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FIG. 2.-Seasonal rainfall at San Francisco, Cal. Intensity or amount in inches. (Normal, 23 inches.)
FIG. 3.-Seasonal rainfall at San Francisco, Cal. Frequency, or number of days. (Average, 71 days.)
A study of the charts in the Monthly Weather Review makes plain the positive character of the relationship between pressure distribution and the amount of rainfall. It will probably be found upon investigation that the frequency of rainfall as well as the intensity bear a direct relation to the pressure distribution as described above. In addition to the charts of the Monthly Weather Review excellent material for a further study of these relationships is to be found in the various international bulletins and synoptic charts of the various meteorological services.
The path of storms will be determined by the position of the great permanent areas. When the North Pacific low extends well to the southward in winter the storm tracks are well to the southward. And conversely if far to the north, the mean paths will also be far north.
MOVEMENT OF AREAS OF HIGH PRESSURE.
By referring to the charts published in the Monthly Weather Review it will be seen that, especially during winter months, areas of high pressure frequently lag in their eastern march over the country between the Sierra and Rocky Mountains. Such conditions are alluded to elsewhere as being generally accompanied by tule fog in the great valleys with temperatures ranging from 30° to 40° at sea level and from 40° to 50° at an elevation of about 1,000 feet. Warm weather with high northerly winds is apt to prevail in southern California during the early part of the winter, while, strangely enough, under somewhat similar conditions of pressure distribution during January and February, cool nights with frost in the morning may be expected in southern California.
During the prevalence of a slow moving area of high pressure it has been found that depressions of moderate depth sometimes develop on what may be called the periphery of the high to the south and southwest. During the months of November, December, and January under such conditions rain will begin falling without much warning along the coast from Point Conception southward. The forecast official can from the very nature of things give but little advance warning of such disturbances.
MOVEMENT OF AREAS OF LOW PRESSURE.
STORMS OF THE WEST PACIFIC OCEAN.
A discussion of these storms falls more properly under the province of the Hydrographic Office; but so interesting are the few facts which are fairly well established that brief mention must be made of them. The typhoons of the West Pacific have been studied with much care and skill by the staff of the observatory at Manila, and particular mention must be made of the work of the director of the observatory, P. José Algué, S. J., and P. Juan Doyle, S. J., subdirector of the observatory. Algue's "Baguios ó Ciclones Filipinos" and Doyle's "Tifones del Archipiélago Filipino y Mares Circunvecinos" are the best contributions to the subject.
Excellent directions for mariners and much information of a practical character is to be found in The Law of Storms in the Eastern Seas, by W. Doberck, director of the observatory at Hongkong. The observatories at Tokyo and Shanghai have also collected much material relating to storm development and motion on the Asiatic coast. Reference should also be made to the work of Dr. Paul Bergholz, director of the meteorological observatory at Bremen, who has given special attention to the storms of the East Indies. A translation of an address by this author on "The origin, paths, and limiting zones of the typhoons of the Orient," by Professor Abbe, was published in the Monthly Weather Review for September, 1899, and an abstract of this follows:
TYPHOONS OF THE PACIFIC OCEAN.
The following table shows at a glance a classification of the hurricanes of the Pacific Ocean:
Typhoons of the Pacific Ocean.
Trend of first
Northwest by north.
Northeast by east.
aNamely, when passing the small circle of latitude for Manila.
In general, the paths of these hurricanes are all parabolic. The average trend of the paths or the inclination to the meridian is much larger when they pass the latitude of Manila (14° 35′ north) than the average trend in the first branch. This is particularly noticeable during the months of the first group, and this evidently results from the fact that the latitude of the intersection [vertex?] of the path agrees very nearly with the latitude of Manila.
If now, with the assistance of the charts of isobars, we determine the conditions under which the cyclones are formed in the different months and groups of months, we find:
(a) The paths of the hurricanes of the Pacific Ocean in the first group start from the region between two areas of high pressure, one of which lies over the continent, the other over the Pacific Ocean. They lead toward the center of low pressure that occupies a portion of Bering Sea. The hurricanes of the China Sea keep within lower latitudes, namely, those which are reached by the limiting isobars of the Asiatic center of high pressure. In proportion as the centers of high pressure flatten out and withdraw during the period from January to March, so these extreme isobars retreat toward the north, and consequently the paths of the hurricanes extend farther north.
(b) With reference to the second group, the charts of monthly isobars show that the hurricanes of the Pacific Ocean in April and May move between the extreme isobars of the high-pressure areas of the Pacific Ocean and Asia.
(c) The paths of the hurricanes of the China Sea keep south of the isobar 760 mm., belonging to the highpressure area of Asia and the low-pressure area of Hindostan. In October and November, in proportion as the Asiatic high-pressure area develops, these are pushed more and more into lower latitudes; moreover, the development of the area of low pressure in Hindostan is an index to these paths.
(d) The hurricanes of the Pacific Ocean, especially in October, pass along the broad zone between the Philippines and Japan, on the one hand, and the isobar of 760 mm. surrounding the high-pressure area of the Pacific. In November this zone becomes narrower by reason of the further development of the continental area of high pressure. The hurricanes of the Pacific Ocean belonging to this group also pursue paths toward the depression in the extreme north, which bears north-northeast from Manila.
(e) It is characteristic for the months of the third group that from June to September, at least to the middle of the latter month, the center of high pressure withdraws from the coast of Asia, and finally disappears. In connection with this the paths of the hurricanes attain higher latitudes, and those of the Pacific Ocean recurve very near the meridian of 125° east, therefore nearer to the Philippines than in the previous months. A single exception offers in the case of the hurricanes of the second half of September, whose recurving points are from 5° to 8° farther east. All hurricanes in the Pacific Ocean have as their ultimate destination the northern center of low pressure. The paths of the hurricanes of the China Sea trend more toward the north in proportion as the high pressure area of the continent moves northward, and do this, therefore, up to the end of August and the beginning of September; if, however, the low-pressure area moves toward the south about the beginning of September, then also the paths of these cyclones must follow it. Some of the July hurricanes after recurving follow paths going very nearly northward; they cross over the Yellow Sea and travel toward a small center of low pressure that has developed in Siberia.
The reader who is interested in the movements of the storms of the Asiatic coast may read with profit Bulletin H, Weather Bureau, 1900, on West Indian Hurricanes, by Prof. E. B. Garriott, since the typhoon on the Pacific Ocean and the hurricane of the West Indies are storms of the same general character and have much in common.
During the years 1895, 1896, and 1897 the Weather Bureau office at San Francisco made an attempt to trace the path of a disturbance across the ocean. By means of a large number of ship reports obtained through the courtesy of the Hydrographic Office the approximate path of a disturbance was determined. On the accompanying chart there is shown the path of a disturbance covering a period extending from November 20, 1895, to January 14, 1896. This path was determined by Prof. W. H. Hammon.
It may also be of interest to present a section of the same disturbance in its passage just preceding and following the point of recurvature.
To illustrate the difficulties of forecasting on the Pacific coast owing to the absence of reliable data over the ocean, the accompanying charts showing a forecast official's map with the conditions existing on the morning of January 13, 1896, and the real condition as subsequently determined by platting the observations of ships traversing the North Pacific. It is interesting to notice that a forecast of "rain" for the ensuing thirty-six hours might with the fuller knowledge obtained later have been changed to a long-range forecast covering a period of many days. As a matter of fact, there was no rain reported at San Francisco during the first twelve days of the month of January, 1896; but rain fell on every day from the 13th to the 21st and the total rainfall for this month was 8.14 inches, or nearly double the normal January rainfall.
LOW AREAS ON THE PACIFIC COAST.
Professor C. Abbe, in Monthly Weather Review, November, 1896, states that
The daily chart for the northern hemisphere accompanying the bulletin of international simultaneous meteorological observations, 1875-1887, has long since familiarized the student with the fact that areas of low pressure frequently pursue very long paths for many consecutive days in their circuit around the north temperate regions. Those that start in the equatorial portions of the Atlantic or Pacific, after passing northwest and curving to the northeast, finally move east-northeastward between the forty-fifth and sixty-fifth parallels. Others start in the temperate regions, and without moving to the westward or recurving, pursue nearly the whole path in an east-northeast direction. In describing the history of areas of low pressure the authors who have contributed chapters to the successive Monthly Weather Reviews for nearly twenty-five years past have usually kept in mind the fact that "lows" which first appear in Washington, Oregon, Montana, and Alberta, or British Columbia, have probably originated at some point far to the west, and if occasionally the description of such a storm begins by speaking of it as originating over our North Pacific slope region, this is a slip of the pen which the reader may generally interpret without being misled by it.
On several occasions I have pointed out the fact that the isobars, and therefore the winds at a considerable distance above the earth's surface, have very little resemblance to the isobars and winds at sea level. In fact, the normal isobars at an elevation of 5,000 meters (which represents a surface a little above the summits of the Rocky Mountains) present a grand oval depression whose longest axis extends from the United States toward the northnorthwest over the Saskatchewan and the Arctic regions to eastern Siberia. By studying a polar projection of the northern hemisphere we perceive that the whole upper circulation of winds and clouds and the general movement of areas of low pressure and high pressure are related to this distribution of pressure in the upper layers of air. When a storm center moves from Japan to the North Pacific, or from the latter to our Pacific coast, or from Alberta and Oregon southeastward, or from Texas and Kansas northeastward, it is describing some portion of a circuit about this great upper region of low pressure. It is simply a special whirl gliding about in the maelstrom that occupies one-half of the northern hemisphere. The axis of this oval polar maelstrom probably changes its position with considerable regularity, oscillating slowly to and fro; therefore the paths which the smaller disturbances describe will vary simultaneously with that; sometimes the storms will move far to the south either in America or in Russia in order to circumnavigate the southern extension of the longer axis of the oval; but will thereby diminish in intensity and almost die out. Sometimes a new whirl will start at the southern end of the oval; sometimes all the paths of the low areas will lie on the northern border of the United States and Canadian weather charts because the polar maelstrom has altered its dimensions and locations. Even the great subpermanent areas of low pressure in the North Atlantic and North Pacific are subordinate to the greater area of low pressure at the upper level and its attendant winds.
These remarks were made in discussing the following notes by Professors McAdie, Henry, and Hammon bearing upon the origin and paths of areas of low pressure on the Pacific coast:
A little study of weather types on the Pacific slope makes it plain that certain conditions traverse the country from the Pacific; thus to take at random the month of January, 1895, some of the deep lows that might have been supposed to originate over Manitoba or farther west over Assiniboia, Alberta, and British Columbia did not so originate, but are storms that can be traced distinctly in their onward march from the northern-central Pacific Ocean northwestward, often recurving and doubling in their paths, but preserving identity. Passing south of Sitka they