82 
MCADIE. 
This last form is most common in “spring showers.” The 
front of storm clouds of wide extent sometimes shows a 
great arch stretching across a portion of the sky which is 
uniformly lighter in color. 
10. Stratus (S.). Lifted fog in a horizontal stratum. When 
this stratum is torn by the wind or by mountain summits 
into irregular fragments they may be called fracto-stratus. 
The above description is from Mr. A. Lawrence Rotch’s 
translation of the minutes of the Conference, in the “Ameri¬ 
can Meteorological Journal,” December, 1894, the Conference 
having requested that all translations should be made under 
official supervision, 
It will be seen that this classification takes some account 
of the cloud’s altitude; and in differentiating clouds formed 
by diurnal ascending currents in calm air, generally of the 
summer cumulus type, from the clouds formed by widespread 
general uplifting of the vapor, some of the nimbus forma¬ 
tions, this classification takes some account of cloud origin. 
In both of these directions the new system is preferable to 
the old; but the criticism can be fairly made that in the 
matter of cloud origin the new classification does not go far 
enough. 
There are many different ways in which a cloud can be 
formed. The ordinary summer cumulus cloud is formed by 
a slow and somewhat limited ascensional movement. Noth¬ 
ing like this, however, takes place in the formation of the 
familiar billow clouds. Here one layer of air glides over an¬ 
other of different density, and waves result, the condensation 
marking very prettily the wave action. Again, many of the 
lowermost clouds are formed by contact cooling. The so- 
called ground fog occurs when the ground has been cooled 
by radiation, and you notice that the cloud grows from the 
ground upward. Finally, in some of the cumulo-nimbus or 
thunder clouds there can be little doubt that electricity plays 
some part in the formation and rapid enlargement of the 
cloud. This monarch of clouds is noteworthy in several 
