau and mountain region of the Cordilleras; (3) a narrow strip on the Pacific coast, lying west of the Sierra Nevada and the Cascade range. These three divisions are of very unequal size and importance. The first embraces about three-fifths of the entire country, and contains fully nineteen-twentieths of its population ; the second is also much larger than the third, containing not much less than a million square miles, but is very sparsely peopled. The third is more densely peopled than the second, but small in area, although its limits are not capable of being accurately defined. These three divisions will here be designated the Eastern, Cordilleran, and Pacific.

In the Eastern division the passage from one type of climate to another is gradual and uniform, though rapid. The difference in climate between the eastern and western coast of the Atlantic was long ago noticed and commented on. It was George Forstcr who first controverted the prevailing idea that the New World in general was colder than the Old, and recognized the analogy between the climates of the eastern coasts of the Atlantic and Pacific. Humboldt afterwards investigated the facts and published a tabular statement, which, as enlarged by Hann, is here presented (Table III.):—

== TABLE ==

From the above table it will be seen that the difference between the mean annual temperature of places in high latitudes on the opw«ite sides of the Atlantic is very large, and that it diminishes as we go south. About lat. 30° the two sides of the Atlantic have nearly the same mean temperature, the difference in climate being very great, but chiefly dependent on differences in the amount of precipitation. Nearly the whole area of the United States is included between the annual isothermals of 44° and 68°—a difference of 24°, the corresponding difference of latitude being about 15°. The average change of temperature is, therefore, 1°'6 for each degree of latitude,—the most rapid change of temperature with the latitude known in any region of anything like equal extent. The causes of the rapid increase of temperature in going south along the Atlantic seaboard are the position of the Gulf of Mexico, the high temperature of its waters, and the increasing predominance of south-westerly winds. From these circumstances the southern portion of the Atlantic coast of the United States is decidedly warmer than the regions corresponding to it in latitude on the west side of the Pacific, while farther north places in the same latitude on the west sides of the two oceans have approximately the same temperature. This similarity of temperature on the corresponding sides of the Atlantic and Pacific is the result of causes now easily understood, the chief being the position of the mass of the land with reference to the direction of the prevailing w:inds. From the Atlantic coast to the eastern base of the Cordilleras the isothermal curves for the year are nearly parallel, and have a general east and west course, being only interrupted in this regularity and deflected to a certain moderate extent in passing across the Appalachian chain, which nowhere rises high enough to give a chance for permanent accumulation of snow. These curves, of course, are roughly parallel to the coast-line of the Gulf of Mexico, which over a breadth of fourteen degrees of longitude does not vary much from an east and west direction. The region over which a higher mean annual temperature than 68° F. prevails includes nearly the whole of Florida and a narrow strip along the Gulf, which widens rapidly in Texas, where the trend of the coast-line suddenly becomes nearly north and south. The extreme south end of Florida, which just touches 35°, has a mean temperature of over 72°, the isothermal of 76° being nearly on that parallel. The isothermal of 64°, wdiich meets the Atlantic coast near the borders of North and South Carolina, keeps nearly on the parallel of 34° as far west as about 100° W. long., where it is rapidly deflected southward, in conformity with the direction of the other isothermals, by the gradually increasing elevation of the country when the plateau region is encountered. The isothermal of 60° is nearly parallel to that of 64°, except that it manifests the influence of the high southern extremity of the Appalachians, and is in consequence considerably deflected to the south between the meridians of 83° and 87°. The isothermal of 52° is, to the west of the Appalachians, nearly coincident in position with the Ohio river as far as Cincinnati, and thence follows an undulating course, with a nearly westerly general direction, through Indiana, Illinois, northern Missouri, and along the northern boundary of Kansas to the border of Colorado, where it is suddenly deflected and runs with a nearly southerly course for a distance of fully 500 miles along the eastern base of the Rocky Mountains. Those portions of the country which lie between the isothermals of 44° and 52° are New England, with the exception of Maine and the northern part of New Hampshire and Vermont; New York, excluding the extreme north-eastern corner (the Adirondack region); the Appalachian plateau region on the borders of New York and Pennsylvania; nearly all Ohio; two-thirds of Indiana and Illinois; nearly all Michigan and Iowa; southern "Wisconsin; south-eastern Minnesota; nearly all Nebraska; and the southern half of Dakota. The isothermal of 40° passes through the centre of Maine, cuts off the extreme northern end of New Hampshire and Vermont, then passes out of the United States, re-entering at the west end of Lake Superior, passing through the centre of Minnesota, making a large loop to the south in eastern Dakota and then trending north-westwardly until it passes beyond the boundary line of the United States in 107° W. long.

Within the Cordilleran region, or west of the 105th meridian, the position of the isothermals is largely dependent on that of the mountain ranges, which rise high enough profoundly to influence the climate, though it is only at a few points, especially round the summits of the lofty volcanic cones near the Pacific coast, that they reach the region of perpetual snow. This deficiency of lasting accumulations of snow, however, is in very considerable part due to the smallness of the precipitation. Observations of temperature on the higher ranges are extremely deficient. On Mr Schott's temperature chart (Plate IX.) the isothermal of 44°, which, as already mentioned, east of the Cordilleran region nearly coincides with the northern boundary of the country, encloses within a great southerly-reaching loop the whole of the higher portion of the Rocky Mountains, extending as far south as the 34th parallel, or to about the position in latitude of the isothermal of 60° in the eastern division of the country. The crest of the Sierra Nevada, Cascade, and Blue Mountain ranges is also within the curve of 44°. The highest portion of the Rocky Mountains, as far south as 39° N. lat., is laid down as having a mean temperature lower than 36° F. The whole of the Great Basin and the Columbian plateau is indicated as having a considerably higher temperature than the dominating system of ranges which enclose it on the east and west. Considerable bodies of snow remain on the summits of the ranges during a large part of the year, at least as far south as 39° N. lat. In the plateau region of Arizona, Utah, and Nevada the decline of the ranges, the generally lessening elevation of the region, and the facility of access which the topographical conditions allow to the heated air from the south give a high temperature, and the isothermals form irregularly concentric loops extending from the head of the Gulf of California northwards. The isothermal of 52° reaches as far north as Virginia City, in lat. 39°, and that of 72° extends to Fort Mohave, in lat. 35°.

In strong contrast with the Eastern division, we find in the region bordering on the Pacific a very marked tendency to a parallelism of the isothermals with the trend of the coast; consequently, a very moderate change in the mean annual temperature may be met with over a large range of latitude. The character of the isothermals here is greatly modified by the position of the two parallel ranges, the Coast Mountains and the Sierra Nevada, which enclose valleys of great extent but of low altitude. In general the temperature of the Pacific coast-belt is much more uniform and higher than that of the Atlantic side of the United States. The isothermal of 60° runs nearly parallel with the coast, and not far distant from it, from the southern line of California north through nearly three degrees of latitude. The isothermal of 52° approaches San Francisco in lat. 37° 48', and keeps near the coast to as far north as lat. 47°. A higher mean temperature than 48°prevails over the region adjacent to Puget Sound, at the northern boundary of the country, in lat. 49°, while the mean temperature of the northern part of Maine, between the parallels of 45° and 47°, is below 40°. Thus it may be said with truth that near the Pacific coast we have a difference of only 12° in mean temperature in a range of over sixteen degrees of latitude. And if we pass from the immediate vicinity of the coast in lat. 35° into the San Joaquin and Sacramento valleys, we may range over five degrees of latitude and keep in a region of which the mean temperature is not below 60° and nowhere much higher. The causes of this are the proximity of the great area of water from which the prevailing winds blow, the modification which the temperature of this ocean undergoes near the American coast by the Asiatic coast current and the northern or Arctic coast current, and the position of the mountain ranges near the coast. Uniformity of climate along the edges of the land is still further aided by the peculiar nature of the currents along this coast. The influence of the warm Asiatic current—the Kuro-Siwo—is distinctly felt in raising the temperature as far south as the northern border of California, while farther south the cold Arctic current, which apparently emerges from under the warm current, makes its cooling presence felt along the coast nearly or quite as far as the southern boundary of the country.

The isothermals for the summer months (June, July, and August) are much more irregular than those of the year, especially in the Eastern division. The powerful heating influence of the Gulf of Mexico, swept over in summer by southerly winds, makes itself extremely apparent in the summer isothermals, which bend to the north-west in a most remarkable manner, that of 72° reaching as far as the centre of Dakota, or beyond lat. 45°. A mean summer temperature of 80° and upwards prevails over Florida, a considerable portion of the Gulf States, and nearly all Texas. The belt adjacent to the Ohio, extending north as far as the Great Lakes, south along the Appalachian tableland into Tennessee and the north-western corner of Georgia, and west through Iowa, Nebraska, and northern Kansas, lies between the summer isothermals of 68° and 76°. The summer isothermals along the Pacific coast are much less considerably changed in position and character from their mean annual character than they are on the Atlantic side, for reasons which have been already given, while the irregularity and complexity of the summer curves in the Cordilleran region generally would be very distinctly noticed if the data were at hand and could be exhibited with some detail. An extraordinarily high o temperature prevails in summer in the southern portion of the Great Basin and in the Arizona plateau region, the isothermal of 88° surrounding with its northerly-reaching loop a large area in the lower valley of the Colorado river and extending north as far as lat. 35°. The winter (December, January, February) isothermals in the Eastern division have more of the regularity of the annual curves than have those of the summer. The winter isothermal of 52° coincides very nearly with the mean annual curve of 68°, keeping near and closely parallel to the Gulf of Mexico. The winter isothermal of 32° runs from Cape Cod across Long Island to New York city, and across New Jersey, thence making a large loop to the south so as to surround the Appalachians, and, after ascending northerly again on the west side of that range to near the Ohio, passing through Indiana, Illinois, Missouri, and Kansas, thence descending in a south-westerly direction and sweeping around the Rocky Mountains, and through the centre of the Great Basin in a very irregular course. On the Pacific coast the form of the winter curves closely resembles that of the yearly isothermals. The winter curve of 52° very closely coincides with that of 60° for the year, and the winter curve of 40° runs from near San Francisco, closely parallel to the coast and at a little distance from it, as far as Cape Flattery, or through a distance of over ten degrees of latitude.

The irregular, non-periodic fluctuations of the temperature are of great interest, and without knowing what these are one would form a very false idea of the real character of the climate. It does not appear that these fluctuations greatly affect the general salubrity of the country, but they have a marked effect on the character of the vegetation, as well as on the methods of cultivation. The occasional occurrence of very low temperatures in low southerly latitudes where the mean winter temperature is quite high is one of the most striking phenomena in the climate. Savannah, as Hann remarks, has a mean winter temperature the same as that of London and Cadiz, although this latter city lies 4J° farther north. But the vegetation of the two regions is essentially different, because frosts do not occur in that part of the Spanish peninsula. Orange trees are liable to become entirely frozen everywhere in the United States except in southern Florida; this is not the case in Spain. The cotton plant is a perennial in the south of Spain, while, on the other hand, the stem and branches are killed ever}' year by frost in the United States, so that the fields have to be annually replanted.

The following table (IV.), from data arranged by Hann, gives an idea of the range of temperature in various parts of the country :—

== TABLE ==

The region of lowest winter temperature is that along the eastern border of the Rocky Mountains in the northern portion of the country, where the temperature not unfrequently sinks so low as to freeze mercury. The lowest temperatures observed in this region, as given by Schott, are—at Fort Sanders, in Wyoming, - 50°; Fort Ellis, Montana, - 53°. A temperature low enough to freeze mercury is occasionally observed in Wisconsin and Michigan, and on the borders of Canada and New York. The hottest region is that along the lower portion of the Colorado and Gila rivers in Arizona and the adjacent part of California.

An excellent illustrative example of the suddenness and severity of the "cold waves" which occasionally pass over the country is afforded by the facts gathered by the Signal Service in regard to an occurrence of this kind in January 1886.





The barometer was high from the Rocky Mountain region to the Pacific coast on the 2d, and from that date to the 5th a" slow north-easterly movement of this high area was observed; after the 5th there was an apparent increase of this high area from the region of the Saskatchewan valley and Manitoba, On the afternoon of the 6th the observers in Wyoming, Colorado, Nebraska, Kansas, and Missouri were warned of the approach of a cold wave," accompanied by a " norther," and of a probable fall of temperature of 20° to 25° in tile next twenty-four hours. The centre of greatest barometric pressure remained north of Dakota from the 6th to the 12th, but the cold wave had readied the Gulf coast and Florida before that date, causing in many places a lower temperature than lias been observed in many years, and in some a lower one than had ever before been known. In Kansas many persons were frozen to death, and the loss of stock was very great; at Dodge City the wind blow witli a velocity of 40 miles an hour, the thermometer averaging during the day 10° below zero. In Mahaska county, Iowa, from the 7th to the 11th twenty persons perished with the cold, and much stock was lost. Similar reports came from other parts of Iowa. In Memphis, Tennessee, the thermometer fell to 8° below zero. In Nashville, from the 9th to ttie 10th, the cold was the severest on record. In New Orleans the cold wave struck the city at 3 A.M. on the Stli, and the thermometer stood at 15°*2 on the morning of tire 9th. At Indianola, Texas, the coldest weather experienced for several years occurred from the 8th to tile 13th ; on tile 12th snow fell to the depth of 3 inches. At Galveston the cold was the greatest ever known, the mercury falling to 11°, being a fall of 54° in less than eighteen hours. A heavy snowstorm set in on the morning of the 12th, covering the ground to the depth of 6 inches, and causing much loss and suffering. At Mobile, Alabama, the minimum on tile morning of the 9th was 11°, and at Montgomery, 5°-4. In Florida the cold was very severe ; ponds were frozen over, and much fruit frozen on the trees. At Atlanta, Georgia, tile mercury fell to 2°-4 below zero. At Savannah it stood at 12°, the lowest ever recorded at that place. At Charleston, S.C., it stood at 10°'5 ; ice 3 inches thick formed on the ponds. On the morning of the 11th, the curve uniting points of which the temperature was zero ran from Dakota south nearly to Arkansas, thence across to the Atlantic, passing south of Knoxville, and up the coast to Nova Scotia. On the St Lawrence and beyond it to the north-west, the mercury stood at from 10° to 30° below zero. This cold wave was remarkable, not only for its severity, but because it extended so far to the south and caused so much damage. Tile whole country east of the Rocky Mountains was brought under its influence. Of the rapidity of its progress an idea can be formed from the statement that the first warning was issued from the Signal Office at 12h. 2m., January 7th, for the extreme north-west, and that for New England just two days later. This area of high barometer moved cast-ward, after the 12th, to the Atlantic coast, following the coast-line, passing over Nova Scotia, and disappearing to the eastward on the 16th.

It appears from Prof. Loomis's working over of the records of the Signal Service that throughout the greater part of the United States there is occasionally observed a difference of as much as 40° between the maximum and minimum of the same day, and that there are a few places where such changes are remarkably frequent. These places seem to be all west of the 95th meridian, and at or near the base of the Rocky Mountains. Thus, in 1874 there were thirty-eight stations at which a difference of 40° on the same day between the maximum and minimum temperature was observed. At Colorado Springs (5935 feet) this happened fifty-six times, at Denver (5135 feet) forty-five times, and at Cheyenne thirty-three times ; at seventeen stations it happened only once. At Denver, 15th January 1875, the thermometer fell 48° in one hour; and an observer "who is pronounced perfectly reliable" reported a fall in temperature at that [dace of 36° in five minutes. These changes of temperature felt at Denver were the concomitants of ti considerable storm, which came from the north-west, and whose centre passed about 250 miles east of that place.

The occasional occurrence of " hot waves" which sweep over large areas of country, raising the temperature much above its normal height, is one of the most striking and most disagreeable features of the climate of the country, and especially of its northern and north-eastern portions. There is rarely a year in which one or more of these abnormal occurrences are not observed; and, although they do not usually last more than two or three days, they are sometimes prolonged for a month or more, in a succession of heated periods with little or no interval between them. Thus, for example, in July 1885 the thermometer at West Las Animas, Colorado, rose on the 15th to 105°'2 ; at Albany, N.Y., on the 17th, to 96°-6 ; at New London, Conn., on the 18th, to 92°'4; in New York city, on the 21st, to 95°'9 ; in Baltimore, Md., on the 20th and 21st, to 98°'3 and 98°'7 ; at Dubuque, Iowa, on the 20th, outdoor work was suspended on account of the intense heat. Again, a little later, in Dayton, Washington Territory, on the 28th of the same month, the temperature rose to 102°-6 ; at Milwaukee, Wis., on the 2Sth, to 92°'8 ; at Fort Sully, Dakota, on the 29th, to 104°-o ; at Yankton, Dakota, on the 30th, to 100°'7 ; at Dubuque, Iowa, on the 30th, to 97°'l; at Des Moines, Iowa, on the 30th, to 100°'1. All through the country many cases of sun-stroke occurred, eighteen fatal cases having been recorded in Balti-more during the week ending with the 25th.

The prevailing winds, as in other regions lying in the latitude of the return trades, are westerly. The extreme southern part of the country is just on the border line where the influence of the causes by which the trade-winds are originated cease to be felt. In the autumn, however, in the southern Atlantic States there is some approach to the conditions of the trade-wind region. At that season the winds in Florida and along the northern edge of the Gulf are decidedly north-easterly as far as 33° N. hit. Farther south the Florida Keys and the northern Bahamas belong, to a certain extent, to the trade-wind region.

Along the whole extent of the Atlantic coast region westerly winds predominate during the entire year, but they are chiefly south-westerly in summer and north-westerly in winter. In the following table (Y. ) the direction of the summer and winter winds is given in percentages of the total amount, for the districts named :—

== TABLE ==

In the region between the Mississippi and the Appalachians, southward as far as the Cumberland range and north to Lakes Michigan and Huron, south-westerly and westerly winds prevail during both summer and winter. There is an extensive region in the south-west of the United States, embracing an area equal to about one-third of the whole country, in which the winds of summer are chiefly southerly, varying between south-east and south-west, while in the winter they are mostly north and north-west. This region extends from the extreme south-east of California, through Arizona, New Mexico, southern Utah, Texas, Arkansas, eastern Colorado, eastern Wyoming, Kansas, and Nebraska, to Missouri. Farther north, in Wisconsin, Minnesota, and northern Michigan, south winds prevail in the summer, but in winter there is no such marked predominance of northerly and north-westerly winds as in the region to the south-west. The influence of Lake Superior is clearly indicated in northern Wisconsin, where the prevailing winds in summer are from the lake and in winter from the land. On the Pacific coast the winds have a decidedly westerly character; but in the winter this preponderance is much less marked than in summer. On the coast of Washington Territory south-east is the prevailing direction, these winds being probably the south-west winds of the Pacific coast deflected by the mountains which lie close upon the ocean. In the interior of Washington Territory south-west is the prevailing direction in both summer and winter. Oil the California coast the winds are very strong and steady from the north-west in the summer, but more to the south-west in winter. In summer the intensely heated plateau to the east draws the air from the Pacific, which blows with violence through every depression in the coast ranges towards the heated land-mass. There is no " wind-gap " in the Coast ranges from the Columbia river to Santa Barbara so deeply and widely cut as that of the Golden Gate at San Francisco. At this point the cool winds from the sea find entrance to the Great Valley of the Sacramento and San Joaquin, and the mass of air thus set in motion spreads itself out fan-like after passing through the Gate, so that the prevailing winds in those valleys are in summer always from the Bay of San Francisco towards the mountains. The hotter the weather in the interior the more violent is the wind at San Francisco. But this condition is limited to the daytime. At night the rapid cooling of the higher plateau checks or stops altogether the indraught of air, and an almost entire calm prevails at San Francisco, while the cool air flows in a gentle breeze down the slopes of the mountains, in a reverse direction from that which it had during the daytime. In the winter the westerly direction of the winds in this region is still greatly predominant, but the prevailing westerly current of air is not intensified in its motion as it is during the summer. Over the plateau and mountain region included between the Sierra Nevada and the Rocky Mountains the surface winds are irregular, being governed by the topography of the country; but the upper currents are, in general, from the west. In the southern part of this region, in the valley of the Gila and the lower Colorado, there is a large area which is intensely heated in summer, and towards which the winds blow from the lower region to the south, and especially from the Gulf of California. Here the predominance of southerly winds in summer is very great; but the mountain ranges to the west have so declined in height in this southern region that westerly winds are nearly or quite as common as northerly ones. Farther east and north-east, as has been seen, the preponderance of northerly winds in winter is very great.

In reference to precipitation the territory of the United States may be divided into two nearly equal portions by the meridian of 100°, the region to the east of that meridian being one of sufficient and pretty regularly distributed rainfall, while that to the west is irregularly and insufficiently supplied, with the exception of a narrow belt on the Pacific coast, over a part of which the precipitation is irregular, but fairly sufficient, while another portion is very abundantly supplied with moisture.

Regions of less than 20 inches of precipitation must be essentially pastoral, or, where the amount falls considerably lower, uninhabitable or even deserts. For regions where the precipitation is between 20 and 25 inches cultivation of the soil may be on the whole possible, but will be liable to serious drawbacks, since the smaller the rainfall the greater the liability to a series of years when it will fall below the mean, with partial or total failure of the crops and consequent suffering. Of course, in regions favourably situated for artificial irrigation much may be accomplished in the way of making up for deficient precipitation. If in the light of these preliminary remarks we consult Mr Schott's rainfall charts of the United States we find that the whole of the Eastern division of the country is well supplied with moisture. The isohyetal of 26 inches, which may be taken as approximately the dividing line between a sufficiently and an insufficiently watered area, crosses the northern boundary to the north-west of Lake Superior, runs south-westerly to the 97th meridian, which it strikes in about the latitude of St Paul (45°), and runs thence very nearly south, with a slight westerly inclination, so that when it reaches the northern border of Texas it has advanced westward as far as the 99th meridian, near which it remains through four degrees of latitude, to the parallel of 31°, when it again advances about four degrees to the westward, and then runs south-easterly to the Gulf of Mexico, near the mouth of the Rio Grande. As thus indicated, the isohyetal line of 26 inches leaves to the east, or in the moister region, a large part of Minnesota, the eastern edge of Nebraska, rather less than half of Kansas, most of the Indian Territory, and about half of Texas. The line of 20 inches crosses the northern boundary of the country at about the 97th meridian, and runs south with moderate undulations, gaining a little in westing, so that in the centre of Texas, on the ¿1st parallel, it is in about longitude 102°. Thence its course is south-easterly to the Gulf, in a course nearly parallel to the isohyetal of 26 inches, and at a very short distance from it. The isohyetal curve of 32 inches, or that marking the western limit of abundant precipitation, is in general pretty nearly parallel to that of 26 inches, and not far distant from it, so that in general it may be said that we pass from a region where precipitation is abundant to one where it is decidedly insufficient in traversing a belt of country having an average width in longitude of about three degrees. The only important exception is that towards the north the distance between the isohyetal lines widens rapidly, that of 32 inches having an almost easterly course along the southern shore of Lake Superior and the northern of Huron. Moreover, there is in the lines of 26 and 32 inches a marked loop running to the south-east, so that almost the whole of Minnesota is brought within the area over which the precipitation ranges between 20 and 32 inches, considerably the larger portion having over 26 inches. The position of the curve of 32 inches is such that a small part of eastern Wisconsin, a portion of eastern Michigan, and a small irregularly shaped belt in New York south of Lake Ontario lie in a region of less than that amount of rainfall.





The regions of largest precipitation arc those bordering on the Gulf of Mexico and the Atlantic. Along the Gulf the rainfall between the meridians of 85° and 92° exceeds 56 inches in amount, and the curve of 56 inches extends northward so as to embrace a portion of Arkansas, Tennessee, Georgia, and South Carolina. There is no part of the Atlantic coast, except the extreme end of Florida, where the precipitation is as large as 56 inches. At various points the average is above 50, as in eastern North Carolina, the line of 44 inches running nearly parallel to the coast, and not far from it, as far south as lat. 37°, when it bends westward]}'. The greater part of the Eastern division of the United States thus enjoys a sufficient but not over-abundant amount of precipitation, namely, that coming within the limits of 32 and 44 inches. Small areas in several of the States, however, have somewhat over 44 inches of rainfall. In the region of sufficient and in places abundant rainfall thus designated there is, on the whole, no such thing as a clearly-defined rainy season. Along the Atlantic sea-coast from Portland to Washington, through the Hudson river valley, Vermont, northern and western New York, in the Ohio valley from western Pennsylvania to Missouri, south to Arkansas and down .the Mississippi to its mouth, the rainfall is pretty uniformly distributed throughout the year. There are, however, local peculiarities in the distribution. Thus, in the Atlantic sea-coast region, as far south as Washington, there are three nearly equal maxima, about the middle of May, August, and December. In the region adjacent to the Hudson river valley through to western New York two maxima are indicated, one early in July and one about the middle of October, while there is one principal minimum, early in February. In the Ohio river valley, west to Missouri, there is one principal maximum and one principal minimum, the former early in June, the latter early in February. In the lower Mississippi valley and in that of the Red River there is one principal maximum and one principal minimum, the former early in December, the latter about the middle of October; there is also a secondary maximum in July, and a secondary minimum in June. In the Mississippi delta and along the Gulf coast eastward in Alabama and Mississippi there are two maxima, the principal one about the end of July, the secondary one early in December, while there are a principal minimum early in October and a secondary one towards the end of April. Along the upper Mississippi, in central Minnesota and part of Wisconsin, there is a decided tendency to a condition of summer precipitation and winter drought; there are two maxima, a principal one about the end of June and a. secondary one about the middle of September, and a principal minimum about the beginning of February. This is a similar condition of precipitation to that prevailing in the Hudson river valley and westward, except that in the upper Mississippi region the range is much larger. Again, on the Atlantic coast from Virginia south to Florida there is also a strongly-marked prevalence of summer rains, there being one maximum of very large range late in July or early in August, with two small adjacent minima about the middle of April and late in October. There are also subordinate maxima in March and December.

On the Pacific coast the increase in the amount of precipitation as we go northward is a very marked feature of the climate. Thus at San Diego the mean of the series from 1850 to 1874 is given at 9'31 inches; that of San Francisco, for nearly the same years, at 21 '49 ; that of Astoria at 77'61. Along the coast of California, as well as in the interior of that State in the valley and on the western slope of the Sierra Nevada, there is an almost entire absence of rain during the summer months, and a strongly marked maximum in December. Farther north, with the great increase in the total annual amount of precipitation already noted, there is also an increase in the rainfall of the summer, which amounts in the extreme north-western corner of Washington Territory to 10 or 12 inches during the three summer months. A large portion of the precipitation in the higher region of the Sierra Nevada is in the form of snow, of which the amount in different years appears to be very variable. Indeed the same thing may be said of precipitation in general on the coast of California. The largest amount of rainfall at San Francisco during the years 1851 to 1874 is given by Mr Schott as 36'02 inches, the smallest 11-73. All through the Cordilleras, from the summit of the Sierra Nevada east to the Rocky Mountains, the statistics of the precipitation are meagre, and have been very irregularly taken. The amount in general is quite small. No doubt the precipitation on the higher portions of the Cordilleran mountain ranges is considerably higher than it is in the valleys, as is indicated by the records kept by the Signal Service at the station on the summit of Pike's Peak (14,134 feet), the average for 1874-80 being 31'65 inches. In the Cordilleran region generally the fact that the precipitation is larger on the mountain ranges than it is in the valleys, and that it is chiefly in the form of snow, is a matter of great importance. When the ranges are lofty and wide enough to collect and store away a large supply of snow, this by i;s melting furnishes water enough to irrigate the slopes and valleys, so that they can be cultivated; when, on the other hand, the ridges are low, they, as well as the valleys at their bases, are absolutely sterile.

Those abnormal disturbances of the atmosphere which are accompanied by rain and wind may be classed under two heads,—ordinary storms, and those of destructive violence, or tornadoes. The former extend over wide areas, and are ordinarily attended by no evil ' results ; the latter are limited to comparatively narrow belts, and are often very destructive. The ordinary storms of the United States begin with the formation of areas of low barometer, which are first heard of in the far west or south-west, and move towards the east or north-east with a velocity averaging for the entire year, as shown by Loomis's investigation of the Signal Service Records for the years 1872-84, 28'4 miles per hour, the velocity being greatest in February and least in August, the former velocity 50 per cent, greater than the latter, and the velocity varying also very greatly for the same month in different years, the average velocity for the entire year being about two-thirds greater than it is in Europe. The direction in which these storm centres advance in the remote western stations—as, for instance, Bismarck, long. 100° 38' ; Fort Sully, long. 100° 36'; Breckenridge, long. 96° 17'—is towards a point considerably south of east, but at the more eastern stations it is a little north of east. In general, probably about half the storms of the country advance from the extreme north-west in great curved lines beginning with a south-easterly direction, and passing out of the country in a direction a little north of east, or, in general, following a track nearly parallel in position to the Great Lakes and the St Lawrence. The remainder of the storms of the Atlantic coast region begin in the south-west and travel north-east, or begin in the south and follow the coast-line pretty closely. In general the area of rainfall attendant on the advance of the centre of low barometer is in advance of the progress of that centre nearly in the direction of its average progress. The diameter of the rain area is variable, often much over 1000 miles. In the case of the great rain storms happening between the years 1873 and 1877, as investigated by Loomis, there were found to be, in many cases, quite a large number of independent rain centres prevailing simultaneously within the general rain .area. In one case there were as many as eight of these, and there were only nine cases in which there was not more than one area in which the rainfall exceeded half an inch. The average distance of the principal rain centres from the centre of low pressure was about 400 miles.

The occurrence of tornadoes in the United States is a matter of importance on account of their frequency and their destructiveness, and much has been published in regard to them. A large amount of information will be found in a publication of the Signal Service, prepared by Mr J. P. Finley, and issued in 1882. These storms are not limited to any one month or season ; but they are most frequent in summer, especially in the months of June, April, July,
and May, and least so in the months of December and January. Of 600 tabulated by Mr Finley, occurring from 1794 to 1881, 112 were in June, 97 in April, 90 in July, 81 in May, and only 9 in December and 7 in January. They are most frequent in the afternoon, between noon and six o'clock; the hour in which the greatest number occurred was that from 5 to 6 P.M. The course of more than half of the 600 (310) was from south-west to north-east, and
only 38 moved in the opposite direction. Only 46 had a course directed from the eastern side of the meridian towards the western. The width of the path of destruction varied from 40 to 10,000 feet, the average being 1085 feet. The velocity of progression of the storm-cloud, in 130 cases in which this item is given, varied from 12 to 60 miles per hour, the average being 30 miles. The time consumed by the tornado in passing any given point varied from 10 seconds to 30 minutes, the average of 50 occurrences being 6'52 minutes. The velocity of the wind within the cloud vortex was variously estimated at from 70 to 800 miles an hour. The whirling motion of the cloud was invariably from right to left. Of 600 tornadoes investigated, 134 were reported as being "unusually destructive." Of these 64 occurred within the States of Kansas, Illinois, Iowa, and Missouri, and this region, lying adjacent to the Mississippi river, seems to be that in which the conditions are most favourable to the development of these phenomena. There are also two areas—one in Georgia and one in New York—where tornadoes are more frequent than they are elsewhere in the eastern States. Of the destructiveness of these tornadoes some idea may bo formed from the statement that in many of them buildings and everything else projecting from the surface are levelled to the ground, fragments of the materials thus uptorn being carried often to great distances. In the tornado of April 18, 1880, the effects of which were felt along a path more than a hundred miles in length through Illinois and Missouri, in one town over which it passed, 65 persons were killed, over 200 wounded, and more than 200 buildings were demolished. The loss of property in two counties of Missouri was over a million dollars.

The series of destructive storms which took place on the 19th of February 1884 is probably the most remarkable occurrence of this kind which has taken place in the United States since the country was settled by the whites. The loss of property was not less than $3,000,000 to $4,000,000, while 800 persons lost their lives, and about 2500 were wounded. From 10,000 to 15,000 were rendered homeless, as many as 10,000 buildings having been destroyed. Great quantities of live stock also perished. A central area of barometric depression moved between 7 A.M. of the 18th and 7 A.M. of the 19th from Fort Keogh to the vicinity of Chicago; at the same hour on the 20th it was about 150 miles north-west of Montreal. On the 19th, at 7 A. M., another extremely elongated area of barometric depression had been formed, extending almost north and south across the whole United States, and having its centre near Davenport, Iowa. Towards this centre the winds blew from north and south, the isotherms indicating very great contrasts of temperature between the areas of northerly and southerly winds, this condition of things being an invariable precursor of tornado development. The two centres of barometric disturbance were, as is commonly the case in occurrences of this kind, widely separated. At 3 P.M. of the 19th the centre of the north and south trending barometric depression was near Indianapolis, the contrasts of temperature remaining extreme, and violent winds developing themselves at various points south of Indianapolis, especially along the Ohio river from Cairo to Louisville, in the vicinity of Nashville, and in northern Alabama. At 11 P.M. of the same day the barometric trough had diminished somewhat in intensity, and the entire area of disturbance was passing rapidly off to the north-eastward. Between 3 F.M. and sundown the area devastated was chiefly in eastern Alabama and northern Georgia. Before 11 P.M. the destructive storms in North and South Carolina had reached their maximum violence; those in southern Virginia were most destructive about midnight. The Signal Service charts for the day indicate about thirty distinct areas of violent tornadoes, most of them between the eastern border of Alabama and the southern boundary of Virginia.


Footnotes

Page 803
1 The name Cordilleran is preferred for the Western division, because thereby any confusion is avoided which might arise from the fact that the people of the eastern States are still more or less inclined to call any portion of the region lying to their west by that name. No grouping in which all the States and Territories are included can be entirely satisfactory; but in that here suggested they are, while geographically connected, in most respects pretty closely allied to each other by their physical, climatic, and agricultural characters.


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