362 SECTIONAL TRANSACTIONS.—A. 
Friday, August 8. 
12. Si Napier Suaw, F.R.S.—If the Harth went Dry. 
The phenomena of the general circulation of the atmosphere depend funda- 
mentally upon warming at the surface by the sun’s rays and on cooling these 
by outward radiation ; but the dominant factor of weather is the modification 
due to water-vapour in the air. In this paper, in order to clear ideas, the 
reader is invited to regard these two aspects of thermal influence as distinct, 
and to consider the effect of ‘dry heat’ alone. We thus form an idea of what 
the general circulation would be if there were no water-vapour at all in the air. 
The subject is hypothetical, inasmuch as the actual circulation is generally 
affected by the condensation or evaporation of water, but its discussion is not 
necessarily’ sterile. It is an exercise in some important points of thermal 
economy; in deserts the conditions postulated are approximately realised, and 
yet winds, dust-storms, and ‘dust devils’ are not infrequent there; and in 
the large part of the atmosphere where the temperature is below 270¢ the 
relative amount of water-vapour, though not by any means without function, 
is too small to play the dominant role. 
It is assumed that ‘ dry’ air (except for dust) would be perfectly transparent. 
Radiation received by a perfect absorber normal to the sun’s rays would be 
135 kilowatts per square dekametre (subject to small variations of the ‘solar 
constant), and the loss of heat from a surface radiating perfectly (subject to 
local variation on account of dust) would be .572 x (¢/100)4 kw., and range from 
9 kilowatts per (10 metre)? for 200 ¢ to 46 for 300¢. A table is given of the 
temperatures (between 200 ¢ and 402 ¢) at which the loss from a radiating surface 
would balance the income for given solar altitudes. 
The technical discussion is in five sections :— 
1. A survey of the thermal processes operative in the absence of water- 
vapour : (a) the katabatic effect of inclined surfaces cooling in the polar night; 
(6) the slow thermal convection, upward, by the building up of layers of dry 
air in convective equilibrium over flat solarised surfaces (incidentally the ques- 
tion of superheated air is dealt with); and (c) the mixing of superposed layers 
by eddy-motion. 
2. An estimate of the flow of air necessary to keep a steady state of tempera- 
ture on a polar slope under assumed conditions during prolonged nocturnal 
radiation. A possible value of 300 km. per hour offers a justification for the 
use of the term ‘ dust blizzard’ as descriptive of the weather. 
3. An estimate of 2km. as the probable daily height of a layer in convective 
equilibrium under a tropical sun. 
4. Diagrammatic sections of surfaces of equal temperature and of equal 
potential temperature for sunrise and sunset at solstice and equinox. A per- 
manent stratosphere, nibbled daily by a convective troposphere, is presupposed 
for the purpose of estimating its probable temperature, which is near 300 ¢. 
The incidental curiosities of temperature are set out. 
5. The pressure and winds consequent upon the temperature are sketched, 
with the conclusion that a polar front would still be operative and a general 
circulation not dissimilar in some of its main features from the present form. 
13. Sir Freperic Srupart.—The Variableness of Canadian Winters. 
_In normal seasons North Pacific cyclonic areas usually move south-eastward 
with their centres well off the coast until at about the latitude of Northern 
British Columbia they enter the continent, while anticyclonic conditions of 
eee intensity with low temperature prevail in Yukon and the Mackenzie 
iver, 
In certain years, however, the Pacific cyclonic areas are less intense and 
enter the continent further south, while great anticyclonic developments occur in 
the far north and sweep south-eastward over Canada, accompanied by severe cold 
waves, which not infrequently reach the Atlantic coast. These conditions lead 
\o abnormally cold winters in Canada. 
In other years the North Pacific cyclonic areas appear to be of such intensity 
—! oof lla 
