​CLIMATE (OF.climat, from Lat.clima, Gk.κλίμα,klima, region, slope, fromκλίνειν,klinein, to incline). A word used in meteorology to indicatethe summation or general result of all the solar and terrestrial influences that affect animalor vegetable life. It is possible, in fact, to disregard the relation to life, and consider only the meteorological phenomena as such, or the phenomena that affect any phase of our activity. Thus, one may ask, How does the climate favor navigation by sailing vessels, or the use of the wind as a motive power? In one region theclimate may favor the development of a certain disease; in another, it may favor the developmentof special varieties of plants or animals. The specific features that favor the growth of either plants or animals, enabling them to make a specific spot their home, are oftentimes soobscure as to elude our observation and record; therefore, climatology is, in many respects, still an unsatisfactory study; but it has made such progress in the past fifty years as to have become exceedingly important to many classes ofindustries, as well as to physicians, naturalists, and agriculturists. Some varieties of plants are so dependent upon the nature of the soil in which they grow that Dr. Milton Whitney, of the United States Department of Agriculture, has advocated defining climatology as that which concerns the soil around the roots of the plant; but this is too narrow a view of the subject.

According to the usage of classic Greek,climate concerned principally the temperature of a place as regulated by the altitude of the sun at midday. As this varies with latitude, theancients divided the known globe into zones two degrees broad in latitude, each of which was supposed to have its climate. At the present time, by combining the accumulated work of thousands of observers, we divide the globe into irregular regions, each of which differs from its neighbor in some important climaticcondition as to temperature, rainfall, pressure, moisture, or the inclination of the sun and the amount of cloudiness. In the extensive works of the most eminent writers on climatology,especially those of Dr. Julius Hann, of Vienna, a large number of meteorological items areenumerated as being essential to a complete study of the climate of any place. These items include not merely the mean temperature, rainfall,cloudiness, the barometric pressure and relativehumidity, but also the variations of these quantities,viz. their highest and lowest values each day, or month, or year, and the liability to suddenrises or falls. For navigation and the use of windmills, we need to know the average velocityof the wind, and perhaps especially the numberof hours during which the wind exceeds a specified limit. With reference to the growth or importation of tender plants, the agriculturistneeds to know the mean dates of the last frost of spring and the first frost of the autumn, the difference between which is ordinarily called the growing season. Since the establishment of the fact that the germination of seed, the growth of the plant, and the ripening of the harvestrequires a certain amount of heat or molecular energy, efforts have been made to determine the thermal constants for many plants, and for each phase in growth. This ‘thermal constant’ isusually expressed as the sum total of the average daily temperatures when such temperatures are above 42° F. There is also a ‘rainfall constant’ peculiar to each species of plant, the nature of which has been investigated by Linsser, who has shown that plants are able, by gradual evolution, to change their own thermal and aqueousconstants, and eventually adapt themselves to a change in climate. Linsser's laws serve as a guide to those who would transplant a species from one part of the world to another of different climate.

In the study of climate with reference tonavigation, we have to consider the frequency ofdestructive storm-winds. Charts showing this factorhave been published for all the oceans and seas by the hydrographic offices of England, France, Holland, Russia, and the United States. In addition to this, for the special benefit of sailingvessels, Galton has shown how to prepare charts showing, for each square degree, the progressthat a vessel of a certain size and rig would make if her sails were set so that she should go in a certain direction. From the point of view of insurance, both life insurance and fire insurance,the destruction by wind, hail, and lightninghave been studied; these data, being plotted on charts, show the climate of the country from that point of view. Perhaps the most general idea of the distribution of climate is given by charts which show the frequency per month or year with which storm-centres pass over a given locality, and the direction in which they pass. A map of such frequency for the eastern portion of the United States was first published in theStatistical Atlas of Gen. Francis A. Walker in 1874, and the most extensive publication of this kind was published in 1893 asWeather BureauBulletin A, showing the frequency of storm-paths for all parts of the Northern Hemisphere. The wind, rain, and temperature are so distributed around a storm-centre that, when its location is known, the distribution of all the others can be closely estimated. In general, in the Northern Hemisphere, the regions that lie to the south of the paths of the storm-centres have prevailing warm, moist, southerly winds followed byoccasional sudden changes to cool, dry, westerly winds. This change occurs with every passing storm-centre, but the prevailing weather is clear and pleasant. Stations lying on the north of the paths of the storm-centres have prevailing easterlywinds, with cloud and rain followed by cool northwest winds; but the time occupied by the trying easterly winds is proportionately larger.

It is difficult to describe or exhibit theclimatic peculiarities of any region without the use of charts. Elaborate publications of this kind, for United States weather, have issued from the Weather Bureau at Washington; theClimaticCharts for the Years 1870-99 show the normal precipitation for each quarter of the year, the normal percentage of sunshine, the normalbarometric pressure, reduced to sea-level, the normal temperature of the air at the surface of the earth, the mean maximum and mean minimum temperatures, the highest and lowest recorded temperatures. In addition to these, charts of first and last frost and of prevailing winds have also been published. The ordinary populartextbooks on meteorology are very largely occupied with climatology, properly so called. Of these, that by Prof. Frank Waldo (New York, 1896) is probably the most complete for America; the treatises of Angot,Traité élémentaire de​météorologie (Paris, 1899) and Hann'sHandbuch derKlimatologie (Stuttgart, 1893) are the most complete for European data. But in almost all respects, the most careful work of the kind ever published is entitledAtlas of Meteorology, vol. iii. of Bartholomew'sPhysical Atlas (London, 1899). In this we have a general text on climatologyaccompanied by about four hundred maps illustratingthe climate and the weather of all parts of the globe for each month and for the whole year, and also an admirable bibliographical list of more important modern publications on thissubject. A table of about forty columns of numerical data would seem to be necessary in order topresent the complete idea of climate as imagined by Hann, in his great text-book on Climatology; but most of these are included in the plates and diagrams collected in Bartholomew'sPhysicalAtlas.

Perhaps the most important feature controllingplant-life is the relative distribution of temperature and rain from month to month duringthe year. Climatic types have been elaboratedby Harrington, Henry, and others, based upon this distribution of rain. Thus, in one region we have the prevailing summer rains; in another, the prevailing winter rains; while in still other places, the rains are divided into two seasons with dry weather between. Professor Hinrichs introduced the idea of a climaticdistinction based upon the law governing the numberof light and heavy rains that had fallen within a given space in a year's time. As the largest falls occurred least frequently, and so also the smallest falls, there is some intermediate rainfall that is most likely to happen. By countingup these different quantities, one obtains a series of numbers that may be represented by the equation of probabilities, and the constant term in this equation becomes the so-called“Hinrichs Climatic Factor.”

The influence of climate on crops is a matter of continued investigation in the variousagricultural experiment stations throughout thecivilized globe, and the reader may refer to theExperiment Station Record, published regularly by the United States Department of Agriculture, for the latest information on the subject. A summary of this work has led some authorities to the conclusion that cereal crops are raised successfully only by means of careful specialcultivation, so that the resulting crop is not so much an evidence of the influence of climate as of the influence of human skill and husbandry in modifying and assisting climate. In the interior of continents, the clear, dry air facilitates great ranges of temperature, both diurnal and annual; the soil is dry, evaporation rapid, and delicate plants do not survive the rigors of cold and drought. On the other hand, an oceanic or insularclimate is more uniform as to temperature, moisture, and cloudiness, and more favorable to the development of animals and plants. Theinfluence of climate in disease is principally secondaryin that climatic conditions affect the growth of germs, fungi, and noxious animals, through which man suffers.

There is no well-authenticated case of anappreciable change of climate within the past two thousand years. The researches of Eginitis on the climate of Greece seem to establish thisprinciple beyond doubt. Neither is it possible that any change on the surface of the earth due to man—such as deforestation, reforestation,agriculture, canals, railroads, or telegraph lines—canhave had anything more than the slightest local effect, if any, on climatic phenomena that depend upon the action of the whole atmosphere. On the other hand, it is probable that appreciable changes have taken place in the course of the very long intervals known as geological periods or æons. The phenomena of the flora, the fauna, the erosion, and the geological stratification, all agree in showing that there have been times when the Lake Region and the Saint Lawrence Valley, the Middle States and New England, werecovered with ice and glaciers; a similar condition has prevailed over northwestern Europe. Such changes may have been produced by changes in the elevation of the land and distribution of the ocean, by periodic changes in latitude, by changes in the composition of the earth's atmosphere, or by changes in solar radiation. All of these are plausible causes; but at present there is no agreement of authorities as to the real cause of the changes in so-called geological climate. To these changes in the continents and the climates, we may plausibly attribute the development of a great variety of flora and fauna, the migratory habits of birds, the traditions of the earlyhistory of the human race, and the extinct plants and animals of paleontology. SeeEvolution.

One of the most evident causes of the differencesof climate is the relation of the wind to the land and ocean. When the prevailing wind is from the ocean, the land experiences moist and usually cloudy or rainy weather. This is due essentially not so much to the temperature of the water as to the mere fact that water of any temperature will evaporate largely into the air, and fill it with moisture. Thus, it is an error to say that the climate of Great Britain and westernEurope is affected by the Gulf Stream, or that the climate of California and British Columbiais controlled by the Japan Current; in both these cases it is the moist ocean wind that brings cloud and rain, and the amount of this latter is not influenced in the slightest degree by the Gulf Stream or the Kuro Siwo. Another important consideration in climatology is the relation of the wind to the mountain ranges. Thus, on the windwardside of a range, there is ascending air which causes damp weather with cloud or rain; whereas,on the leeward side of a mountain range there is descending air, which is always dry and clear, and frequently quite warm.

The relation of climate to physiography has been essentially a relation of cause and effect. The surface features of the land, as we now know them, present to us hills and valleys which we may easily recognize as the result of erosion by wind and water, continued for many ages, and assisted by frost and the varying hardness of the different kinds of rock and soil. These features, as we now see them, are usually all that remain after a depth of many thousands of feet of soil and rock has been broken down and carried into the sea. Geology tells us what strata and masses must, at one time, haveexisted; but physiography shows how all this material has been carried away by the action of the frost, wind, and rain, which constituteprominent features of the climate.

Among the works that treat of meteorological climate, the first place must be given toBartholomew,Atlas, vol. iii.Meteorology (London,​1899). For the United States specifically,consult the various publications of the Weather Bureau, aud Waldo,Elementary Meteorology(New York, 1896). For the world in general, consult: Hann,Handbuch der Klimatologie(Stuttgart, 1897); Woeikof,Die Klimate derErde (Jena, 1887); and Hann,Lehrbuch derMeteorologie (Stuttgart, 1901).