September 19; 19 12] 



NATURE 



69 



radiation is the chiej source of condensation nuclei, 

 and if normal ultra-violet radiation is a necessary 

 condition for normal precipitation of aqueous vapour 

 from the atmosphere, and if during periods of 

 diminished activity in this part of the solar spectrum 

 we are to expect the heaviest precipitation over one- 

 hait of the world, then it logically follows that the 

 existence in excess of condensation nuclei produced by 

 ultra-violet solar rays is a preventive of rain. 



Not to pursue farther for the moment this function 

 of ultra-violet radiation, it may be confidently stated 

 that orthodox meteorologists will view with suspicion 

 the simple conception of rapid and general distribu- 

 tion of vapour through the atmosphere embraced in 

 Dr. Ramsauer's supposition. According to a well- 

 established law of physics, the quantity of vapour 

 which may exist in a given space is conditioned upon 

 the temperature of the vapour and the pressure 

 exerted upon it; consequently, in speaking of the 

 aqueous vapour of the atmosphere, it may be said, 

 without widely deviating from the truth, that the 

 temperature of a given volume of air is the controlling 

 factor determining the quantity of vapour which may 

 exist therein. It has been well established by observa- 

 tion that the spontaneous diffusion of vapour in the 

 air is a rather slow process, and that its general dis- 

 tribution through the atmosphere is chiefly effected by 

 the winds and the currents of the general planetary 

 circulation. 



Dr. Ramsauer suggests no explanation as to the 

 manner in which the vapour from the northern was 

 translated to the southern hemisphere. The assump- 

 tion that it was by the process of diffusion implies 

 supersaturation in the upper air; and as to that condi- 

 tion we must hold, with Dr. Shaw, "that it has never 

 been demonstrated." Numerous observations in the 

 loftv atmosphere made in practically all parts of the 

 world, bv means of balloons and kites, show that the 

 quantity of vapour decreases in nearly a geometrical 

 progression as the elevation above the surface increases 

 arithmetically, so that even at a moderate height a 

 region is reached in which the water equivalent of 

 its vapour content may convenientlv be expressed in 

 barrels instead of billions of tons — a quantity in- 

 adequate to account for excessive rains over a large 

 portion of the earth. With such free distribution of 

 vapour in the atmosphere as Dr. Ramsauer suggests, 

 it would be difficult to account for the arid regions of 

 the earth ; since atmospheric overturninj^s are most 

 frequent and most violent over such regions, enough 

 vapour would be condensed to convert them into fer- 

 tile regions. Indeed, the arid regions furnish evidence 

 of the most conclusive kind that the lowest stratum of 

 the atmosphere constitutes the "physical laboratory 

 where rains and dense clouds are made," since they 

 are all flanked to windward by mountain barriers 

 which abstract the life-giving vapour from the air. 



It is highlv probable that during the earth's dawn 

 the vapour of the atmosphere was saturated, as it is 

 supposed to be now on Venus, but the irre.gjularities 

 introduced bv continental unheavals have been the 

 means of draining the moisture to such an extent that 

 the air over all continental interiors and in all regions 

 higfh aloft is now prevailingly dry. 



If it be assumed that a mere preponderance of 

 vapour over a given region would cause a rapid out- 

 flow to regions where the air is less charged, then 

 there might arise a number of complicated conditions 

 much more difficult to explain than the "weather of 

 iqii." For instance, to cite an extreme case, the 

 mean vapour tension over the desert of Sahara is 

 about the same as that over central Europe and 

 southern England; conscquentlv it must be in excess 

 over the desert during probably half the time, and it 

 might therefore be concluded that much of the rain 



NO. 2238, VOL. 90I 



and verdure of central Europe and England is due to 

 the vapour coming to those regions from the heart of 

 the African desert. The relative conditions over Europe 

 and the desert are chronically similar to those which 

 characterised the weather of Europe and South 

 America during 191 1. 



If the semi-annual exchange of vapour between the 

 hemispheres really took place, as suggested by Dr. 

 Ramsauer, it would probably cause disaster through- 

 out the northern half of the world during periods of 

 diminished ultra-violet radiation. For while the 

 southern hemisphere might easily accommodate the 

 vapour emanating from the limited water surfaces 

 during the northern summer, si.x months later, with the 

 boundless southern oceans seething under the perihelion 

 sun, the quantity of vapour sent across the equator 

 would quickly impart a decidedly martian aspect to the 

 northern hemisphere. 



In discussions of summer w^eather, an important 

 phvsical fact is generally overlooked, one which is 

 rarely referred to with proper stress even in treatises 

 on meteorology, viz. the remarkable increase in the 

 tension of saturated aqueous vapour after passing the 

 temperature of 30° C. The energy of solar rays fall- 

 ing upon the ocean surfaces, the chief source of vapour 

 supplv, is almost wholly used up in converting the 

 water into a gaseous state, so that the temperature 

 of the water surface varies but slightly during a 

 season ; whereas the energy of the rays falling on 

 land areas is chiefly exhausted in heating the soil 

 and dust particles suspended in the air, and thus in- 

 directly heating the air in contact with those solid 

 bodies and causing land temperatures in the lower 

 atmosphere to varv through wide limits. So, while 

 the ocean gives off a nearly constant supply of vapour 

 at a uniform and relatively low temperature during 

 the summer, the so-called capacity of the continental 

 air for vapour increases enormouslv once the mid- 

 summer temperatures are reached, making it increas- 

 ingly difficult for atmospheric overturnings to bring 

 about condensation unless rains have been sufficiently 

 distributed over the land during the late spring and 

 earlv summer to compensate for this large saturation 

 deficit which would otherwise be caused by intense 

 solstitial insolation. 



This relation between temperature and vapour ten- 

 sion sufficiently explains the barren islands of tropical 

 mid-oceans, as well as the clear skies and " dry spells " 

 of the great majority of summers ; and it also points 

 to the red end as the portion of the solar spectrum 

 chiefly responsible for "the w-eather of igii." 



I have no desire to question the statement that ultra- 

 violet ravs do produce condensation nuclei ; the por- 

 tion of £)r. Ramsauer's first letter which struck_ me 

 as open to debate is the statement that ultra-violet 

 solar radiation is the chief source of condensation 

 nuclei in the atmosphere. One of the ablest investiga- 

 tors of the day has recently said, in treating of another 

 branch of terrestrial physics, "it is in any case to 

 observation that we must turn as the touchstone by 

 which to try theory." During the past nine years_ I 

 have made many observations of the relative intensity 

 of ultra-violet solar radiation in the northern hemi- 

 sphere, on the ocean, and in the southern hemisphere, 

 with the hope of witnessing some phenomenon in 

 nature that might furnish evidence of this power nf 

 ultra-violet rays so conclusive as the results obtained 

 in laboratorv experiments. Only once have I had 

 reason to think (for a short time) that mv efforts were 

 being rewarded. 



.'Mthoucfh Dr. Ramsauer concludes that ultra-violet 

 solar radiation was at a minimum phase durinc tqii. 

 durintr the first half of March of that vear I obtained 

 the hicrhest v.nlues for all the vears of my measure- 

 ments. About TO a.m., March ii, just after the 



