1170 
[101, p. 4] says they are an almost sure precursor of 
rain. 
Altocumulus and Rain. In addition to the observa- 
tions on mackerel sky, other studies of Ac have been 
made. After Ac at the morning observation at Poders- 
dam in northwest Bohemia, Schindler [86] found that 
rain followed on the same day (14 hours) in 42 per cent 
of the 700 cases and by the end of the following day in 
68 per cent. 
TasLe V. MAcKEREL SKY AND PRECIPITATION 
Cases with precipitation within 
Cases 
12 hr 24 hr 48 hr 
number j 1 
number per cent) number per cent 
Mackerel sky 17 10 15 (88) 17 ‘| (100) 
(Martin [67]) 
Undulated Ac 16 — 10 (62) | — — 
(Sweetland [98]) 
Altocumulus Castellatus as a Prognostic of Rain and 
Thunderstorms. Ley [60] seems to have been the first 
(1882) to note that morning castellatus is commonly 
followed by afternoon showers, and afternoon castellatus 
by night thunderstorms when, as he said, radiation from 
Ac tops favors convection. A tabulation of subsequent 
investigations 1s given in Table VI. 
TasLe VI. AurocumuLUS CAaSTELLATUS AND THUNDERSTORM 
Thunder- 
Thunderstorm | Sto™™ on 
Region Acc on same day Same ror 
(per cent) cop owibe 
(per cent) 
Lake Constance Strong Nearly 100 — 
(Peppler [76]) 
England (Douglas Strong _ 69* 
[41]) 
Bohemia (Schindler In morning = 67 
[86]) 
N. of Alps (de Quer- 46 cases 7A* 87* 
vain [387]) 
Blue Hill Observa- 12 cases 66T = 
tory (Sweetland | (Apr—Sept.) 
[98]) 1894-1896 
Blue Hill Observa- 51 cases 24F = 
tory (Brooks [19]) (May-—Sept.) 
1946-1949 
* At station or within 100 mi. 
{ At station or nearby, no rain in some cases. | 
In the rainier climates of western and central Europe, 
higher percentages of storms are noted. The greater 
frequency of Acc observations now than fifty years ago 
seems to indicate that many Ac, formerly not desig- 
nated castellatus, are now thrown into that class, diluting 
the apparent prognostic value that is logically attached 
to this indicator of a steep lapse rate. In the more recent 
Blue Hill study, the thunderstorm expectancy for Acc 
days (24 per cent) was nevertheless greater than the 
expectancy for all days of the period involved (14 per 
cent). Schinze and Siegel [87, Figs. 67a and b] have 
CLOUDS, FOG, AND AIRCRAFT ICING 
indicated by cross-section diagrams and charts of tem- 
perature and humidity vs. height how Acc and floc 
are formed and precede Cb. 
Altocumulus Lenticularis—A Sign of Wet Weather. 
The appearance of Ac lent usually means an increase in 
humidity in middle levels commonly owing to the ad- 
vection of moist air in a wind of increasing speed. Such 
clouds appear first over mountains where the obstruct- 
ing masses locally force the wind to ascend. Thus it 
is that new caps on mountains or lenticulars in their 
lee are early signs of coming wet weather. Rink [80] 
describes a striking case of the ‘“Moazagotl Wetter- 
wolke,” a standing foehn cloud in a horizontal whirl 
in the lee of the Riesengebirge, that mdicated rain 
inside twenty-four hours. 
De Quervain [37] found that pzlews was accompanied 
on the same day or followed on the next by Cb in 15 
out of 21 cases. 
Cirrocumulus. Various indications by Ce are cited by 
Angot [2], who says it means fine weather in Britain 
(and, he could have added, New England [23]), but 
the opposite in southern Europe, especially Italy. In 
the tropics, he says, it has no significance. Schindler 
[86] found Cc more a fair- than a foul-weather cloud in 
Bohemia, with rain on the same day 33 per cent of the 
time and by the end of the next day 59 per cent (both 
9 per cent less than for Ac there). Clayton’s figure for 
twenty-four: hours was 86 per cent. Bruckmayer and 
Seiler [20] corroborate Angot’s findings. 
Directions of Motion, Point of Convergence. Ley [60, 
pp. 120, 125, 126] early systematized the prognostics of 
directions of cloud motion, presenting them in qualita- 
tive terms in two groups—for Cz and for Cs. For the 
latter, he used only C's rad and the direction of the con- 
vergence, or V point on the horizon, which he found 
was an average of 15 degrees to the right of the direc- 
tion from which the clouds move. Clayton [23] found 
that 50 per cent of the motions are within 5 degrees of 
the V point. Rules are given for each of the eight 
directions. Thus, “Cirrus from N.W., when not tending 
to form Cirro-filum, is an indication of temporary fine 
weather, especially in summer’; and “‘A V point N.W. 
is an unfavourable symptom, and when it occurs with 
or just after an increase of barometric pressure, is an 
indication of a sudden decrease of the same with rain 
and wind.” 
Henry, Bowie, Cox, and Frankenfield [49, p. 287] 
found four helpful rules: (1) Cz from the southwest in 
the lower Mississippi Valley and Texas indicate that 
precipitation is likely to occur in that region or to the 
northwest 24—48 hours after their first appearance; (2) 
Ci from the southwest over Arkansas and Oklahoma, 
changing to C's overcast, indicate that rain will probably 
soon extend into the middle Mississippi Valley; (3) 
Ci from the northwest in the southeast quadrant of a 
high seem to indicate that the high will increase in 
strength and move slowly; and (4) when C7 stripes lie 
northwest to southeast [probably indicating usual pre- 
storm motion from the northwest], the chance of rain 
is greater than when their direction is northeast to 
southwest [the usual orientation at the rear of a storm]. 
