Popular Science Monthly/Volume 86/February 1915/Popular Misconceptions Concerning the Weather
| POPULAR MISCONCEPTIONS CONCERNING THE WEATHER |
By ANDREW H. PALMER
UNITED STATES WEATHER BUREAU
THE weather is perhaps the most widely discussed of all topics of conversation. It is not unnatural that it should be of such general interest, since every man living upon the surface of the earth is influenced by this feature of his environment. Moreover, atmospheric air is itself one of the elements necessary to sustain human life. So commonplace a subject as the weather, therefore, needs no definition. Ever since man first appeared upon the earth the weather has been an ever-present influence—its changes have affected his actions as well as his very mode of life. It is only during the past century, however, that any real progress has been made in a scientific knowledge of the weather, the influences to which it is subject, and the effects resulting therefrom. Only a beginning has thus far been made in that direction. Meteorology, the science of weather, and climatology, the science of climate, have progressed slowly, and for this reason various misconceptions and superstitions concerning weather and climate have persisted even to the present time. While much is still to be learned about the atmosphere it is already possible to disprove many of these false notions. It is the purpose of this paper to enumerate and briefly to discuss twenty-five of the more common of these misconceptions. No new facts will be presented—the aim simply being to make clear the fallacies underlying these misconceptions in terms of principles generally accepted by meteorologists and climatologists.
The supposed influence of the moon, the planets or the stars is probably the most widespread of all popular misconceptions about the weather. Manifestations of these fallacies are seen in a great variety of ways, including long-range forecasting, the planting and the harvesting of crops, and various events in the husbandry of cattle during periods determined by phases of the moon, etc., all of these being examples of a belief in the relation of heavenly bodies and human affairs. The textbooks in geography still used in many of the common schools frequently combine a brief discussion of astronomy and meteorology in the introductory chapter, thus laying the foundation for considerable confusion in the minds of the children. Moreover, the ancient science of astrology still has a few disciples among the uninformed, as far as the weather is concerned. Meteorologists, however, are now unanimous in the opinion that the influence of the moon, the planets and the stars (not including the sun) is practically nil, when terrestrial weather is considered. It should be remembered, in this connection, that heat is the fundamental force determining weather—the form of energy outweighing all others combined. When it is stated that the sum total of all the heat energy received from all heavenly bodies (not including the sun) is so slight that one of the most delicate of instruments is required for its measurement, it is apparent that their influence upon our weather is negligible. The moon, about which most misconceptions of this character center, is without doubt the direct cause of ocean and atmospheric tides, and there are places along certain coasts where ocean tides produce periodic tidal breezes. Aside from the indirect effects here enumerated astronomical influence upon weather is practically of no consequence. The untruth of the proverb which states that the moon tends to drive away the clouds is explained partly by the fact that a clearing of the sky at night is not ordinarily observed unless the moon is above the horizon, and partly by the fact that after sunset there is a cessation of the ascending currents which result in the formation of clouds of the cumulus type, the clouds already formed soon dissipating.
Contrary to a fairly general impression, there is no apparent relation between earthquakes and the weather. Scriptural allusions to destruction of life and property often associate earthquakes and violent storms as though they were of common origin, and the idea has persisted, to some extent, even in modern writings. In general, it may be said that earthquakes are caused by forces at work within the earth, or at least beneath its surface, such as the slipping of the crust along a fault plane, or the movement of molten matter or steam beneath the hard crust. On the other hand, weather changes result from the effects of forces at work within the atmosphere itself, primarily as a product of energy coming through space from the sun. Various investigators have attempted to discover a relation between barometric pressure of the atmosphere, earth tides and local disturbances of the crust. Aside from this possible indirect relationship there is no Imown coordination of earthquakes and the weather.
Nor is there any marked relation between magnetic phenomena and the weather. Magnetic storms, or disturbances in the magnetic state of the earth, frequently occur without any apparent effect upon the weather. That there is a relation between magnetic phenomena in the earth, auroras, and solar disturbances, particularly sunspots, there can no longer be any doubt. The aurora borealis, seen in northern latitudes, and the aurora australis, seen in southern latitudes, are believed to be caused by electrical discharges in the rarefied strata of the earth's upper atmosphere. Aside from the visible manifestations of such discharges, observers have sometimes noticed sounds, and, upon rare occasions, odors which were thought to have resulted therefrom. However, the aurora has not yet been satisfactorily explained. With the exception of the aurora, there is no known relation between terrestrial magnetism and atmospheric phenomena.
The question as to whether or not forests affect weather and climate has been much debated. Recent investigations have brought out the following facts: Whatever influence forests have upon meteorological conditions is purely local, and even that influence is not marked. In one case it was found that the mean annual temperature within a forest was only a few tenths of a degree cooler than at a point a half mile or a mile outside the forest border, the greatest difference amounting to 2° F. The relative humidity was at times 7 per cent. greater within the forest than in the open country. In the United States the wholesale destruction of forests, which has been going on since colonial times, has not been accompanied by any marked increase or decrease in rainfall. On the other hand, the reforestation of large tracts in central Europe and in northern Africa during the past century has not resulted in an appreciable effect upon the precipitation observed during that period. Forests are the effect rather than the cause. There is still considerable confusion in the public mind concerning rainfall and flowoff, when the supposed influence of forests is considered. Deforestation has undoubtedly increased the frequency and the intensity of floods in small constricted districts, notably in certain mountain valleys, but where the removal of the forest cover over large areas has been followed by cultivation of the soil the rate of flowoff has remained unchanged. From hydrographs of the principal rivers of the United States it is apparent that high waters are neither higher nor low waters lower than they were fifty years ago, and they are neither more frequent nor of longer duration now than they were then. Notable floods like that of Paris, France, during the spring of 1910, and that of the Ohio Valley in the spring of 1913, are the result of a number of causes, in which the excessive rainfall was in no way related to the presence or absence of forests, and in which the rapid flowoff was more dependent upon the frozen soil than upon the recent removal of the forest cover. That the flowoff is more rapid when the ground is frozen explains the greater frequency of floods during spring than during any other season of the year. Moreover, forests tend to preserve the snows of winter, as well as to retain the fertile elements of the soil from washing away. While forests are thus of importance to the agriculturalist and the engineer, they are of little concern to the student of the weather.
The deep-seated notion, held by many individuals, that the climate is changing is often referred to in expressions like "old-fashioned winter," "the storms we used to have," and "the deep snows when I was a boy," etc. Subjective phenomena like these are of interest to the psychologist, and it remains for the meteorologist simply to prove that the notions have no basis in fact. When one plots the seasonal or the annual temperatures or snowfalls, or any other elements of climate, using reliable records as far back as they are available, it is apparent that the curves show no appreciable change of climate within the life of any man now living. The explanation for fallacies of this nature must be given in terms of psychology. Present winters do not seem to be as severe as "old-fashioned winters" because of better housing and heating conditions, more efficient clothing, improved methods of transportation, with multiplied comforts and conveniences. The retired farmer living in a steam-heated city apartment building in which there are double windows is apt to exaggerate the severity of past winters when he may possibly have seen the snow drift through cracks in a log house. Moreover, a snowfall of three feet looks considerably deeper to a boy four feet tall than it does to him when he becomes a man six feet in height.
There is no known relation between the weather of one season or year with that of the following season or year, various opinions to the contrary, notwithstanding. The records of the Weather Bureau do not show that a relatively dry spring is followed by an unusually hot summer, or that an abnormally cool autumn is followed by a severely cold winter. Neither can it be shown that cold years or warm years occur in groups of two or three, as is sometimes maintained. While well-marked cycles are recognized in various solar disturbances, particularly sunspots, there is no similar cycle apparent in the weather of seasons or of years. If there are cycles in the weather they must be measured in terms of tenths of units, and they are therefore of no practical importance.
Neither is there any indisputable connection between the weather of one day and that of subsequent weeks or of seasons. Tradition has it that the presence or absence of sunshine on Groundhog Day, February 2, determines whether or not winter conditions shall continue during the following six weeks; that a showery Easter Sunday is followed by seven showery Sundays; and that a rainy St. Swithin's Day, July 15, portends forty consecutive days of rainfall. No basis can be found for these traditions in available records. True it is that springlike conditions come considerably earlier some years than during other years, but such conditions are not related to the weather of February 2. Moreover, spring and summer are the seasons of greatest and most frequent rainfall over the central portion of the United States, but the frequency of rain is not related to the conditions prevailing on Easter Sunday or on July 15.
In the use of the terms cyclone and tornado there is considerable confusion, and the terms are used indiscriminately. As used by the Weather Bureau the term cyclone refers to an area of low barometric pressure with winds blowing counter-clockwise and spirally inward toward its center, or point of lowest pressure. Marked "Low" on the weather map, the cyclone is variously called storm, depression, or disturbance. Cyclones vary greatly in size, some being as large as the whole Mississippi Valley, while others are no larger than New England. In the United States they usually move from west to east at an average rate of 300 miles a day, the rate being faster in winter than in summer. In general, the wind velocity varies directly as the barometric gradient, that is, the rate of change of barometer as measured outward from the center. Cyclones are regions of clouded sky, with more or less precipitation, and as they pass alternately with the "Highs" in endless procession across the northern and central portions of the country they produce the frequent weather changes which are characteristic of these regions. In winter, when they are most common and follow the more southerly routes, they bring warm weather at the front and cold weather at the rear. A tornado, on the other hand, is a violent local storm of the thunderstorm type, with whirling and ascending winds of extremely high velocity, causing destructive effects over paths varying in width from a few feet to a few miles. They occur during the summer half-year only, and usually during the hottest part of the day. Not only are they always associated with thunderstorms, but they may be considered overdeveloped storms of that class. While cyclones and tornadoes thus have many common characteristics, custom has identified the use of the terms with certain meanings. Cyclone, as a general term, refers to any whirling mass of air, while tornado, the special term, refers to a particular kind of whirl. However, as used by the Weather Bureau their application is that described above.
The frequent expression in winter that "another storm is brewing at Medicine Hat" seems to be based upon a false association of that station with the origin of our weather. Charts of the weather of the whole northern hemisphere, now made daily at the central office of the Weather Bureau at Washington, show that the cyclones and the anticyclones which determine our weather move from west to east in endless procession. Some of the individual areas may be followed throughout the entire circuit around the earth, while others can be traced for only short distances. Neither Medicine Hat nor any other single station serves as a starting point. However, well-defined storm tracks are now recognized. Certain stations in the Canadian northwest are closely watched for indications of an oncoming storm, which, if it follows the usual route, will enter the northwestern states one to three days later, subsequently passing eastward and finally passing off the Atlantic coast. Because of their positions on the storm tracks, and not because of any center of storm formation, should stations like Medicine Hat be of meteorological interest.
What is popularly known as the equinoctial storm is supposed to occur about the time of the autumnal equinox, September 21, when the sun crosses the celestial equator to the southern hemisphere. East of the Rocky Mountains rain occurs on an average about once in three or four days, while in the North Pacific states it occurs once in every two or three days, taking the year as a whole. Throughout these large areas the latter part of September is a transition period, with autumn conditions replacing those of summer, and occasionally with the first occurrence of a storm of the winter type. The latter are usually characterized by relatively high winds, rain on two or three successive days, and followed by a considerable fall in temperature. Bearing in mind the average frequency of rainy days and of winter storms, it is apparent that it would be abnormal should no rain occur during the week preceding or the week following September 21. The so-called equinoctial storm is a fiction.
Indian Summer is another popular superstition. Characterized by high temperatures, light winds and calms, and a hazy or smoky atmosphere, it is generally supposed to be a particularly pleasant period of indefinite length occurring in October or November. That there is frequently a return of summer-like conditions during the late autumn can not be denied. But to affirm that Indian Summer is a period of several weeks in duration, recurring each autumn, and easily recognized by the occurrence of heat, calms and haze, can not be proved by climatological records. It is a peculiar fact that while the recurrence of summerlike conditions in autumn has given rise to this tradition, and even the name as a season, the similarly frequent recurrence of winterlike conditions in spring has not been popularly recognized. Summerlike periods in autumn and winterlike periods in spring can in every individual case be explained by the weather map in terms of barometric distribution, paths of storms, resulting winds and calms, the height of the sun, the length of days, and the unequal distribution of heat over the continent and the bordering oceans.
Another false notion, particularly common in rural districts, is the belief that various animals, through some particular dispensation of Nature, have a previous knowledge of coming weather changes. As a result, many proverbs have arisen, based upon observations of the behavior of animals. For example, it is sometimes stated that a cold winter is portended when the musk-rat or the beaver builds the walls of his home thicker than usual, or when the squirrels or the non-migratory birds hide large quantities of food during the autumn. Again, the remark is often made that a storm is imminent when the chickens go to roost early or when the house-cat seeks a warm place beside the fire. Even the human feeling of comfort occasionally gives rise to presentiment. Persons afflicted with recurrent rheumatism claim to feel the approach of a storm long before it appears, and people of nervous temperament often affirm that they have forebodings of coming thunderstorms or of rainy spells through a temporary disturbance of their neural equilibrium. Physiologically considered, either from the point of view of man or of the lower animals, these fore-warnings, often verified, have some basis for their existence. The secret of the explanation probably lies in the fact that all weather changes occur in cycles—that is, a more or less constant order of events accompanying every change. With the summer thunderstorm this cycle usually consists of the following: rising temperature and humidity, pressure oscillations, decreasing winds, increasing potential of atmospheric electricity, thickening clouds and consequent growing darkness, distant lightning, rumbling thunder, the lightning growing more vivid and the thunder louder and louder as the storm approaches, a squall of wind coming from the direction of the storm itself, accompanied by a marked fall in temperature, inconstant humidity, large drops of rain, followed by a downpour, often accompanied by hail. Another cycle, covering a considerably longer period of time, is recognized as a precedent of the rains of a barometric depression. Men differ greatly from the lower animals in their sensitiveness to these various stages, and even different individuals, whether in the higher or in the lower orders of animal life, show wide divergence in this respect. As a result, sensitive persons and certain animals feel a coming change because for them the change has already begun, they feel the rising humidity or the changing pressure before others, and they are, in fact, simply changes usually precedent to the larger changes observed by all. However, one or more of these changes may occur without reference to the various other changes, thus explaining why the premonitions are sometimes amiss. But cycles of this nature occur so frequently that traditions of a fair degree of reliability have arisen. It might be added that most of the reliable proverbs based upon the behavior of animals are ultimately concerned with changes of humidity. To such changes certain animals appear to be super-sensitive, while most men are phlegmatic in this respect.
That rain has resulted from the concussions attending the old-fashioned celebration of Independence Day (July 4) or during great battles, particularly those of the Civil War, has long been a popular belief. Even before gunpowder was used for military purposes it was held that rain was produced by the clashing of swords and armor in physical combat. The explanation offered was to the effect that widespread concussions caused the small vapor particles floating in the air to coalesce to form raindrops, the dust and smoke furnishing the necessary nuclei of condensation. Records obtained in all parts of the United States and covering long periods of years fail to show that precipitation is heavier or more frequent upon July 4 or 5 than it is upon July 2 or 3. Moreover, so far as the records are available, the rain accompanying or immediately following great battles is not unlike that which might have been expected in the course of natural events. Bearing in mind the fact, already stated, that throughout large areas rain occurs on an average once in three or four days, and also the subjective fact that rain associated with July 4 celebrations or with battles would doubtless not have been remembered had it not been for such associations, the hypothesis appears to have no foundation. In 1892 the U. S. Government disproved the idea by experiments in which violent explosions of dynamite were produced within clouds by means of kites and balloons, with no rain following as a direct or even as an indirect result. The practise, still followed in various European countries, of attempting to prevent hail by bombarding approaching clouds or of projecting vortex rings of smoke upward, also is without scientific basis. The relatively feeble convectional currents resulting from these artificial attempts to influence the weather are too meager to have any appreciable effect upon the massive convection accompanying storms and are wholly inadequate to influence precipitation.
It is often maintained that cold waves are produced by a descent of cold air from aloft. While it is true that the air aloft is colder than that at the ground, and that up to a height of about six miles there is a more or less uniform decrease of temperature with increase of height, cold waves owe their origin to a number of factors. Nearly all cold waves of the United States occur in the area forming the rear of a passing cyclone and the front of an approaching anticyclone. During the winter half-year this region is characterized by relatively strong northerly or northwesterly winds, clearing skies, decreasing humidity, and the conspicuous fall in temperature. There is a distinct gyratory movement in large disks of air, clockwise, outward from the center, and to a slight extent descending, in the anticyclone, while it is counter-clockwise, inward toward the center, and to some degree ascending in the cyclone. The sharp fall in temperature forming the cold wave is caused primarily by the horizontal transportation of huge masses of cold air from the cold continental interior, and is heightened by the increased radiation from the ground through clear, dry air thus brought in. Vertical currents are probably only of secondary importance in this connection.
In comparing the climates of different places too much stress is generally laid upon mean, and not enough upon extreme conditions of the weather. For example, the average annual temperature, often the only climatological fact quoted in the description of a place, may be very deceptive. Based upon the records of 33 years, the mean annual temperatures of Washington, D. C, and San Francisco, Cal., are practically the same, being 54.7° F. and 54.9° F., respectively. The climates of the two cities are greatly unlike, however. Washington has a semi-continental climate, with daily maximum temperatures in summer often exceeding 90° F., and minimum temperatures in winter frequently going below 0° F. San Francisco, on the other hand, has a semi-tropical climate, with temperatures of 90° F. or over occurring but two or three times in a year, and minimum temperatures below 40° F. being equally rare. In addition, the climates of the two cities differ greatly in respect to the amount and duration of sunshine, cloudiness, rainfall, relative humidity, wind velocity and direction, and the various other elements which constitute climate. The mean annual temperature is therefore an inadequate indication of climatic conditions, and can not alone serve as a basis of comparison.
Too much emphasis is also placed upon the temperature itself—our feeling of comfort is by no means entirely dependent upon the reading of the dry-bulb thermometer. An ideal curve of comfort might show but little resemblence to the thermograph trace. Relative humidity is so important a contributory factor that the wet-bulb rather than the dry-bulb thermometer is often the better indicator. The feeling produced by a temperature of 100° F. experienced in southern Arizona is wholly unlike that accompanying a similar temperature in an eastern city, the difference being due primarily to the marked difference in relative humidity. Other factors also affect one's feeling of comfort, such as sunshine, wind velocity, barometric pressure, and atmospheric electricity. Some time it may be possible to give correct relative weights to each of these factors in determining their effect upon the man in perfect health. While temperature doubtless will receive the greatest weight, the other factors are by no means negligible.
Night air is occasionally referred to as though it is different from day air, and convalescents are sometimes urged to avoid it as dangerous. While there are obvious physical differences between night air and day air there is little diurnal change in chemical composition. Atmospheric air is a physical mixture which when perfectly dry consists principally of nitrogen, oxygen, argon, and carbon dioxide, in which the relative proportions remain fairly constant, and in which the first two named constitute more than 99 per cent. by volume. Up to heights greater than the summits of the highest mountains the percentage of oxygen, an element necessary in the respiration of both plants and animals, shows no appreciable variation. Carbon dioxide, however, which forms but .03 per cent. by volume, or .05 per cent. by weight, of the air, shows both an annual and a diurnal variation. By volume it is 23 per cent. greater in summer than in winter, and is 12 per cent. greater at night than during the day. Since carbon dioxide does not become dangerous until it constitutes considerably more than 1 per cent. of the air we breathe, the change from day to night can not account for the supposedly offensive feature of night air. Water vapor, which never exceeds 4 per cent. by volume of atmospheric air, is important as far as respiration is concerned, because of the diurnal change in relative humidity, the change usually being inversely as the temperature. The actual amount present, however, does not change greatly from day to night. If therefore, night air is dangerous for convalescents, and it probably is not, it is because of physical and not chemical differences between it and day air.
The importance of ozone as a constituent of the atmosphere is popularly overestimated, and the numerous advertisements referring to it as the basis of the health-giving qualities of the air at certain resorts are largely a delusion and a snare. In a molecule of ozone, one of the allotropic forms of oxygen, three atoms of oxygen are held together in such a way that there is but feeble chemical attraction of two atoms for the third atom, which readily leaves the other two to form a compound with some other element. It is because of the latter characteristic that ozone has its peculiar properties. Though there is considerable diurnal and annual range in the amount present in the atmosphere, and also a large difference between that of the air in cities and that in the country or in the free air, the relative proportion, in general, is but one part in a million. In nature it may be formed (1) by lightning discharges, thus explaining the unusual odor sometimes perceptible immediately after a thunderstorm, (2) by the evaporation of water, particularly in clouds or near waterfalls and fountains, and (3) the action of ultra-violet light upon oxygen, probably most effective in the free air above the highest cloud level. However, the healthful properties of the air at various resorts is due primarily to the dryness of the air, the relatively low temperature with small diurnal and annual ranges, the absence of dust and smoke, and the increased amount of atmospheric electricity, and only secondarily to the larger amount of ozone present in the atmosphere.
Since the sun is ultimately the source of all the heat of the atmosphere the question is sometimes raised: "Why is not the upper air, being nearer the sun during the day, warmer than the lower air, which is more distant; in other words, why is there not an increase rather than a decrease in temperature with height?" Records obtained by means of kites and balloons show, among other things, (1) that up to a height of about 6 miles there is a more or less uniform decrease of temperature with height, (2) that the density of the atmosphere decreases rapidly with height, it being half as dense at a height of 3.5 miles as it is at sea level, and (3) that the water vapor is limited to the lower strata, 80 per cent, of it being below a height of 3 miles. The last two conditions explain the first. Partly because of the adiabatic rate of decrease of temperature of a gas with a decrease of its density, and partly because of the ability of water vapor to remove and to store heat energy from the solar rays, the lower atmosphere is warmer than the higher atmosphere.
The source of the water which falls in the form of rain or snow in the United States is erroneously stated in several geographical textbooks to be the Pacific Ocean. Such a statement is doubtless based upon the delusion that the United States, located in a region of prevailing west winds, naturally should receive precipitation from the air which has been moving for thousands of miles across the Pacific, and therefore must have accumulated as much moisture as its temperature will allow it to carry. As a matter of fact, by far the greater proportion (one authority says 90 per cent.) of our precipitation has its source in the Gulf of Mexico and in that part of the Atlantic Ocean lying directly east and southeast of the continent. West of the Rocky Mountains the precipitation comes ultimately from the Pacific, but as the rainfall throughout this large area is deficient, except in western Washington and Oregon, the sum total is small compared with that of the nation as a whole. General and widespread precipitation accompanies the passage of a barometric depression, where the winds in its front, blowing from points between northeast and south, discharge a part of their load of water vapor in the form of rain. The condensation is brought about primarily through the cooling air compressing some of its moisture from it, the lowering temperature being caused by a passage of the air from the relatively warm Atlantic or Gulf to the relatively cold continental interior in winter, or from ascending, expanding, and therefore cooling air during summer. Even so large a water surface as that of the Great Lakes contributes but little to the total rainfall of the United States.
Northeast storms, a characteristic feature of the winters of the Middle and the North Atlantic States, do not come from the northeast, as many infer. The strong, northeast, rainbearing winds do, it is true, bring their loads of moisture from the Atlantic Ocean, but they are simply the indraught of a barometric depression which the weather map shows has come from the west or southwest, usually along a well recognized track. Only upon rare occasions does a storm travel from east to west in these latitudes, and storms of this type, called "flarebacks," are still a stumbling-block in weather-forecasting. In general, a storm or barometric depression is accompanied by winds blowing in a counterclockwise direction and spirally inward toward the center. An examination of the weather map when a northeast storm is in progress will show that the center of the disturbance is southwest or west of the observer, the winds backing to northwest when the center subsequently passes close by or south of the point of observation in its easterly or northeasterly movement.
The tradition that the climate of a city is very different from that of the surrounding country, while partially true, is often exaggerated in the public mind. According to Professor J. Hann, unquestionably the leading authority on climate, city temperatures differ from those of the open country nearby in the following respects: The mean annual temperature of the air in places where there are many buildings in from 1° to 2° F. too high, the differences being greatest in the morning and evening, and least at noon. The diurnal range is smaller in cities, especially in summer. The cooling by radiation, at night, is much greater in the open than in places which are built up. The cooling due to evaporation probably also plays a part. While it has been calculated that the burning of gas and coal in London develops sufficient heat to have an appreciable effect upon the temperature of the air in a stratum 100 feet thick over that city, no progressive increase in the mean temperatures of New York City and Boston can be found to form a parallel with the growth of those cities. The absolute winter minima are much less marked in the interior of cities than in the surrounding open country. A study of certain cold waves showed that the absolute minimum temperatures recorded in the cities of Toledo, Cleveland, Columbus, and Cincinnati, Ohio, were 20° to 25° F. higher than those noted in the country surrounding these cities. From this the investigator concluded that it would be well to put weather stations near, rather than in, large cities, and at a sufficient distance from them to be free from purely local conditions. It should be added that the temperature felt in the city, under the influence of the radiation from the heated walls of buildings and the reflection from the bare ground, is very different from that felt in the country. Other meteorological elements which show difference between city and country are sunshine, which, on account of smoke, is somewhat less in cities than in the surrounding country, and wind velocity. Every large city has one or more tall buildings about which the wind blows with frequent and violent gusts, even on comparatively calm days. As there is everywhere a rapid increase in wind velocity with height, the taller buildings tend to bring down the higher velocities from aloft. It is thus apparent that there is some degree of difference between the climate of city and country, but when due allowance is made for actual and sensible differences, the effect of the local control upon climate is seen to be small.
Concerning the course followed by a thunderstorm, there are many and varied misconceptions. It is often remarked that a thunderstorm, upon coming to a river valley or a mountain gorge, will divide into two parts, one moving up and the other down the valley, in other words, that thunderstorms tend to follow valleys. Another statement is to the effect that the center or most severe part of the storm passes not over the point of observation, but at some distance away. Instrumental observations fail to verify these and similar generalizations. From a study of individual storms based upon the records of many stations it has been found that thunderstorms are most frequently formed in the southern half of a cyclone, where warm and light southerly winds are superimposed by cold and heavy northwesterly winds. In the restoration of equilibrium between these horizontal air masses there is violent vertical convection, accompanied by lightning, thunder, heavy rain, and occasionally hail. Though called local storms they usually advance along well defined convex wave-fronts, which measure from 50 to 200 miles in length, moving broadside in an easterly direction across the country, at about the rate of a fast express train. The horizontal breadth of this line varies from 10 to 30 miles, while the vertical convection extends to heights 5 miles or more above the ground. When one considers the vastness of the mass of air in violent agitation in one of these storms it is apparent that the topography of the ground can have no appreciable effect in determining the course of the storm. Certain it is that throughout the central and eastern parts of the United States, where thunderstorms are a characteristic feature of summer weather, the ground relief is insufficient to influence the courses. Nor is there any foundation for the belief that the storm has a center of extreme violence, which is usually stated to have passed a point either north of or south of the observer. When the storm line is passing an observer from west to east, perspective causes the cloud to appear darker to the north and the south, rather than in the front or the rear of the storm, or even overhead.
Tradition has it that "lightning never strikes twice in the same place." The idea is not only without scientific basis, but the opposite may be nearer the truth, for if the conditions which attracted a lightning discharge are not disturbed by such a discharge there is great probability that they may attract the lightning a second time. In general, any good electrical conductor projecting above material which offers resistance to the passage of electricity will tend to attract lightning. If this projecting conductor is insulated from surrounding material and is anchored deep in the soil, down in the level of permanent moisture, the conductor will protect the surrounding objects. This is the theory of the lightning-rod, which, when properly installed, is a good protector. Though it does actually attract the lightning it may be struck any number of times without damage to things nearby. The Eiffel Tower, in Paris, France, a steel tower 1,000 feet high, has often been struck, six times during the passage of one particularly severe storm. As ample provision is made to conduct the electricity to the earth no serious destruction has resulted. The tradition "lightning never strikes twice in the same place" is therefore more nearly correct when the word never is omitted.
That freezing temperatures are necessary for the formation of hail has sometimes led people to conclude that the hailstones must have necessarily come from the far north, falling as they do on days when the heat has been oppressive. True hailstones occur only with storms of the thunderstorm type, where violent convection extends to heights of five miles or more above the ground. Here the ascending currents are occasionally so strong that they carry aloft, far beyond the level of permanent freezing temperature, particles of moisture already condensed into raindrops. In the average, there is a fall of temperature of 1° F. for every 300 feet of height, so that even in midsummer, when the temperature at the ground is 90° F., one has to ascend but 3½ miles to encounter a freezing temperature. The water droplets, solidifying upon entering the freezing stratum of air, later fall to lower levels, where they may again be caught up by ascending currents to the colder strata above. This process may be repeated a number of times, with the result that the hailstones, upon finally reaching the ground, will show concentric layers of ice and snow. The moisture content more probably came from the Atlantic Ocean, to the east, or the Gulf of Mexico, to the south, rather than from the far north.
The development of meteorology and climatology has been so recent that the general public has not kept pace with the progress. While there are thousands of weather proverbs which are correct generalizations of weather observations extending over many years, a number of traditions have persisted which are apparently without scientific foundation. A few of these, originating in European countries, and doubtless true in their native environment, have proved inapplicable when imported to America. Others are inadequate as they make no distinction between the real and the apparent—between the objective and the subjective. Still others are found wanting because they are based upon fallacious ideas. Instrumental observations, laboratory experiments, and the exploration of the free air have exposed many more misconceptions. Though we have made but a small beginning in a systematic science of the weather, we have advanced far enough to make it possible to eliminate some of the earlier preconceived notions.