1918-19.] The Origin of Anticyclones and Depressions. 77 
cloud-sheets, and variations of diathermancy. As regards the former, air 
which is moving poleward, especially in winter time, is almost certainly 
cooling. Hence arises the continual succession of depressions in the 
winter time in the Northern Antitrades, which practically control our 
winter weather. In summer, anticyclones are likelier owing to the in- 
creased duration of sunshine. In the trade-wind region, where the winds 
move towards the equator, anticyclonic conditions prevail. Where the air 
and underlying earth are cooling, the formation of an extensive cloud- 
sheet, by reflecting back the terrestrial radiation, will retard the cooling 
below and accelerate it above. Thus anticyclonic conditions arise beneath 
the cloud, and cyclonic above. If the cloud is low, the greater mass of air 
is under cyclonic conditions, the expulsion of air from the cyclone will 
exceed the entrance of air to the anticyclone, and hence cyclonic conditions 
will ultimately prevail. If, however, the cloud-sheet forms at a high 
altitude, anticyclonic conditions will result. \_E.g. Desert of Gobi in 
winter.] 
The converse is true if the air and ground are in process of heating, and 
hence (as has been frequently observed) low sheets of stratocumulus are 
often, in early summer, associated with anticyclonic conditions. 
Diminished diathermancy in a layer of air has probably the same effect 
on lower and higher layers as the formation of cloud, but in the layer of 
diminished diathermancy itself there will be a rise of temperature, whereas, 
where cloud forms there must be a reduction. 
Disturbance of vertical equilibrium may arise in the region of most 
rapid heating in an anticyclone (cf. table of average values given above) 
and gives rise to summer thunderstorms. The resultant churning must 
diminish the intensity of the anticyclone. Disturbance by too rapid 
cooling in a cyclone is a rare occurrence owing to the higher speed of wind 
movement in such systems, which more rapidly supplies new air to the 
region of refrigeration. But a few winter thunderstorms may be due to 
this cause. 
After the original causes of the anticyclone or depression have vanished, 
the established inequalities of density will cause the circulation to persist 
for a little. Friction and the conduction of heat are, however, constantly 
withdrawing energy ; while the process of mingling (postulate A) tends to 
destroy the inequalities of density. Average conditions would in time be 
restored, did not fresh thermal disturbances in adjacent regions set up 
fresh wind systems. 
{Issued separately June 19 , 1919 .) 
