ologie (Paris, 1899) and Hann's Handbuch der Klimatologie (Stuttgart, 1893) are the most complete for European data. But in almost all respects, the most careful work of the kind ever published is entitled Atlas of Meteorology, vol. iii. of Bartholomew's Physical Atlas (London, 1899). In this we have a general text on climatology accompanied by about four hundred maps illustrating the 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 this subject. A table of about forty columns of numerical data would seem to be necessary in order to present 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's Physical Atlas.
Perhaps the most important feature controlling plant-life is the relative distribution of temperature and rain from month to month during the year. Climatic types have been elaborated by 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 climatic distinction based upon the law governing the number of 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 counting up 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 various agricultural experiment stations throughout the civilized globe, and the reader may refer to the Experiment 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 special cultivation, 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 insular climate is more uniform as to temperature, moisture, and cloudiness, and more favorable to the development of animals and plants. The influence of climate in disease is principally secondary in that climatic conditions affect the growth of germs, fungi, and noxious animals, through which man suffers.
There is no well-authenticated case of an appreciable change of climate within the past two thousand years. The researches of Eginitis on the climate of Greece seem to establish this principle 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—can have 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, were covered 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 early history of the human race, and the extinct plants and animals of paleontology. See Evolution.
One of the most evident causes of the differences of 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 western Europe is affected by the Gulf Stream, or that the climate of California and British Columbia is 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 windward side 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, have existed; but physiography shows how all this material has been carried away by the action of the frost, wind, and rain, which constitute prominent features of the climate.
Among the works that treat of meteorological climate, the first place must be given to Bartholomew, Atlas, vol. iii. Meteorology (London,
