CHAPTER XXII

ATMOSPHERIC BYWAYS

1. IGNIS FATUUS, OR WILL-O’-THE-WISP

Will-o’-the-wisp is proverbially elusive. It has thus far escaped the fate of the rainbow, deplored by Keats. We do not know its woof and texture, and it is not given in the dull catalogue of common things.

A strong argument in favor of the reality of this phenomenon is found in the great number of names that have been applied to it. There are forty or fifty in the British dialects alone. A myth generally carries its nomenclature with it, as it spreads from one community to another, while a fact of nature may give rise to a variety of local names.

It is certain, however, that a great many different phenomena have been described as will-o’-the-wisp. Some of these are: (1) The phosphorescence of decaying wood (“fox fire”) and other vegetable matter. This is due to luminous fungi. According to H. Molisch there are some forty-five species of fungi, including twenty species of bacteria, that have the property of luminosity. Sometimes the ground under a forest is illuminated on all sides with a soft, white light from decaying leaves. (2) Fireflies, including glowworms (the wingless females of the firefly and the larvæ). (3) Luminous birds and animals. Their luminosity is supposed to be due to parasitic fungi. Certain species of skunk have been described as giving off in the darkness a continuous flame, the head being fiery red, which blends into a bright blue at the tail. (4) Ball lightning. (5) St. Elmo’s fire. (6) Moving lanterns, distant lights of houses, and other lights due to human agency. (7) Burning gas ascending from marshes, stagnant pools, and the like. Marsh gas and other inflammable gases commonly rise from such places, and are often ignited by man, or by lightning, etc. Such fires are sometimes seen by day as well as by night. (8) Burning naphtha springs.

Excluding the numerous reported cases of will-o’-the-wisp in which the phenomenon may be plausibly identified with one of those mentioned above, there remain several cases, some of them reported by very careful observers, which appear to belong to a different category. The reports in question differ somewhat in details, but yield the following composite description:

Small luminous bodies, “about as large as your fist,” or “the size of a candle flame,” are seen hovering a few feet above the ground; not only over marshes and pools, but also over dry land. Sometimes they are stationary; at other times they appear to drift with the wind, or even to move independently. They appear and disappear, after the manner of fireflies. They do not set fire to objects with which they come in contact, and are believed to be without sensible heat. Their color is most often described as bluish, but may be yellow, purple, green, etc.; rarely pure white. They are without odor and without smoke. Traditionally they are associated with graveyards, but in very few of the cases heretofore recorded were they actually seen in such places. The popular idea that they flee from the traveler who tries to approach them and follow him when he seeks to avoid them is also unsupported by the evidence thus far adduced.

One of the most circumstantial accounts of these objects is that published in the Belgian journal “Ciel et Terre” for July-August, 1920, by a retired army surgeon, Jules Rossignol, who observed them repeatedly in the autumn of 1908 in and about some marshy woods near Grupont. They were generally seen to rise from the ground, at first in the shape of little white clouds, which changed to luminous globes on attaining an altitude of a dozen yards, and returned by a circular path toward the ground. They lasted from one to several minutes before disappearing in the air.

It is astonishing that the phenomenon of _ignis fatuus_, though reported from so many parts of the world, has not yet been made the subject of direct scientific examination. Nobody has ever studied its light with the spectroscope, for example. Chemists have, indeed, attempted to reproduce the phenomenon, yet the chemical explanations of it that have appeared in reference books down to a recent date are quite untenable. It has sometimes been attributed to marsh gas (methane, CH4), and sometimes to phosphureted hydrogen (phosphine, PH3). But marsh gas, besides not being spontaneously combustible, diffuses too rapidly in the air to produce the effects described, while phosphureted hydrogen, though it takes fire spontaneously in the air, produces thick wreaths of smoke when burning and has a powerful odor--features never reported in connection with will-o’-the-wisp.

At least two more plausible explanations of _ignis fatuus_ have been offered in the last few years. Mr. F. Sanford (“Scientific Monthly,” Oct., 1919) believes that it is due to “swarms of luminous bacteria which are carried up from the bottom of the marsh by rising bubbles of gas.” A Belgian chemist, M. Léon Dumas (“La Nature,” Dec. 11, 1909), claims to have produced little luminous clouds, corresponding to the traditional descriptions of will-o’-the-wisp, by combining the two gases sulphureted hydrogen and phosphine. Both these substances are produced in the decay of animal matter, especially of the brain and spinal cord. The body of an animal, buried in some wet place, would accumulate the two gases under pressure in the skull and spinal canal, and their escape, simultaneously, would fulfill the conditions of M. Dumas’ experiments.

(At the request of the present writer, these experiments were repeated at the Bureau of Standards, in Washington, with only partial success. Further trials with these and other gases due to putrefaction are desirable.)

2. THE RAIN TREE

From time to time the newspapers publish accounts of a wonderful tree, said to grow wild in Peru or elsewhere, from the leaves of which falls a continuous shower of rain, even in the driest weather. The writers generally urge the introduction of this tree in regions where the rainfall is deficient, and a so-called “rain tree” has actually been sold for this purpose by nurserymen.

The story of this tree is very old. Early voyagers reported finding it in the East Indies, Guinea, Brazil, and especially the island of Ferro, in the Canaries. Nowadays the name “rain tree” is applied especially to a magnificent tree of tropical America generally known to botanists as _Pithecolobium_ (or _Enterolobium_) _saman_. One of its common names is “guango.”

That many plants spontaneously exude moisture under suitable conditions is well known. The phenomenon is called “guttation.” The moisture drawn up from the roots is usually transpired from the leaves in the form of invisible water vapor; i. e., it is evaporated on passing into the air. If, however, the humidity of the surrounding air is sufficiently high, or its temperature sufficiently low, to check evaporation, the water will collect on the surface of the plant in liquid form, and may ultimately trickle to the ground in considerable quantities. Guttation occurs chiefly at night, or in cloudy or foggy weather. In a very dry climate it does not occur at all; and for this reason, even if the so-called rain tree could be successfully introduced in such a climate, it would not help solve the problem of irrigation.

The dripping of moisture deposited on plants by drifting fog is another common process that may have contributed to the legend of the rain tree. A classic example of this process--technically called “fog drip”--is that described by Dr. R. Marloth, who has made actual measurements of the abundant moisture captured by the vegetation of Table Mountain, South Africa, from the driving clouds of the southeast trade winds during the nearly rainless summer months. Mr. Madison Grant, writing of a similar phenomenon witnessed in the redwood forests of California, tells us that “these forests are sometimes so wet that the dripping from the high crowns is like a thin rain, and in summer it is oftentimes hard to tell whether it is raining or not, so saturated with moisture are the foliage and the trunks when the fog darkens the forest.”

A copious production of “honeydew” by plant lice, scale insects, etc., may be at the bottom of some of the rain-tree stories. F. E. Lutz, in his “Field Book of Insects,” writes of “weeping trees,” which drip fluid of insect origin, and he says of the honeydew secreted by the pear psylla (_Psylla pyricola_): “When the psyllas are numerous the leaves and fruit become coated with this sticky substance and it even drops from them like rain and runs down the trunk.”

The following account of the Peruvian rain tree, quoted from the traveler Spruce, was published in “Nature” of Feb. 28, 1878, by Prof. Thiselton Dyer:

“The Tamia-caspi, or rain tree of the eastern Peruvian Andes, is not a myth, but a fact, although not exactly in the way popular rumor has presented it. I first witnessed the phenomenon in September, 1855, when residing at Tarapoto. I had gone one morning at daybreak, with two assistants, into the adjacent wooded hills to botanize. A little after seven o’clock we came under a lowish spreading tree, from which with a perfectly clear sky over-head a smart rain was falling. A glance upward showed a multitude of cicadas, sucking the juices of the tender young branches and leaves and squirting forth slender streams of limpid fluid.”

3. DEW PONDS

The Down country of southern England is one of the few places in the world where the people go to the hilltops to seek water in dry weather. On the summits of the Downs are found many artificial shallow ponds, most of them very old. Some, indeed, date back to prehistoric times. The bottom of these ponds consists of a layer of puddled chalk or clay and is impervious to water, so that there is no loss by seepage. As the ponds are not fed by springs or surface drainage, and as lack of rain does not cause them to dry up, it is popularly believed that their maintenance depends upon dew. Hence they are called “dew ponds.”

Kipling mentions them in his poetical description of Sussex:

We have no waters to delight Our broad and brookless vales-- Only the dew pond on the height Unfed, that never fails.

The leading authority on dew ponds is Mr. Edward A. Martin, who has written a book about them. Mr. Martin’s experiments have demonstrated that dew can make no important contribution to the water supply of these ponds. The rainfall on the hilltops is somewhat higher than in the valleys, and the greater part of the water in the ponds is undoubtedly derived from this source. The real key to the mystery, however, is found in the wet fogs that drift in from the sea. The process of “fog drip,” which we have mentioned in connection with the rain tree, supplies the deficiencies of the rainfall, and the name “mist ponds,” occasionally applied to these bodies of water, is more appropriate than “dew ponds.”

It remains, however, an interesting paradox that, in time of drought, the farmers of the Downs drive their cattle to the hilltops to be watered and send their carts uphill to procure water for household use in the valleys below. Was it, perhaps, in this topsy-turvy region--where uplands are called “downs”--that Jack and Jill went “up the hill” on their ill-starred water quest?

4. BLOWING WELLS

Wells that predict weather changes are local curiosities in many parts of the world. Such wells are not uncommon in the United States. If the well is open at the top, its manifestations consist of occasional disturbance of the water and the discharge of numerous bubbles. If it is covered, a strong current of air is, at times, emitted from any small orifice in the cover. This may be strong enough to lift and blow away light objects placed over the aperture. Its emission is frequently accompanied by a loud whistling or roaring sound. Such occurrences are supposed to betoken an approaching storm. These wells are called “blowing wells”; sometimes “weather wells” or “barometer wells.”

In certain cases an indraft of air is sometimes observed; i. e., the well alternately “sucks” and “blows.”

As a rule these phenomena correspond to fluctuations in barometric pressure, and therefore are, in a rough way, indicative of changes in weather. It is obvious that a body of air inclosed in the earth and communicating by one or a few small openings with the air above will set up outdrafts and indrafts in adjusting its tension to that of the latter. The amount of air contained in the well itself would not suffice to produce the violent effects observed and it is therefore assumed that the typical blowing well taps a subterranean reservoir of air, probably filling the interstices of sand and gravel beds. When the pressure of the external air is diminished, some of the imprisoned air escapes. For a given body of inclosed air, the smaller the channel or channels by which it emerges, the stronger the outdraft. When the barometric pressure outside increases, the current of air flows in the reverse direction. In winter the indraft of cold air in such a well sometimes causes the water to freeze, even at a depth of 100 feet or more below the surface of the ground, and is therefore a source of inconvenience to the owner.

Various other circumstances may give rise to the bubbling and blowing of wells. Carbon dioxide and other gases dissolved in the well water account for the bubbling of many wells, and this process is more active at times of low barometric pressure, because at a given temperature, the amount of gas that the water can hold in solution varies with the pressure.

Another cause of the blowing of a well is, in some cases, a sudden rise in the water level (“water table”) in the surrounding ground, as after a heavy rainstorm. If the ground is overlain by an impervious stratum, the air imprisoned between this stratum and the surface of the ground water over an extensive area will escape with violence through any available channel, such as that supplied by the well.

Lastly, cases have been described in which subterranean air currents arise from the friction of rapidly flowing underground streams, setting up permanent indrafts or outdrafts through wells communicating with such streams.

5. CURIOUS SHOWERS.

Showers of blood, sulphur, manna, frogs, fishes, and what not figure in all the old chronicles, and are still frequently reported. Many occurrences of this kind are recorded in Camille Flammarion’s book “The Atmosphere,” Dr. E. E. Free’s “Movement of Soil Material by the Wind” (U. S. Bureau of Soils, Bulletin 68), and Mr. W. L. McAtee’s article “Showers of Organic Matter” in the “Monthly Weather Review” for May, 1917.

The power of the wind to whirl objects aloft is a matter of familiar observation. McAtee tells of seeing a silk hat lifted from its owner’s head and blown over a ten-story building in the city of Washington. The vortex of a tornado or a waterspout furnishes the most favorable skyward route for things that belong on _terra firma_. Objects weighing scores or even hundreds of pounds are lifted by these whirls. Within a mile or so of a tornado a shower of cart wheels or cook stoves would not necessarily constitute a “prodigy.” A chicken coop weighing 75 pounds has been carried four miles and a church spire seventeen miles. Oersted tells of a waterspout at Christiansö, on the Baltic, that emptied the harbor to such an extent that the greater part of the bottom was uncovered, while McAtee says that “waterspouts have been observed to accomplish the comparatively insignificant feat of emptying fish ponds and scattering their occupants.”

There is, in fact, no mystery about the way in which terrestrial objects of many sorts get into the air; nor, considering the force of the winds and their occasional strong vertical components, is it strange that such objects sometimes travel a long way from home before they return to earth.

There are, however, a great many cases of reported showers in which the objects did not really fall, as supposed. McAtee gives the following account of these spurious showers in the “Monthly Weather Review”:

“_Insect larvæ._--The rains of insect larvæ that have been investigated have proved to be merely the appearance in large numbers on the surface of the ground or upon snow of the larvæ of soldier beetles (_Telephorus_), or sometimes caterpillars, which have been driven from their hibernating quarters by the saturation of the soil by heavy rains or melting snow.

“_Ants._--Accounts of showers of ants have usually been founded on incursions of large numbers of winged ants, which of course needs no assistance from the elements to follow out their habit of swarming forth periodically in immense numbers.

“_Honey; sugar._--Showers of honey and of sugar are popular names for what scientists know are exudations of certain plants, or of plant lice which feed on a great variety of plants and whose product is often known also as honeydew.

“_Grains._--Showers of grain, usually considered miraculous, have in most cases been determined to be merely the accumulation by washing during heavy rains of either the seeds or root tubercles of plants of the immediate neighborhood.

“_Manna._--An account of manna ‘rains’ certainly pertains to the discussion of showers of vegetable matter, for the substance manna consists of lichens of the genus _Lecanora_ but in none of the numerous recorded instances of manna ‘rains’ is there any direct evidence that the substance really fell from the sky. These lichens form small, round bodies that are easily blown over the surface of the ground and accumulate in depressions; they are very buoyant also and hence easily drifted into masses during the run-off of rain water. Manna ‘rains’ have not occurred except in countries where these lichens are common, and as for statements of their falling down upon roofs or upon people, or for any other proofs that they really rained down, I have seen none.

“_Blood rains._--The most frequently reported showers that are spurious, at least in name, are the so-called blood rains. In all times the phenomena going under this name have frightened the people and have been taken as portents of terrific calamities. One of the famous plagues of Egypt was a bloody rain which prevailed throughout the whole land, continuing three days and three nights. Homer and Virgil both allude to blood rains, and, in fact, the general subject of preternatural rains was a favorite with the older writers.

“But scientific investigation has done away with the element of mystery in these phenomena and has explained, with the others, the rains of blood. Some blood rains have been found to be the meconial fluid ejected by large numbers of certain lepidoptera simultaneously emerging from their chrysalides; other red rains are due to the rapid multiplication in rain pools of algæ and of rotifers containing red coloring matter; “red snow” results from the presence of similar organisms. But in no case have they rained down, except in the sense that their spores or eggs have at some time been transported, probably by the wind. The precipitation of moisture furnishes favorable conditions for their rapid development and multiplication.”

Most of the reported showers of blood, however, have probably been rainstorms in which the rain was colored with reddish dust. The occurrence of such dust in the atmosphere is very common in some parts of the world, as we have stated in a previous chapter. It has been asserted that rain which fell at Oppido Mamertina, Italy, May 15, 1890, actually contained blood, believed to be from birds.

Showers of supposed “sulphur” are due to pollen, chiefly from pine trees. The air in the vicinity of pine forests is sometimes filled with clouds of this material and the wind carries it for many miles. It is reported that a pollen shower at Pictou, Nova Scotia, in June, 1841, was so heavy that bucketfuls were swept up on a ship.

In the case of alleged showers of “paper” the material has been found to be the crusts of dried algæ, which form on the surface of the ground exposed by the evaporation of the water of shallow ponds.

6. THE YEAR WITHOUT A SUMMER

The Weather Bureau is a bureau of information, and one of the ways in which it strives to give a good account of itself is by answering endless questions about “the year without a summer.” This title has been given to the year 1816.

Blodget, in his “Climatology of the United States,” tells us that all the summers from 1811 to 1817 were cold in this country, and that in every month of the summers of both 1812 and 1816 snows and frosts occurred in the Northern States. It is the latter summer, however, that has lived in popular tradition. The year 1816 is known further as “poverty year,” or “eighteen hundred and froze to death.” It acquired the name of “mackerel year” in New Hampshire, where people ate mackerel as a substitute for pork, little of which was fattened on account of the extreme scarcity of corn. Western Europe, also, had a cold summer in 1816, and the year as a whole seems to have been a cold one over a great part of the world.

Sources of information about the cold summer in this country, besides Blodget’s book above mentioned, include Perley’s “Historic Storms of New England,” which devotes a whole chapter to the subject, and Charles Peirce’s “Weather in Philadelphia.” Peirce tells us that at Philadelphia “there was ice during every month of the year, not excepting June, July, and August, There was scarcely a vegetable came to perfection north and east of the Potomac.” According to the “Monthly Weather Review,” citing the recollections of James Winchester, of Vermont: “It is said that in June of that year snow fell to the depth of three inches in New York, Pennsylvania, and New Jersey on the 17th; five inches in all the New England States, except three inches in Vermont. There was snow and ice in every month of this year. The storm of June 17 was as severe as any that ever occurred in the depth of winter; it began about noon, increasing in fury until night, by which time the roads were impassable by reason of snowdrifts; many were bewildered in the blinding storm and frozen to death.... There was a heavy snowstorm August 30th.... The year 1816 had neither spring, summer, nor autumn. The only crop of corn raised in that part of Vermont that summer was saved by keeping bonfires burning around the cornfield night and day.”

At the time of its occurrence the frigid weather of the summer of 1816 was popularly attributed to sun spots, which were big enough to be seen with the naked eye in May and June. A present-day hypothesis on the subject has been mentioned in our chapter on atmospheric dust. The dust cloud from the eruption of Tomboro, in 1815, was so vast that for three days there was darkness at a distance of 300 miles from the volcano.

A proximate cause of the cold summer is perhaps to be sought in an unusual intensity and extent of the area of low barometric pressure which is more or less permanently located in the vicinity of Iceland and, as one of the principal atmospheric “centers of action,” has a great deal to say about the weather of the countries adjacent to the North Atlantic. Dr. C. F. Brooks has called attention to the fact that the Arctic navigator, Scoresby, found unusually mild and open weather that summer in the seas east of Greenland. This would be explained by strong southerly winds, forming part of the “counter-clockwise” circulation around the Iceland low; and if the same pressure system extended its influence to our shores, persistent cold northwest winds might be expected to result over the northeastern United States.

7. INDIAN SUMMER AND THE ICE SAINTS

Indian summer _weather_ is an undeniable fact. Every inhabitant of the northern United States and southern Canada is familiar with the mild, calm, hazy state of the atmosphere that frequently occurs in the autumn, sometimes following a brief period of unseasonable cold known as “squaw winter.” It is, however, one thing to recognize the existence of a certain type of weather as characteristic of our autumns, and quite another to admit that one definite spell of such weather occurs more or less regularly from year to year. One true summer, and only one, comes to pass each year, and occupies an approximately fixed place in the calendar. Even the so-called “year without a summer,” which we have just described, was merely a year in which the regular annual rise of the temperature curve was less marked than usual. Indian summer, on the contrary, has never been tied down to a particular part of a particular month. In his notes on the meteorological conditions at Concord, Massachusetts, during the ten years, 1851-1860, Thoreau records the occurrence of Indian summer weather on dates all the way from September 27 to December 13; a range of 77 days.

The belief in the definite occurrence, year after year, of what has sometimes been called the “after-summer” is not peculiar to America. It prevails also in Europe, where this supposed period of renewed warmth has been assigned to certain dates, owing in part to its association with the names of particular saints in the calendar. These dates vary widely, however, from one region to another, ranging from August 15 (Julian calendar), the beginning of the “young women’s summer” of Russia, to November 15, St. Martin’s day, a date popularly associated with after-summer in Germany, Holland, France, Italy, and sometimes England.

The supposed tendency of particular types of weather to occur at about the same period every year, independently of and often in sharp contrast to the regular march of the seasons, has been described by R. Abercromby under the name of “recurrence,” and there is a large literature on the subject; especially in connection with periods of unseasonable temperature. While Indian summer is the most discussed example of recurrence in the American weather calendar, in the Old World more attention, both popular and scientific, has been devoted to a frosty period supposed to recur in May. With the elaboration of the ecclesiastical calendar, the frosts in question became definitely associated with the days dedicated to Saints Mamertus, Pancras, and Servatius (May 11, 12, 13), or, in south-central Europe, Saints Pancras, Servatius, and Boniface (May 12, 13, 14), hence known as the Ice Saints. These saints and their days are called in French _saints de glace_, and in German _Eisheiligen_, _Eismänner_, or _gestrenge Herren_.

Yet other examples of the elusive phenomenon of recurrence are the “January thaw” of New England, the April “blackthorn winter” of England, and the June “sheep-cold” (_Schafkälte_) of Germany, dangerous to newly shorn sheep.

In the middle of the last century the cold weather of the Ice Saints was variously ascribed to the melting of the ice and snow of high latitudes, the passage of periodic meteor showers between the earth and the sun, and other far-reaching terrestrial or cosmical causes. FitzRoy believed that the liberation of latent heat in autumn during the formation of ice in the circumpolar regions was accountable for Indian summer. A review of the whole body of literature concerning supposed recurrent irregularities in the annual march of temperature will be found in the “Monthly Weather Review” (Washington) for August, 1919.

Whether recurrence, in Abercromby’s sense of the term, is a real phenomenon is still an unsettled question. Many periods of unseasonable weather occur in the course of each year, and it is easy for the uncritical observer to identify one of them with the Ice Saints, another with Indian summer, and so on. About the best that meteorologists can do at present is to explain each particular instance of such weather by reference to barometric and other conditions shown on the daily weather map.

GLOSSARY

The vast vocabulary of meteorology is very inadequately represented in ordinary dictionaries, and has never been made the subject of a comprehensive special glossary. The writer of this book has been gathering material toward such a glossary for some years, and from the material now in hand it appears that an approximately complete English meteorological dictionary, embracing both scientific and nonscientific terms relating to the atmosphere and its phenomena, would contain upward of fifteen thousand definitions.

From this statement it will be evident that the brief glossary herewith appended is of the most fragmentary character. It includes only a selection of the _meteorological_ terms found in the present book. It does not, in general, include terms pertaining primarily to physics, chemistry, astronomy, physiology, etc., even though they figure to some extent in meteorology, as all such terms used in the book are more or less satisfactorily defined in the latest editions of the large American dictionaries.

_Absolute Extremes._--The highest and lowest values of a meteorological element (especially temperature) that have ever been recorded at a station; known, respectively, as the _absolute maximum_ and the _absolute minimum_. (The term is sometimes improperly applied to the highest and lowest values for a specified year.)

_Aeroclinoscope._--A semaphore formerly used in Holland for displaying weather signals.

_Aerology._--The branch of meteorology dealing with the “free” atmosphere; i. e., all parts of the atmosphere not near the earth’s surface. Aerological investigations are made with kites and balloons, and also include observations of clouds, meteor trails, the aurora, etc.

_Afterglow._--1. The glow in the western sky after sunset. 2. A renewal of rosy light on mountain peaks after the first sunset illumination has faded; also called _recoloration_. This is one stage of the _Alpenglow_.

_After-summer._--A renewal of mild weather in the autumn; called Indian summer in America, St. Martin’s summer, etc., in Europe.

_Alpenglow._--Successive appearances and disappearances of rosy light sometimes seen on mountain peaks in clear weather after sunset or before sunrise.

_Altimeter._--A barometer used for measuring altitude.

_Alto-cumulus_; _Alto-stratus_.--Forms of cloud. (See Chapter VI.)

_Anemogram._--The record traced by a self-registering anemometer.

_Anemometer._--An instrument for measuring the force or speed of the wind.

_Aneroid Barometer._--A barometer consisting of a thin-walled metal vacuum-box, which changes its shape with changes of atmospheric pressure. The movements of the box are communicated, by levers, to an index or (in the barograph) to a recording pen.

_Anthelion._--A rare species of halo, consisting of a brilliant, usually white image of the sun opposite the latter in azimuth. (This term has also been applied to the _glory_, q. v.)

_Anticrepuscular Rays._--The continuation of the crepuscular rays converging toward a point in the sky opposite to the sun.

_Anticyclone._--An area of high barometric pressure and its attendant system of winds. (Cf. _cyclone_.)

_Antitrades._--Term formerly applied to the prevailing westerly winds of middle latitudes, but now more frequently applied to the westerly return-currents lying over the trade winds. Some writers prefer to call the former the _antitrades_ and the latter the _countertrades_.

_Antitwilight Arch._--The pink or purplish zone of illumination bordering the shadow of the earth (_dark segment_) in the part of the sky opposite the sun after sunset and before sunrise.

_Arcs of Lowitz._--A pair of rare halo phenomena. These arcs are directed obliquely downward from the parhelia of 22 degrees on either side of the sun toward the halo of 22 degrees.

_Astraphobia._--A pathological condition experienced by certain persons before and during thunderstorms.

_Atmometer._--An instrument for measuring evaporation; also called _atmidometer_, _evaporimeter_, etc.

_Aureole._--(See _corona_. 1).

_Aurora._--A luminous phenomenon due to electrical discharges in the atmosphere; probably confined to the tenuous air of high altitudes. It is most commonly seen in sub-Arctic and sub-Antarctic latitudes. Called _aurora borealis_ or _aurora australis_, according to the hemisphere in which it occurs. Observations with the spectroscope seem to indicate that a faint “permanent aurora” is a normal feature of the sky in all parts of the world.

“_Backstays of the Sun._”--A sailor’s name for crepuscular rays extending downward from the sun.

_Baguio._--The name current in the Philippines for a tropical cyclone.

_Ballistic Wind._--A military term applied to a fictitious wind which, if affecting a projectile throughout its flight, would produce the same total effect in deflecting it from its course and altering its range as do the various winds that it actually encounters.

_Ballon-sonde._--A sounding-balloon.

_Bar._--A unit of pressure equal to 1,000,000 dynes per square centimeter. A bar = 100 _centibars_ = 1,000 _millibars_. A barometric pressure of one bar is sometimes called a “C. G. S. atmosphere,” and is equivalent to a pressure of 29.531 inches of mercury at 32 degree F. and in latitude 45 degrees.

_Barisal Gun._--Same as _brontide_.

_Barocyclonometer._--One of several instruments that have been devised for locating tropical hurricanes without the aid of a weather map.

_Barograph._--A self-registering barometer.

_Barometer._--An instrument for measuring the pressure of the atmosphere. The two principal types are the _mercurial_ and the _aneroid_. The _microbarometer_ is used to show minute changes of pressure. Certain forms of hygroscope are popularly miscalled “barometers.”

_Barometer Well._--Same as _blowing well_.

_Barometric Tendency._--The change of barometric pressure within a specified time (usually three hours) before one of the regular observations.

_Beaufort Scale._--A scale of wind force, originally devised for use at sea, but now used also on land. The scale runs from 0 = calm to = hurricane. Many other scales are similarly employed in the noninstrumental observation of wind force.

_Bioclimatic Law._--A phenological law, announced by Dr. A. D. Hopkins, according to which periodical events of plant and animal life advance over the United States at the rate of 1 degree of latitude, 5 degrees of longitude, and 400 feet of altitude every four days--northward, eastward, and up-ward in spring, and southward, westward, and downward in autumn.

_Bishop’s Ring._--A large corona due to fine dust in the atmosphere. It has been seen after certain great volcanic eruptions, especially that of Krakatoa, in 1883.

_Blizzard._--A violent, intensely cold wind, laden with snow.

_Blowing Well._--A well which emits a strong current of air from any small opening in its cover during a fall of barometric pressure. During a rise of barometric pressure such wells are sometimes observed to “suck.” Wells that are thus responsive to barometric changes are sometimes called “barometer wells” or “weather wells.”

_Bora._--A cold wind of the northern Adriatic, blowing down from the high plateaus to the northward. Also, a similar wind on the northeastern coast of the Black Sea.

_Brave West Winds._--The boisterous westerly winds blowing over the ocean between latitudes 40 and 50 degrees S. This region is known as the “roaring forties.”

_Bright Segment._--The broad band of golden light that, in clear weather, borders the western horizon just after sunset and the eastern just before sunrise.

_Brontide._--A sound resembling a distant muffled detonation, usually indefinite as to direction. Brontides are rather common in certain parts of the world. They are called _mistpoeffers_ on the Belgian coast, _Barisal guns_ in the Ganges delta, _bulldag_, _desert sounds_, or _Hanley’s guns_ in parts of Australia, _gouffre_ in Haiti, _Moodus noises_ at Moodus, Connecticut, _Nebelzerteiler_, _Seedonner_, _Seeschiessen_, etc., in Germany, _baturlio_, _boniti_, _bombiti_, etc., in Italy. These sounds are probably of subterranean origin in most cases.

_Bump._--An upward jolt experienced by an aviator, as if running over an obstruction. A bump may be caused by any condition that suddenly increases the lift of the machine, but is perhaps most frequently due to rising air currents. Air in which bumps are experienced is said to be “bumpy.” (Cf. _hole in the air_.)

_Callina._--A Spanish name for dry fog.

_Calms of Cancer_; _Calms of Capricorn_.--The belts of high pressure lying north of the northeast trade winds and south of the southeast trade winds, respectively.

_Center of Action._--Any one of several large areas of high and low barometric pressure, changing little in location, and persisting through a season or through the whole year; e. g., the Iceland low, the Siberian winter high, etc. Changes in the intensity and positions of these pressure systems are associated with widespread weather changes.

_Ceraunograph._--A self-registering thunderstorm recorder.

_Chinook_, or _Chinook Wind_.--A foehn blowing down the eastern slopes of the Rocky Mountains over the adjacent plains, in the United States and Canada. In winter, this warm, dry wind causes snow to disappear with remarkable rapidity, and hence it has been nicknamed the “snow-eater.” (Cf. _foehn_.) The “wet chinook” is a wind of a different character, blowing from the Pacific Ocean over the northwestern United States.

_Circumscribed Halo._--A halo formed by the junction of the upper and lower tangent arcs of the halo of 22 degrees, when the luminary is about 40 degrees or more above the horizon. As the altitude of the luminary increases, the circumscribed halo gradually assumes an elliptical form and finally merges into the halo of 22 degrees.

_Circumzenithal Arc._--A rainbow-tinted halo, often very bright, convex to the luminary and 46 degrees or a little more above it. It is sometimes called the _upper quasi-tangent arc of the halo of 46 degrees_, but the circumzenithal arc and the halo of 46 degrees are rarely seen at the same time.

_Cirro-cumulus_; _Cirro-stratus_; _Cirrus_.--Forms of cloud. (See