Shaw, N.: Centres of Action in the Atmosphere, 
AB 
Centres of Action in the Atmosphere, 
By Sir Napler Shaw, London, 
The sequence of changes in the state of the atmosphere is expressed in a 
series of weather-charts by variations in the distribution of pressure, and these 
in their turn can be regarded as the consequence of redistribution of mass, or 
as the removal of air from one or more parts of the area represented by the 
map and the increase of air in other parts. 
The changes which are thus indicated may be thought of as happening in 
one or other of two ways, or in both ways in combination, First, we may think 
of the motion of a vast air-current in a general cireulation of the atmosphere 
carrying with it limited dynamical systems of low or high pressure which pre- 
serve their identidy and intensity unchanged during the transfer, In this case, 
with regard to any particular locality on the map, we think of the whole of the 
air of the region being swept away in the general motion and replaced by a new 
body of air bringing with it its own local peculiarities. Secondly, we may think 
of the formation of local dynamical systems within the environment of a general 
eireulation whereby mass is lost from one part and consequently increased in 
some other part, near or remote, in consequence of internal changes due to con- 
vection or some other agency which can cause a redistribution of mass. 
In the first case we need not think of any supply of energy being necessary 
except such as is required to maintain the general ecireulation in which the dy- 
namical systems are carried. In the second case we have to think of the local 
transformations of energy which are involved in the process of econvection or 
other agency which causes the redistribution of the mass, It is in respect of the 
second case that we are naturally disposed to use the expression “centre of action” 
as charaeteristic of any particular part of the earth’s surface, Dynamical systems 
embedded in the eurrent of a general cireulation may have “storm centres” asso- 
eiated with certain lines of travel; but any “action” which takes place belongs 
to the travelling system and not to any centre over which the system travels, 
The two cases may be regarded as being in combination when the dynamical 
systems embedded in the ecirculation and travelling with it are at the same time 
subject to loss or gain of mass in consequence of the action of conveetion or 
some other agency. If these dynamical systems passing over some particular 
part of the earth’s surface are found to have their mass-distribution affected 
when they are in that locality, the part of the earth’s surface which has this 
peculiar characteristice may fairly be called a “centre of aetion”, 
The expression was introduced into meteorological terminology by L. Teisse- 
vene de Bort in the following sentenoce, 
«T/ötude de la repartition des pressions et des vents sur le globe m'’a 
conduit dans les travaux precedents, & definir et 2 classer les divers maxima 
at minima de pression que presente l’atmosphere ä la surface du globe, 
J’ai €te amen&6 ainsi A designer, sous le nom de grands centres d’action 
de l’atmosphere, ces aires de hautes et de basses pressions que l’on re- 
}rouve dans presque toutes les cartes moyennes, et dans celles qui repre- 
sentent l’E&tat de l’atmosphere ä un moment donn&» 
We propose to consider the expression from the point of view which has 
been set out in the preceding paragraphs wherein we have been led to regard 
“action” in the atmosphere as the redistribution of mass, and consequently a 
“sentre of action” as a geographical position where agencies exist fo Cause re- 
distribution of mass, 
From the quotation cited from Teisserenc de Bort we gather that he would 
regard as centres of action the salient features, in respeet of high pressure or 
low pressure, in the normal distribution; and that idea has been generally accepted 
by meteorologists who have made use of the expression “centres of action”. Pro-