398 The Philippine Journal of Science 1921 
individuals, and these having been carefully collected and tabulated by the 
Royal Irrigation Department, and compared with the regular statistics 
of recent years, give what appears to be a fairly correct average over a 
long period. 
The comparative smallness of the rainfall in Central Siam is undoubtedly 
due to the influence of the great western mountain ranges which gather 
the clouds of the southwest or rain-bearing monsoon, and cause the precip- 
itation on their summits and slopes of the greater part of the rain which 
would otherwise be distributed more equally over the whole country. 
The rainfall is not entirely confined to the wet season, for in the neighbour- 
hood of Bangkok showers fall at intervals during the cold and the hot sea- 
sons, while towards the west and in Southern Siam the fall amounts some- 
times to several inches during the hot weather months. Snow never falls 
anywhere in Siam, not even upon the highest mountain peaks of the north 
but hailstorms, though of very rare occurrence, are not altogether unknown. 
The beginning of the wet season is usually heralded by a series of severe 
squalls and thunder-storms accompanied by heavy rain, which sweep 
down from the western heights and sometimes cause damage to property 
on the plains. During the months of September and October, heavy gales 
almost of cyclonic violence are met with in the gulf, but accidents to the 
shipping constantly plying there to and from Bangkok are very rare. 
Waterspouts are occasionally seen both at sea and over the flooded marshes 
of the plains. 
Tables 5, 6, 7, and 8, showing the mean rainfall, the mean 
of the number of days in which rain fell, and the mean rain- 
fall on the delta of Chao Phaya River at Samutsakorn and 
Supanburi in Nakorn Chaisri Province, obtained from the 
statistical yearbook, give some idea of the rainfall in the re- 
gions near Ban Mai, Nakorn Chaisri. 
TABLE 5.—Mean rainfall for ten years, 1909-1918, at Bangkok, Siam. 
mm. 
January 10.2 
February 16.9 
March 65.1 
April 30.0 
May 160.0 
June 150.7 
July 158.2 
August 206.2 
September 305.0 
October 241.4 
November 76.8 
December 11.8 
Mean for ten years 1,433.0 
