METEOROLOGY 



155 



'forecasts' of weather, first suggested in the 

 United States about the middle of the 19th 

 century. As regards the British Islands these 

 'forecasts' are nosed on telegrams which are 

 received every morning from about sixty selected 

 stations in Great Britain and Ireland, and on 

 the Continent, which give the exact state of 

 the barometer, thermometer, hygrometer, and 

 rain-gauge, with the direction and force of the 

 wind, and appearance of the sky, at each of these 

 stations at eight in the morning. In the event of 

 there being any storm or other atmospheric dis- 

 turbance at one or more of these places, a full 

 and accurate description of it is thus conveyed 

 to London ; and it is the duty of the officials 

 there to consider the direction in which the storm 

 is moving and is likely to move, so as to enable 

 them to give warning of its approach at different 

 ports by special signals. Hut in addition to warn- 

 ings of storms, daily .'forecasts' of the weather likely 

 to occur in the different districts of Great Britain 

 for the following two days are also issued. As 

 regards storms the problem to lie practically worked 

 out is this : Given telegrams showing the exact 

 meteorological conditions prevailing over the area 

 embraced by the stations, with indications of a 

 storm approaching in a certain direction, to deter- 

 mine, not the probable area over which the tempest 

 will sweep, but the precise localities which will 

 altogether escape, the places where the storm will 

 rage, its continuance, its violence, and the particular 

 direction* from which thc> wind will blow at places 

 visited bv the storm. Weather- registers extend- 

 ing over long periods give no countenance to the 

 ncition that there are regularly recurring cycles of 

 weather on which prediction sufficiently precise 

 and particular to be of service to agriculture and 

 navigation may l>e based. The manner in which 

 good and bad seasons occur in different places with 

 iv-pect to each other shows clearly that they have 

 little direct immediate dependence on any of the 

 heavenly bodies, but that they depend directly on 

 terrestrial causes. Owing to its proximity to the 

 Atlantic, Great Britain is not so favourably situ- 

 ated for the issue of warnings ns the countries of 

 Europe to the eastward. Since 1870 this branch 

 of science has leen prosecuted with remarkable 

 energy ami success by General Myer (familiarly 

 known as ' Old Probability') and his successor in 

 charge of the signal service of the United States 

 War Department. American meteorologists were 

 the first to undertake the representation of 

 isothermals over the ocean ; and to the United 

 States science is also indebted for magnificent 

 contributions and researches in international 

 meteorology. 



The stud'y of meteorology has of late benefited 

 largely by the establishment of high-level meteor- 

 i'-al stations in the United States, France, Italy, 

 Switzerland, Austria, India, Australia, Scotland, 

 and many other countries. The nine arctic expedi- 

 tions in 1882-83 devoted themselves largely to 

 meteorological olwervations. Many first-class mete- 

 orological observatories are now established in all 

 civilised countries at which hourly observations are 

 made. 



On the diurnal phenomena the more important 

 principles of the science are based. Of the sun's 

 rays which arrive at the earth's surface, those 

 which fall on the land are wholly absorbed by the 

 tliin surface layer, the temperature of which eonse- 

 'inently rises. A wave of heat is thence propagated 

 downward through the soil, the intensity of which 

 rapidly lessens with the depth at a rate depending 

 on tli<' conductivity of the soil, till at a depth of 

 about 4 feet it ceases to l>e measurable. Part of 

 the heat of this surface layer is conveyed upwards 

 into the atmosphere by convection currents. But 



as regards the surface of the ocean the case is 

 totally different. Here comparatively little of the 

 heat is arrested at the surface, but it penetrates, 

 as shown by the observations of the Challenger 

 expedition, to a depth of about 500 feet. Hence in 

 deep waters the temperature of the surface is but 

 little heated by the direct rays of the sun, though 

 in shallow waters, owing to the heating of the 

 bottom, the water has a considerable daily range 

 of temperature. Thus, in mid-ocean, from 30 N. 

 lat. to 30 S. lat., the temperature of the surface 

 of the sea does not vary (luring the day quite so 

 much as one degree Fahrenheit. Off the coast of 

 Scotland the daily variation is only 0~3, and in 

 higher latitudes still less. On the other hand, the 

 daily variation of the upper layer of the surface of 

 the land is frequently 50, and in many cases very 

 much greater. Hence the enormously different 

 results which large masses of land and sea respec- 

 tively exercise on climate. The temperature of the 

 air over the ocean is about three times greater than 

 that of the surface of the open sea over which it 

 lies ; but on nearing land it is nearly five times 

 greater. The least daily variation on land is in 

 insular situations, being at Rothesay about 5 ; and 

 the greatest in the Sahara regions of tropical and 

 subtropical countries, where it is in many places 30, 

 rising on occasions to 40 and upwards. The daily 

 minimum temperature occurs some time before 

 dawn ; and as regards the maximum, it occurs 

 from 1 to 4 P.M., according to season and geo- 

 graphical situation, the earlier hour obtaining in 

 arid climates and at true high-level observatories, 

 and the later in climates characteristically humid. 



The absolute humidity of the air, or, as it is 

 usually termed, the elastic force of vapour, is seen 

 in its simplest form on the open sea, as disclosed by 

 the Challenger observations. The minimum occurs 

 at 4 A.M. and the maximum at 2 P.M., thus approxi- 

 mating closely to the diurnal march of the tem- 

 perature ; on nearing land a secondary minimum 

 prevails from 10 A.M. to 4 P.M., due doubtless to 

 the drier descending aerial currents which take the 

 place of the currents that ascend from the heated 

 surfaces of the land. The relative humidity is 

 widely different from the vapour pressure, and 

 presents features of the simplest character. The 

 maximum occurs from midnight to 4 A.M. ; or when 

 the temperature is lowest the air is nearest to 

 saturation. On the other hand, the minimum is 

 ajxjut 2 P.M.; or when temperature is highest the 

 air is furthest from saturation. This feature of the 

 humidity characterises all climates. When the air 

 is by terrestrial radiation cooled below the dew- 

 point dew is deposited, and when the temperature 

 is below 32 hoar-frost is the result. 



The diurnal oscillations of the barometer show 

 two maxima and minima the maxima occurring 

 about 9 to 10 A.M. and 9 to 10 P.M., and the minima 

 from 3 to 4 A.M. and 3 to 4 P.M. Since the tem- 

 perature of the surface of the tea does not vary 

 quite one degree during the day, and since these 

 oscillations occur equally over the open sea as on 

 land, it conclusively follows that they are independ- 

 ent of the temperature of the part of the surface of 

 the globe on which the air rests. Generally speak- 

 ing, the amount of the oscillations decreases with 

 latitude. Taking latitude with latitude, the 

 amounts are greatest over land surfaces which are 

 greatly heated during the day and cooled during 

 the night, and least over the anticyclonic regions 

 of the great oceans lying to the westward of the 

 continents from about 20 to 40 N. and S. lat. 

 The characteristics of these anticyclonic regions is 

 a vast descending current down their central 

 spaces. This air necessarily increases in tem- 

 perature with its descent, and consequently is 

 further removed from saturation ; and it is prob- 



