Some day the meteorologists of the world will join forces to produce a great encyclopÆdia of climate. No work of science is more sorely needed, but the magnitude that it would, ideally, assume is simply staggering.

Few people realize the multiplicity and complexity of climates. It is a common occurrence for a prospective traveler or a business man to write to a meteorological establishment requesting, for example, a description of the climate of South America. Of course, no such thing exists. A continent does not have a climate, but a multitude of climates. Even to set forth, in general terms, the more important types of climate that prevail between Cape Horn and Panama is no small undertaking. Moreover, general descriptions often fail to supply the needs of those who make inquiries about climate.

Suppose, instead of the wholesale order above mentioned, the meteorologist receives the relatively modest request to describe the climate of Buenos Aires or Rio de Janeiro. Is it easy to comply with such a request? That depends. If the information is sought by a tourist who wishes chiefly to know whether he will need light or heavy clothing at a specified season, or whether his excursions are likely to be hampered by frequent rains, we can enlighten him in a few brief paragraphs. If the inquiry comes from a manufacturer who aspires to invade the South American markets, we must know, before replying, what kind of goods he purposes to export, and just how they are affected by climatic conditions. Are they liable to injury by high or low temperatures, dryness or humidity? Does the demand for them depend, as in the case of rubber coats, upon the prevalence of rain, or, as in the case of electric fans, upon the occurrence of hot weather during at least a part of the year? For each branch of the export trade certain elements of climate are important, and the more detailed and explicit the information that can be obtained about them the better. Suppose, again, climatic data are desired by a horticulturist who has to solve the problem of introducing a South American plant into the United States. In order to find the best environment for it in this country, he should know something about the climate of its original home. The data he requires are, however, different from those sought by the tourist or the manufacturer. Is the plant’s habitat a region where frosts occur? How long is the growing season? Is the rainfall rather evenly distributed over the year, or are there definite dry and rainy seasons? Such are some of the questions he will ask. For the purposes of medical climatology a different set of data will be sought. The astronomer, selecting a site for a new observatory, will ask about freedom from clouds, and also about the pureness and steadiness of the air that insure good “seeing.” The aviator will want information about winds and fog. And so on.

Thus it appears that climate means very different things to different people. Climate has been variously defined as the sum total of weather, average weather, typical weather, etc., but the conception is still somewhat indefinite. We know that, while the weather of any place is subject to incessant changes, its climate persists; but we need not assume that it persists indefinitely. The geological record proves, on the contrary, that vast changes of climate have occurred in the course of long ages. In Antarctica and in Spitzbergen are found deposits of coal, constituting the dÉbris of ancient forests such as could not exist in the climates now prevailing in those regions. There are plenty of other proofs that great climatic changes have taken place from one geological period to another; but what of changes in shorter intervals of time?

An immense amount of zeal and energy has been devoted to the study of supposed changes of climate. Evidence of such changes is sought, on the one hand, in a painstaking examination of weather records (a process often involving the tabulation of hundreds of thousands of figures), and, on the other, in the collection of geographical and historical data bearing on the question. There have been numerous reports of the gradual drying up of African and Asiatic lakes, of the discovery of ancient ruins indicating that prosperous agricultural communities once flourished in regions that are now deserts, and of various other tokens that marked vicissitudes of climate have occurred within historic times. A recent ingenious method of studying climatic variations is to measure the successive annual rings seen in cross sections of old trees. Thick rings are supposed to have been formed during periods of abundant rainfall, and thin rings when the rainfall was deficient. This method has been applied to the giant Sequoias of California, some of which are more than 3,000 years old.

The net result of a wide range of investigations appears to be that, on the whole, climate has everywhere been remarkably constant since the dawn of human history. There is much evidence that, in certain regions, there have been alternate increases and decreases—recurrent oscillations—of temperature, rainfall, etc., but there is little evidence of progressive changes in one direction.

In contrast to the uncertainty that still prevails in the scientific world on the subject of climatic changes is the confidence with which the average layman may be heard to assert that such changes have taken place within his own recollection. The popular idea that climate has changed perceptibly within a single human lifetime is a world-wide delusion, and one that has, apparently, always flourished. In the United States we hear of the “old-fashioned winter,” with its unlimited sleighing, and also of a marked increase or falling off in the rainfall in certain districts. It is an interesting fact that a century and more ago Americans were indulging in the same sort of retrospections.

In the year 1770, when Benjamin Franklin was president of the American Philosophical Society of Philadelphia, a paper was read before that society entitled: “An Attempt to account for the Change of Climate which has been Observed in the Middle Colonies of North America.” It is published in the first volume of the society’s Transactions. Barring the long s’s and the use of the word “colonies,” the greater part of it might have been addressed to the owners of automobiles and Liberty bonds. We are told of a “very observable change of climate,” remarked by everybody who has resided long in Pennsylvania and the neighboring colonies. “Our winters,” says the author, “are not so intensely cold, nor are our summers so disagreeably warm as they have been.” These changes he ascribes to the clearing and cultivation of the country.

Another firm believer in old-fashioned winters and old-fashioned summers was Thomas Jefferson. In his “Notes on Virginia,” written in 1781, he says:

“A change in our climate is taking place very sensibly. Both heats and colds are become much more moderate, within the memory even of the middle-aged. Snows are less frequent and less deep. They do not often lie, below the mountains, more than one, two or three days, and very rarely a week. They are remembered to have been formerly frequent, deep, and of long continuance. The elderly inform me, the earth used to be covered with snow about three months in every winter.”

Samuel Williams, who published a “History of Vermont” in 1794, uses almost identical language in reference to the climate of that State. “Snows,” he says, “are neither so frequent, deep, or of so long continuance as they were formerly; and they are yet declining very fast in their number, quantity, and duration.” That these changes, he adds, “are much connected with and greatly accelerated by the cultivation of the country cannot be doubted.”

What are the facts? When the statements above quoted were written few regular records of the weather had been maintained for any length of time in this country. The earliest instrumental record was begun at Charleston, S.C., in 1730. Much information was, however, available concerning the dates of harvest, of the formation and breaking up of ice in rivers and harbors, and other events dependent upon the weather, which, if anybody had taken the trouble to collect and analyze it, would have dispelled the universal belief that marked changes of climate had recently taken place. Nowadays it is much easier to refute the common assertion that the climate has changed within the memory of living men. The meteorological history of our country for more than three-quarters of a century has been recorded from day to day by a host of careful observers in every State of the Union. The records show that, while the weather of one year has often differed strikingly from that of the next, there has been no real change in climate. “Old-fashioned” winters, for example, were neither more nor less common half a century ago than they are to-day.

Our memories of past weather mislead us, chiefly because we remember the exceptional weather and forget that which commonly prevailed. Other circumstances may contribute to the illusion. Thus many people who now live in cities, where modern appliances make them more or less independent of the weather, passed their childhood under the more primitive conditions of the country.

If climates were not fairly constant for long periods of years, it would be a waste of time to compile the climatic statistics that, as we have seen, are wanted by so many different kinds of people for so many different purposes. Such statistics are based upon past events, but their practical value depends upon the fact that, within certain limits, they are a safe guide to the future.

The climatic data for any place are a sort of digest of the meteorological observations that have been made there, special emphasis being given to those features of the meteorological record that bear important relations to the life and activities of mankind. Temperature and rainfall are the leading elements of climate; others are wind, humidity, evaporation, cloudiness, etc. We have not space to enumerate here all the kinds of data found in elaborate climatic tables; but in order to illustrate how the records of a meteorological station are utilized in compiling climatic statistics and to show what complications may arise in this process, we shall consider the question of temperature alone.

The instruments used in measuring temperature have been described in another chapter. From these instruments are obtained the current temperature of the air, the wet bulb temperature (used to compute the humidity), and the maximum and minimum temperatures of the day. Readings are made at fixed hours, known as “term hours.” At regular stations of the United States Weather Bureau the term hours for the observation of all the meteorological elements are 8 a.m. and 8 p.m., Eastern Time, and an observation of temperature, humidity, and clouds is made at noon. In most other countries tri-daily readings have been the rule, though in Europe four or more observations a day are now taken at many stations in order to supply the frequent weather bulletins required by aeronauts. Important stations are generally equipped with thermographs, which make a continuous record of temperature.

Theoretically, the mean temperature of any day is the average of 24 hourly observations, from midnight to midnight. In practice, the mean is generally computed from the observations at the term hours, or from the maximum and minimum. Having obtained the mean daily temperature for each day of a month, the average of these values gives us the mean monthly temperature. The average of the mean temperatures for the twelve months of the year is the mean annual temperature.

These data for each day and month, and for the year—sometimes also for other intervals, such as five-day periods, or “pentads,”—are computed year after year, and eventually the values for all the years of the record are averaged to form what are called “normals.” We thus obtain, for example, for a given station, the normal temperature for January 21, the normal temperature for the month of March, the normal annual temperature, etc.

All this is a mere beginning toward the complete discussion of a body of temperature observations for the purposes of climatology. We have still to obtain from the readings of the maximum and minimum thermometers the normal maximum and minimum temperatures and range of temperature for each day, each month, and the year; also the “absolute” maxima, minima, and ranges (i. e., the extreme values that have occurred during the entire record) for corresponding intervals of time. These data furnish answers to such questions as: What was the lowest temperature ever recorded on January 21? What is the lowest on an average January 21? What is the average range of temperature in March? What was the highest temperature ever recorded, on any day, at the station?

Having thus disposed of the extremes and ranges, we may compute what is called the “variability” of temperature, i.e., the average difference between the means of two successive days in a given month, and the corresponding average for the entire year. These data are of considerable importance in medical climatology. We may also compute the frequency of occurrence of various values among the temperature data above enumerated. The most frequent value is often quite different from the average value. Many climatologists compute the number of days, in an average year, on which the temperature rises to 77 degrees (Fahr.) or above (“summer-days”), and the number of days on which the temperature does not rise above the freezing point (“winter-days”). Especially valuable in agricultural regions are data of the average and extreme dates of the last frost in spring and of the first frost in autumn. These define the length of the “growing season.” Statistics of the temperature of the ground at the surface and at various depths below the surface are also of agricultural interest.

From the foregoing outline it will be seen that a bewildering variety of climatic statistics may be computed merely from observations of temperature, and the same is true of the other elements. Moreover, the list set forth above is by no means exhaustive even for temperature. In fact, there is almost no limit to the number of ways in which the raw material of climatic data—i. e., the original records of observation—may be grouped, averaged, or otherwise treated in order to bring out certain features of the climate that may conceivably serve some useful purpose. The reader will now be able to understand why a treatise on the climate of a single locality often fills a substantial volume.

The numerical data contained in such a work are generally supplemented by text descriptions and by various graphic devices, such as curves showing the normal fluctuation or “march” of a weather element during a day, year, or other interval. Works which deal with the climates of larger areas, such as whole countries, are usually accompanied by climatic charts. These charts furnish a quick and easy way of getting a general idea of the climate of a region. Among the more important climatic charts are the following:

1. Temperature (isothermal) charts. These include charts showing the distribution of normal temperatures for months, seasons, and the year; normal range of temperature for similar periods; highest and lowest temperatures ever recorded at the different stations; etc. Lines known as isotherms are so drawn as to pass through places having identical values of the element in question (mean temperature, highest temperature, etc.).

2. Rainfall (isohyetal) charts. These show the distribution of rainfall (including snowfall, expressed in equivalent depth of water); especially for each month and for the year. Other charts may show the average number of rainy days; average snowfall (actual depth, not water equivalent); seasonal distribution of rainfall; etc.

3. Wind charts. These are drawn in various forms, to show the prevailing wind directions, the frequency of winds from different directions, the average force of the winds, etc. Charts of the winds at different levels above the earth’s surface will eventually be drawn for the use of aeronauts, but such charts are still in a tentative stage.

As in the case of tabulated climatic data, the number of charts that might be drawn to bring out different features of climate is practically unlimited. Sunshine, cloudiness, humidity, barometric pressure, and the frequency of various special phenomena, such as thunderstorms, hail, tornadoes, droughts, etc., are all charted in some of the more extensive works on climate. Large atlases have been published to portray the climates of certain countries. The study of the actual distribution of climates over the earth, as distinguished from that of climate in general, is sometimes called climatography.

Climates are variously classified, usually on the basis of one or more of the climatic elements, but sometimes with reference to their effects. The most familiar classifications refer to temperature. We speak of tropical, temperate, and polar climates; but in using these terms it should not be forgotten that other things besides latitude control the distribution of temperature. Location with respect to the ocean or other large bodies of water is almost equally important. A land surface grows warm by day and in summer, and grows cold by night and in winter, much more rapidly than a water surface, and the adjacent air varies in temperature accordingly. Hence we have a classification of climates as marine and continental. The former, under the influence of oceanic winds, have a moderate range of temperature, while the latter are subject to extremes of heat and cold. With increase of altitude temperature is diminished, but rainfall is generally increased. The distribution of rainfall is also determined to a great extent by the paths of cyclonic storms. Such are a few of the many things that control the complex distribution of climates.

People who never travel far from their own homes usually cherish quite erroneous ideas regarding the climates of distant lands. It is hard for most Americans to realize, for example, that the Isthmus of Panama, in the heart of the tropics, never experiences temperatures nearly so high as those which occur every summer in the United States. A citizen of South Dakota, where the mercury, in the shade, frequently rises above 100° Fahr., and has been known to reach 115°, will be inclined to revise his definition of the term “tropical” when he learns that at Colon, the Atlantic terminus of the Canal, a temperature as high as 90° is decidedly exceptional, and that the maximum reading during a period of six years was only 92°. In thirteen years Canal Zone vital statistics showed only two deaths from sunstroke and twenty-one non-fatal cases of heat prostration among a population of 120,000. It will also surprise most Americans to learn that the highest natural air temperatures that have been recorded anywhere on earth were not observed near the equator, but in a California desert. At a place on the edge of Death Valley, rejoicing in the ironical name of Greenland Ranch, a temperature of 134° Fahr. was registered in July, 1913. The thermometer which furnished this remarkable reading was a tested instrument, installed in a standard screen over an alfalfa sod, and not exposed to the reflected heat of the desert. At the same place the temperature reached 100° or more on 548 days in four years. Outside of the United States the highest temperature ever recorded at a meteorological station was 127° Fahr. at Wargla (Ouargla), Algeria.

The lowest temperatures encountered by polar explorers are considerably higher than those experienced each winter by the inhabitants of northern Siberia. The “record,” so far as instrumental observations go, is held by the town of Verkhoyansk, at which the temperature fell to 90° below zero (Fahrenheit) in February, 1892. Strange to say, this “winter cold pole” of the earth has warm summers. At Verkhoyansk the temperature sometimes rises to 80° above zero, or higher. At Yakutsk, Siberia, the thermometer has been known to fall to 84° below zero in winter and to rise 102° above zero in summer; a range of temperature exceeding the interval between the freezing point and the boiling point of water!

Another climatic paradox is that experienced by mountaineers who, in scaling peaks mantled in eternal snow, often suffer with the heat, on account of the intensity of solar radiation in the pure, dry air of high altitudes. At the health resort of Davos, in the high Alps (altitude 5,250 feet), invalids sit out-of-doors without wraps in midwinter, and, indeed, are sometimes driven into the shade to escape the too ardent rays of the sun. At the same time the temperature of the air itself may be far below freezing, and the ground covered with snow.

Certain parts of the world are often loosely described as “rainless,” but, as we have stated elsewhere, there is actually no spot on earth at which rain (or snow, in the polar regions) has never been known to fall. In the driest part of the Sahara—the Libyan Desert, between Dakhel and Kufra—the explorer Rohlfs experienced a drenching rainstorm of three days’ duration in 1874. Neither is the Sahara, in spite of its proverbial heat, exempt from touches of real winter. Snow is a common occurrence in many parts of this desert, even at moderate altitudes. On the higher Saharan peaks snow lies on the ground all winter, and is sometimes found, in sheltered spots, in summer. Occasional falls of snow occur in all parts of Algeria, and several falls have been recorded in Lower Egypt. When all is said and done, the whole fabric of what now constitutes the science of climatology leaves much to be desired. Climate is of practical interest, first of all, on account of its effects on human life and health, and secondly because of its influence upon the crops that are the mainstay of man’s material prosperity. Under both these heads climatic data, as now commonly presented, ignore certain atmospheric activities of the utmost importance. For biological purposes no description of a climate can be regarded as even approximately complete that does not furnish, for the region under discussion, a detailed account of the different kinds of radiation received from the sun, their intensities and fluctuations; and there are few places in the world at which even a beginning has been made in the collection of such data. Again, the phenomena of atmospheric electricity, including radioactivity, are probably of real climatic significance, but we are still in the stage of speculation with regard to this subject. Possibly there are still other elements of climate, now wholly neglected, that will figure prominently in the climatology of the future.