THE USE OF CLOUDS IN FORECASTING 
an aid in interpreting cloud sequences. A cross section 
showing the clouds and their movements as a storm 
passes a single station [11] indicates trends but is not 
synoptic. Conover and Wollaston [27] have recently 
prepared a detailed study of the cloud systems of a 
winter cyclone based on a large amount of data from 
all reporting stations, both surface and upper air, which 
was reduced to maps of the eastern United States at 
3-hour intervals throughout several days. These give a 
true picture of the cloud development from the origin 
through the various stages of growth of a cyclone into 
a full-fledged snowstorm. 
Cloud Indications of New Developments. Cloud ob- 
servations, synoptically mapped, are particularly val- 
uable for indicating the intensification of a low before 
this trend is apparent in the isobars or surface winds. 
Often middle clouds afford the first sign of the forma- 
tion of secondaries, as Miller [69] and George [46] have 
indicated. Miller pomts to the sequence of C3, Cu, 
or Cm7, then Cy2 (As becoming Ns) in the earliest 
stages of the development of middle or south Atlantic 
coastal storms. This cloud development is easily recog- 
nizable in Type A storms (new lows, off the coast, 
usually) but in Type B (secondaries) the new develop- 
ment will be superposed on the lateral sky of the pri- 
mary, but will be recognizable either by being separated 
from clouds of equal development nearer the center of 
the primary or by the apparent progress of this cloud 
development at a greater rate than the actual advance 
of clouds into the region. 
Cloud and Weather Types. Deppermann [86] has clas- 
sified Manila weather types more or less directly ac- 
cordmg to cloud types. His major divisions are Pure 
Trade, Trade, Northers, Convective Trade, Mild SW 
Monsoon, Frontal SW Monsoon or SW Monsoon Sector 
(with typhoon quite distant), and Typhoon types, each 
with one to seven subdivisions, making twenty-seven 
altogether. These he has related to an elaborate classifi- 
cation according to frontal types of eighteen main 
divisions, each with one to seven subdivisions. The map 
gives the frontal type for the day and the reference table 
then presents the cloud and other weather features to 
be expected. 
In 1894, Ley [60, see pp. 202-204] proposed some- 
thing of this sort: that chart books be prepared, showing 
the distribution of weather, and probabilities for each 
of a number of synoptic situations. These chart books, 
he proposed, should be distributed to users of the fore- 
casts and be consulted when the synoptic type was 
identified by telegram from the forecaster. 
Besides the cloud photographs of the various states 
of the sky in different parts of a cyclone published in 
the Atlas of Clouds [53], an excellent set of such pictures 
has been published by Miss Douglas [40, see Figs. 142— 
162, pp. 108-116], and a set of drawings by Brooks [16]. 
Cloud Indications of How an Approaching Low Will 
Pass a Station. As the clouds of an approaching storm 
gather, how can the observer apply the knowledge 
gained by statistical studies and the idealized or actual 
charts and cross sections of the distribution of cloud 
forms mentioned above? The problem is to forecast 
1175 
the track the center will take to one side or the other, 
the nearness of approach of the center, and the general 
intensity of the storm. As the clouds thicken steadily 
from C7 to Cs and from C's to As, an observer naturally 
expects further development to Ns with its rain or 
snow. There are usually counterindications if such will 
not be the case, or if the precipitation will come late, 
amount to little and end soon, or will come in two brief 
periods separated by several hours of mild, more or less 
sunny weather. If the northern horizon remains clear 
until As overspreads most of the sky, the storm will 
probably be: passing to the south without bringing pre- 
cipitation. If the northern horizon is slow in clouding 
up but becomes covered by the time the cloud sheet is 
principally As, there will probably be some precipita- 
tion but not much. In both cases, the temperature will 
stay low. 
If the cloud sheet after increasing to As breaks into 
As and Ac or Ci den and the sky above is seen to have 
lost most of its Cs, the storm is either weakening or 
passing on the north. In the latter case, the east or 
northeast wind will veer to southeast and south, unless 
a secondary is developing to the southwest or south, 
in which event a new cloud system will appear and go 
rapidly through the north-of-the-center sequence, al- 
ready described. 
Through the southern margin of a storm, the Cz 
appear first in the west or northwest. They increase 
hesitatingly. The wind blows from the southeast and 
does not become strong. The Cz thicken to broken C's 
and these to broken As, with local patches of Ac. There 
may be a little rain, and then the wind will turn to 
south as the warm front passes. The warm, muggy, 
sometimes showery weather that follows will last for 
some time before the cold front sweeps over from the 
northwest or even north-northwest, perhaps with a 
thunderstorm, but, except for the squall, with less wind 
afterward. If the low has already occluded, there will be 
but one brief rain, if any. 
The approach and passage of a squall line or cold 
front is the most spectacular phenomenon of a trip 
through a low south of the center when the action is 
strong. A typical sequence is as follows. In the north- 
west, there appears a long bank of dense, towering 
clouds, with Cz, Cs, and As in front. The bank soon 
stretches from west to north, then from west-southwest 
to north-northeast, and rises higher in the sky. The 
wind is still increasing. Thunder may now be audible. 
The high clouds cover most of the sky. The wind begins 
to weaken as a long, low arch of gray cloud appears 
across the northwestern sky, which is darkening and 
growling ominously. The arch rises and lengthens at an 
alarming rate, as the wind dies to a calm. Now, stretch- 
ing from the northeastern to the southwestern horizons, 
the storm collar, with ragged pendants, reaches the 
zenith as the thundersquall strikes suddenly from the 
northwest. A downpour follows, with lightning and 
sharp thunder, and the squall diminishes. After half 
an hour or more, the sky lightens in the northwest, and 
the rain soon ceases. As the massive cloud bank retreats 
in the southeast a cool, dry northwesterly wind begins 
