The climate of Bangkok has been studied in some detail, as it is the site of superior and long-established meteorological facilities and has been the center of educated Thai and Western observers. (Average annual precipitation statistics for Bangkok go back to 1859.)
The average precipitation in the city is 59 inches yearly. The bulk of this falls during the southwest monsoon rainy season from May through October, and the month of mean highest rainfall is September when over 12 inches fall. The maximum amount of rainfall received in any month was 24 inches, also in September. The driest September on record had less than seven inches. A small amount of rainfall usually is received during the dry winter months. Very occasionally no rain has been recorded during December but never in any other months.
During the summer six months, rain usually falls from 17-22 days each month. As few as seven days of rain have been recorded in May, June, and August, when rainless or nearly rainless periods may last a week or more, either (in May and June) because of the later arrival of the southwest monsoon, or (in June and August) because of interruptions in it. Interruptions have also occurred in July and September. The rainiest month on record occurred during a September when rain fell 29 days, but July, August, and October are close behind with 27-28 days of recorded rainfall. The maximum recorded in any 24-hour period was over seven inches, also attributable to September. However, heavy falls of 3-5 inches in 24 hours have been received throughout the year, including all dry season months except December.
The mean monthly temperature in Bangkok varies from a low of 77° in December and January to a high of 86° in May, a mean annual range of only 9°. December has over 17 days when the minimum temperature is under 70° and January has 22 days. However, maximum temperatures will rise above 90° on a number of days in both months. After January the mean temperature begins to rise quickly, and during the hottest period of the year-in late March, April, and May--the temperatures will exceed 90° almost every day. The monsoon rains force the monthly mean temperature down 3°-5°, and daily temperature rises above 90° only about two out of every three days during the rainy season; the minimum temperature, however, never falls below 70°.
The highest temperature ever recorded at Bangkok was 108°, recorded in both April and May; the lowest was 52°, recorded in December. The mean maximum during April and May is 92°-93° and the mean minimum 77°-78°. The mean maximum and minimum in December (and January too) are 86° and 69°. The mean monthly range of temperature in each of the months from June through October is 12°-13°. This, the smallest monthly range, is not very much less than the highest mean monthly temperature range, in December and January, of 17°.