ATMOSPHERIC PRESSURE 9 
Table 11. Comparison of computed and observed amplitudes, cg, of 12-hour 
waves of atmospheric pressure at sea 


0°6N 11°7'S 17°98 30°6S 
110°3W 126°7W 130°2W 100°2W 
mm mm mm 
Mean position 

mm mm 
Computed? 0.982 0.915 0.837 0.606 0.405 
Carnegie 0.924 0.809 0.759 0.479 0.278 
Difference 0.058 0.106 0.078 0.127 0.127 
P.E., Carnegie 0.030 0.024 0.035 0.051 0.093 
4 After Simpson 
Table 12. Monthly distribution, number of days, atmospheric-pressure observations within 
each latitude range, Carnegie, 1928-29 

: coum: 
17 40 32 
vA 
ences between Simpson’s values and the Carnegie values, 
in three cases out of five, are greater than twice the 
probable errors of the latter. A comparison of the two 
sets of data indicates that the constants of Simpson’s 
formula are too large for accurately representing con- 
ditions over the ocean. These differences, it is true, 
may in part be due to seasonal effects. 
Simpson’s values were intended to represent a year- 
ly mean, whereas the Carnegie values translate a rela- 
tively few days of observation unsystematically distrib- 
uted over a few months (table 12). For example, the 
Carnegie values at latitudes 30° 36’ south and 38° 30’ 
south are probably lower than the yearly mean value for 
these latitudes, since the Carnegie observations in this 
region were made during the southern summer months 
of December and January, when the amplitude of the 12- 
hour wave is at a minimum. 
The small quantity of available pressure data from 
oceanic areas does not justify time spent in constructing 
a formula which would express the amplitude of the 12- 
hour wave at any season and position over the ocean. It 
is possible that these differences between the results of 
the Carnegie and those of Simpson may be partly region- 
al in character, and therefore not representable by sim- 
ple formulas. It seems probable, however, that Simp- 
son’s formula gives amplitudes several hundredths of a 
millimeter too high for oceanic areas. 
Data from thirteen islands fairly well distributed 
with regard to latitude [14] have been compared with the 
Carnegie results at corresponding latitudes. The num- 
Latitude range 
GS se re 
25°N | 15°N 15°S | 25°S 
558 355 soc aa 2 4 8 

21 2 
con 2 22 
1 6 snc 
7 2 no oct 
7 eae 
15 s6¢ 
13 4 sa6 
2 22 14 5 2 
see ane 12 9 
44 29 45 33 22 9 
ber of island stations suitable for this study was limited 
by the fact that data, in order to be comparable, had to 
fall within the same months as the observations made by 
the Carnegie. This comparison of the 12-hour wave at 
islands with the data from the Carnegie is presented in 
table 13 and is illustrated graphically in figure 8. 
The 12-hour pressure waves at each of the island 
stations, except Lerwick, Mauritius, Mangarewa, and 
Samoa, show amplitudes greater than the mean ampli- 
tudes over the ocean at corresponding latitudes. There 
is reason to suspect that the amplitude computed from 
the Carnegie observations at the mean position, 20° south, 
is too large for this comparison, since it is greater than 
the amplitude at either Mauritius or Mangarewa, which 
are in about the same latitude. This is probably due to 
the unsymmetrical monthly distribution of the days in- 
cluded in the mean value for the Carnegie. The number 
of days recorded for November, February, and March 
was five, four, and twenty-two respectively. Since the 
amplitude of the 12-hour wave varies with the season 
and is greatest at the equinoxes, the mean may be heavi- 
ly overweighted by days of fairly high amplitudes as com- 
pared with the mean for the islands where the monthly 
distribution of days is about the same. On the same ba- 
sis, however, it is not possible to explain the large am- 
plitude at latitude 60° north compared with the smaller 
amplitude at Lerwick. Of the seventeen days of obser- 
vation on the Carnegie, fourteen were in July and three 
in August, a time when the amplitude is at a minimum in 
these latitudes. Moreover, with only one exception, 
