Abbott Lawrence Rotch

“ The wave, or ripple cloud, has been explained by von Helmholtz and von Bezold to be due to the undulations in a horizontal current producing alternate rarefaction and condensation of its water-vapour through changes of temperature. Still another cause of low-lying clouds is the cooling of the air to its dew-point by contact with a cold surface, such as the earth when cooled by radiation during a clear night, or the polar currents of the ocean. Fog is often formed in this way, which we call stratus cloud when it rises above us. ”

“ If we could see our earth from outside its atmosphere, the light reflected from the upper surfaces of the cloud-belts would probably make them appear bright. From the markings on a planet that are known to be caused by condensation, a French meteorologist, M. Teisserenc de Bort, believes that the circulation of its atmosphere can be inferred, for wherever on the surface of the planet bright spots are seen, there the vapour of rising currents should be condensed. ”

“ At 15,000 feet the voyagers began to breathe oxygen, which had been used beneficially by Sivel and Crocé-Spinelli in a high ascent the previous year. At 24,000 feet Tissandier wrote in his notes: "My hands are freezing. I am well. We are all right. Haze on horizon with small rounded cirrus. We are rising. Crocé pants. We breathe oxygen. Sivel shuts his eyes, Crocé does the same." Five minutes later: "Sivel throws out ballast, temperature -11° Cent., barometer 300 millimeters." After this, Tissandier became so weak that he could not turn his head to look at his companions. ”

“ The mean height of the cumulus is about a mile, but the tops of the cumulo-nimbus, or thunder-shower cloud, sometimes penetrate into the cirrus level. Generally the base of the nimbus, or rain cloud, is only 2300 feet above the ground, and it frequently sinks below the top of Blue Hill, which is only 630 feet above the sea. The poetic saying, that "Earth wraps her garment closer about her in winter," has [Pg_57] a scientific basis, for the average height of all the clouds is greatest in summer and least in winter. ”

“ But in the average, the temperature falls as much for three hundred and thirty feet of elevation as it does for a change of seventy miles on the earth's surface north or south of the equator. When dry air rises, because it is heated and thereby is made lighter, the laws of thermo-dynamics show that, by reason of its expansion, its temperature is decreased 1° Fahrenheit for each one hundred and eighty-three feet that it ascends, and, by compression, its temperature is increased as much if it is made to descend the same distance. ”

“ In practice the ascensional force usually diminishes at first from various causes, such as the escape of gas, its cooling, and the deposit of moisture on the outside of the balloon. To penetrate the clouds, therefore, it is necessary to have a considerable excess of ascensional force, but above the clouds, since the heating effect of the sun increases greatly with altitude, the gas in the balloon is warmed much above the surrounding air, and so the theoretical altitudes are exceeded. ”

“ The so-called "luminous clouds" seen at night during the same period, and which were probably these same dust particles still illumined by the sun, were found by trigonometrical measurements to have about the same altitude. Although it is computed that at a height of seventy miles the air has less than one-millionth of its density at sea-level—which is about the density of the air remaining in the exhausted bulb of an incandescent electric lamp—it is there sufficiently dense to render meteors luminous by friction after they with great velocity enter our atmosphere. ”

“ This appears natural if we consider the importance of the weather to the ancient pastoral nations, which, from the open-air life and keen perceptive faculties of their people, were well fitted to study natural phenomena. The beauty and grandeur of many of the phenomena occurring in the atmosphere, and the curiosity excited concerning their causes, probably contributed to interest people in them. Meteorology appears to have been first treated systematically, as distinct from astronomy and astrology, by the Greek philosopher, Aristotle, more than 2000 years ago. ”

“ In the tropics especially, the luxuriant vegetation, which diminishes on mountain slopes and higher up gives place to snow, must have been visible proof of the decrease of temperature with altitude, for, as Professor Daniell remarked, mountains are a gigantic registering thermometer having for the freezing-point the line of perpetual snow. The invention of instruments for measuring temperature and barometric pressure made possible the quantitative observations that have supplied the data for deducing the laws governing the atmosphere. ”

“ In regions traversed by the balloon where the pressure of the air is reduced to a few inches of mercury, where there is a complete absence of water-vapour, and at heights to which terrestrial dust does not extend, the measure of the quantity of heat sent by the sun towards the earth is freed from almost all the errors which we encounter on its surface. ”

“ The adiabatic rate of change is seldom observed on mountains because of their influence upon the currents of air in contact with their flanks, or even in balloons, on account of imperfect measurements, but, as will be explained in the closing chapter, the adiabatic change of temperature is confirmed by the observations with kites, which furnish the best method of obtaining the temperature of the free air up to moderate heights. The adiabatic cooling of rising currents of air is another reason for the rapid decrease of temperature with height up to a mile or more. ”

“ It was soon perceived that not only did the level of the mercury in the tube change with height, but that it oscillated continually at the same place, and from its observed relation to the state of the weather its name "weather-glass" is derived. In 1650 the weight of the air was demonstrated in another manner by Otto von Guericke, burgomaster of Magdeburg, who by means of an air-pump of his invention performed the experiment, which Aristotle had tried unsuccessfully, of weighing a vessel full of air and the same vessel exhausted of air. ”

“ For several years the struggle for supremacy in the attainment of the greatest heights was keen between the scientific men of both countries, but a truce was declared at Paris in 1896, and since then both nations have worked together harmoniously. The friendly meeting of French and German physicists at Strassburg in 1898 to agree upon the details of co-operation, typified the union of nations through science, and while it is true that the atmosphere has no boundaries and cannot be pre-empted, let us hope that the common aims of science will ultimately obliterate even political boundaries. ”

“ The upper air alters its temperature from diurnal and seasonal causes much more slowly than the lower air, and a mile above the earth the daily change of temperature, apart from the passage of "warm and cold waves," is less than one degree. At a height of six miles above the earth a temperature much below zero constantly prevails, while, at ten miles, 80° below zero has been recorded in a balloon—this is approximately the temperature prevailing winter and summer above pole and equator. ”

“ The continuous line, representing the day observations, shows a uniform fall of temperature at the adiabatic rate to the cloud level. During the night, the lower part of the broken line bends decidedly to the left, showing a body of relatively cold air near the ground, caused by radiation. There is a rise of temperature with increasing altitude above the ground up to a certain height, and afterwards a comparatively uniform fall as high as the clouds, if they exist; but the rate of fall with increasing altitude, shown by the upper part of the diagram, is slower at night than during the day. ”

“ The measurements in Sweden showed that the middle and upper levels of clouds are higher than in America, but that they move less rapidly. This may be because the surfaces of equal temperature in the air are higher in the United States than in Sweden, on account of the direction of the upper currents, while the greater velocity of our high clouds corresponds with the more rapid movement of areas of low and high barometric pressure over the United States. ”

“ In the average, the velocity of the currents increases, from the lowest to the highest clouds, at the rate of about three miles an hour for each 1000 feet of height, but near the ground the increase with height is faster. It has been found that the velocity of the lower clouds is less than the velocity of the wind on a mountain of the same height, which may, perhaps, be explained on the supposition that the mountain acts like a dam to accelerate the flow of air over it. ”

“ The kite fails when there is little or no wind at the ground, but it seems possible in such cases to lift the kite into the upper air, where there usually is wind, by attaching it to a small balloon that, after the kite can support itself, shall be detached automatically. While the height to which kites can rise is limited, and the limit is probably being approached, judging from the less gain of altitude in recent flights, yet it seems reasonable to expect that, with favourable conditions, a height of at least three miles will be reached. ”

“ The arrows fly with the wind and are proportional in length to its velocity, the dotted arrows indicating the probable flow of the air through the cyclones and anti-cyclones that are indicated by the circles, their axes being assumed to be nearly perpendicular to the earth's surface. Above the cumulus it will be observed that the wind in the cyclone tends to come from the south-west in front and from the north-west in the rear, while in the anti-cyclone the contrary is the case, indicating a deflection of the westerly upper current to the right in cyclones and to the left in anti-cyclones. ”

“ Howard believed that clouds are formed by the aqueous vapour which rises from the earth, and that the globules which compose them are solid, and are not filled with hydrogen gas as had been maintained by Deluc and De Saussure. Howard classified the clouds as we do to-day, according to their appearance, into three principal types, viz. stratus, cumulus, and cirrus, which represented also low, middle, and high clouds. Stratus is the sheet of low-lying cloud which forms at night, and commonly rests on the earth; cumulus is the heaped-up cloud of the day-time ”

“ We have seen that the ascent of human beings to heights of six miles is attended with difficulty and danger, and even with apparatus for supplying the life-sustaining oxygen, man can hardly hope to reach much greater altitudes. Consequently, to obtain information about the atmospheric strata lying above six miles, that is to say, those facts which require to be ascertained in the medium itself, we must employ the so-called ballons-sondes, carrying self-recording instruments but no observers. ”

“ More has been gained from the mountain stations in Europe, notably from those in the Austrian Alps, which have furnished data for Dr. Hann's splendid discussions of the thermo-dynamics of the atmosphere. While mountain stations present the only means of obtaining continuous observations at a considerable and constant height, still they have serious drawbacks. Not only is the distribution of mountains over our globe irregular, but since they form part of the earth's crust, terrestrial influences affect all observations made upon them. ”