1749 1784 1809 1822-3 1852 1862 1869 1874 c. 1882 1891 c. 1893 1893 1894 1894-5 1896 CHAPTER XI THE METEOROLOGICAL OBSERVATORY: 1899-1902 1901 THE UPPER AIR Dr Alexander Wilson of Glasgow raised thermometers by kites. Dr John Jeffries and the aeronaut Blanchard made the first balloon ascents for meteoro- Pilot-balloons: Thomas Forster watched the movement of small balloons filled with Rev. George Fisher and Captain Sir Edward Parry raised self-registering thermo- Four balloon-ascents by John Welsh at Vauxhall for the Committee of Kew Obser- M. Bonvallet sent up paper balloons with post-cards attached. March 21. First registering balloon-ascent by Hermite and Besançon. The first continuously recording instrument was raised by kite at Blue Hill Observatory. Discovery and demonstration of the stratosphere by Teisserenc de Bort and Assmann. Berson and Süring reached 10,800 m in a balloon. Dines flew kites from a steamship. Teisserenc de Bort arranged for kites to be flown day and night, when possible, for nine 1901 1902 1902-3 1904 Rotch made the first registering balloon ascents in America. The dates contained in the heading to this chapter have been extracted from the following papers: (1) Gold and Harwood, 'The Present State of our Knowledge of the Upper Atmosphere,' Report to the British Association, 1909; (2) The Principles of Aerography, by A. McAdie: Rand McNally and Co. 1917; (3) 'Upper Air Research,' by C. J. P. Cave, Q. J. Roy. Meteor. Soc., vol. XL, 1914, p. 97. THE VARYING ATMOSPHERE We now proceed to give some account of the equipment which has been devised and brought into operation for extending our knowledge of the structure of the atmosphere above the surface-layer, the region which is generally indicated by the title of the "free air." There are many ways in which observers at the surface can appreciate the variations in the state of the atmosphere above them. Astronomers who are engaged in the study of the sun, planets and stars are always conscious of effects which are attributable to the atmosphere; the twinkling of the stars is one of the most familiar instances. There are many others; the observers of the planet Mars, for example, have to depend on periods of clearness in the atmosphere for the best definition of the planet in their telescopes and on this account visual observations give more information than photographs1. The edges of an image of the sun projected upon a screen present a flickering appearance which is called the "boiling of the limb." Meteorologists have been invited to regard it as furnishing a means of forecasting weather. Such interferences with "good seeing," however, cannot at present be classed as atmospheric measurements. By astronomers they are regarded mostly as things to be avoided or reduced to the smallest possible limits by choosing special sites for their observatories and special times of the day or night for their observations. Observers of solar radiation however make a practice of forming a numerical estimate of the absorption of the atmosphere by separate observations with the special object of making the correction; and since the navigation of the air has become the special care of meteorologists, observations of visibility of distant objects through the free atmosphere have become part of the recognised duty of many meteorological stations. We cannot however discuss here the bearing of such observations upon our knowledge of the structure of the atmosphere further than to mention that a "dust-horizon" (fig. 24), the appearance of a horizontal surface extending to the boundary of vision, is easily recognised by aviators in favourable circumstances. It marks the top of the layer of air, generally but not always the surface-layer, which contains sufficient dust to differentiate it from the layer above by a well-marked boundary. The conditions are favourable when the surface-layers are comparatively free from eddies. The evening and early morning are favourable times. CLOUD-FORMS Among observations which are more definitely within the domain of meteorology in relation to the structure of the atmosphere we must refer to clouds as being one of the most obvious sources of information about the free air. We have already mentioned the amount of cloud as one of the regular observations of meteorological stations. In that respect, as also in respect of the observatories at high levels, the surface-survey and the survey of the upper air overlap. We have now to consider the development of the study of clouds which is concerned with their form, height and motion. The forms of cloud must have been an object of interest from earliest times and the classical languages have a number of names νέφος, νεφέλη, nubes, nebula, nimbus, imber, translated by the word cloud or mist without any specific definition. We have noted on p. 101 the important references in Theophrastus to "streaks of cloud" in a wind from the south and "clouds like fleeces of wool"; the classification thereby indicated was not developed. The credit of classifying the forms of cloud in a scientific manner belongs to Luke Howard2. He named the main classes, cirrus, stratus, cumulus, and gave a definite meaning to nimbus. 1 Sir Frank Dyson, Nineteenth Century, February, 1925. Rev. T. E. R. Phillips, 'Observations of Mars in 1924,' Monthly Notices R.A.S., vol. LXXxv, December, 1924, p. 179. 2 Luke Howard, On the Modifications of Clouds, London, 1803. THE FREE AIR CLOUD-FORMS: PLATES I TO VII. CLOUD-GROUPS OR LAYERS: VIII Disorderly grouping: High, middle IX Linear grouping: Middle and low. XI Tessellation at four levels. XII Continuity at four levels. XIII Nebula at four levels. XIV The Whole Sky: Stereoscopic view. Fig. 24. Dust-horizon at 1250 m. seen from 450 m. above, with clouds beneath it." |