896 
Detection of Tropical Cyclones. Since tropical cy- 
clones develop over water surfaces, where observa- 
tional data are often sparse and occasionally non- 
existent, detection is a primary problem. Ship and 
island observations provide information on surface con- 
ditions, and a few pilot-balloon and radiosonde reports 
are available. In some areas pilot reports from air- 
TROPICAL METEOROLOGY 
area the trade winds will blow from northeast to south- 
east at 15-22 mph. If a ship within the trades reports 
a wind with a westerly component, a tropical dis- 
turbance has formed; the stronger the westerly wind 
the more intense is the storm. If the easterly trade- 
wind velocities are 25 per cent or more above normal, 
a disturbed situation has developed. 
TasBLe J. FREQUENCIES oF TROPICAL CYCLONES BY Monrus 
Jan. Feb. Mar. Apr. May | Jun. Jul. Aug. Sept. Oct. Nov. Dec. Year 
North Atlantic Ocean (1887-1948) 
All tropical cyclones. _-........--.--- 0 0 0 1 A a) |} ta) |) ae) ikes} 4 7.3 
Hurricane intensity.................. 0 0 0 0 2 2 OM lies oul 1 0 3.5 
is peewee eee es ee | ae 
North Pacific Ocean—off west coast of 
Mexico (1910-1940) 
All) tropical ‘eycloness-..-4-+-4+2 +5: = * * 1 8 of || AO) i) ie 1 it) 5.7 
Definitely hurricane intensity....... 4 3 ee 1 2 «2 =) i 5 0 0 2p2, 
North Pacific Ocean—long. 170°E west- 
ward (1901-1940) 
All tropical cyclones}............... A 2 ae atl WO) BoB 2 ee Nees oe Pe at 
North Indian Ocean—Bay of Bengal 
(Dates not known) s 
All tropical cyclones}............... 1 |0 <2 5) 6 8 6 7 @) | LO A 6.0 
North Indian Ocean—Arabian Sea 
(Dates not known) 
All tropical cyclonest.............. ll |} 0 2 3 1 |0 1 2 3 1 1.5 
South Indian Ocean—Madagascar east- 
ward to 90°H (Dates not known— 
possibly 1839-1922) 
All tropical cyclones§............... 18) fie || 2 2 |0 0 0 0 1 2 8 6.1 
South Indian Ocean—Northwest Aus- 
tralia, excluding Queensland (Dates 
not known—possibly 1839-1922) 
All tropical cyclones§................ a 2 ae) 0 0 0 0 0 0 (0) ae 9 
Some (Rasdite QCA. occcocoscuesososauc According to Visher [20], the average number of tropical cyclones is in excess of 
27 per year. However, this includes several more or less separate areas of develop- 
ment. A number of island groups such as Fiji, Samoa, Hebrides, Tongo, and New 
Caledonia average 2 to 3 per year. Data are insufficient for monthly percentages, but 
the tropical cyclones are apparently well concentrated in the period December through 
March. 
* Less than 0.1. 
} Very few of the storms listed in the winter months reach hurricane intensity. The proportion of storms in other months 
reaching full hurricane intensity is unknown. 
{ Few of the midwinter and midsummer storms reach full hurricane intensity, and the midwinter storms may not be true 
tropical cyclones. 
§ The intensities of these storms are not indicated. 
planes provide limited and often none too accurate 
upper-air information. To detect tropical disturbances 
in the early stages, a careful analysis of all observational 
information is necessary, but it is still possible in some 
areas for tropical storms to travel a number of days 
and hundreds of miles over water without detection. 
Surface Indications of a Tropical Storm. Within the 
trade-wind area winds blow with great steadiness with 
respect to both direction and velocity. Over the ocean 
Pressure fluctuates within narrow limits in the 
tropics, with the 24-hr net pressure change usually 
less than the diurnal variati n. The existence of a 
tropical storm may be recognized by a 24-hr fall of 
from 3 to 3.5 mb or more, or by a fall in pressure to 
5 mb or more below normal. 
Precipitation normally occurs in the form of scattered 
showers and occasional thunderstorms, with variable 
middle cloudiness from none.to broken. If unusually 
