Article

Notes on the rainfall of Singapore

From Munshipedia, the MBRAS digital historical encyclopedia

Notes on the rainfall of Singapore

J.J.L. Wheatley published “Notes on the Rainfall of Singapore” in 1881 in the Journal of the Straits Branch of the Royal Asiatic Society (Vol. 7), compiling scattered meteorological records spanning 1820 to 1880 to demonstrate that Singapore’s rainfall is governed by its geographical position and monsoonal circulation rather than by the extent of its forest cover. The article’s central thesis is that the popular alarm over deforestation’s effect on local rainfall is unfounded, as the available data show no material diminution over the preceding four decades.

Summary

Wheatley’s primary objective was practical and archival: to consolidate rainfall data that existed in fragmented form across Government Gazettes, Blue Books, and private registers, making it available for future study. He identified six distinct sources of information, ranging from a single annual figure for 1835 to the continuous registers maintained at the Criminal Prison (from 1869) and at Mount Pleasant by A. Knight (from 1864). By selecting these two latter stations as representative of the town and the country respectively, he constructed a comparative dataset sufficient to test the prevailing local anxiety that the clearing of forests in Singapore and Johor was reducing rainfall.

The article engages directly with a public debate conducted in the local press, in which “oldest inhabitants” testified to the heavier rains of former years and warned that continued deforestation would suspend rainfall altogether. Wheatley’s data refutes this: the average annual rainfall at the Criminal Prison for 1869–1880 (99.96 inches) actually exceeds the earlier Singapore Observatory average of 1841–1844 (92.697 inches), and Knight’s seventeen-year average (93.94 inches) remains comparable. He further demonstrates that the number of wet days has not declined in any meaningful way, and that the extreme years in both directions (drought and excess) are well within the range of natural monsoonal variability.

The second half of the article turns to the monsoonal framework. Wheatley rejects Dr. Randell’s proposed artificial division of the year into three equal four-month periods, arguing instead for a natural division based on the North-East and South-West monsoons. His analysis of the Prison register by monsoon quarter reveals that the first half of the North-East monsoon (November–January) is uniformly wetter than the corresponding period of the South-West monsoon, while the second half (February–April) is drier, and that the total North-East monsoon rainfall exceeds the South-West. He attributes this asymmetry to the North-East monsoon crossing an unbroken expanse of water, whereas the South-West monsoon’s moisture-laden clouds are partially intercepted by Sumatra.

Key Findings

  • Average annual rainfall at the Criminal Prison (1869–1880): 99.96 inches; at Mount Pleasant, Knight’s register (1864–1880): 93.94 inches; Singapore Observatory (1841–1844): 92.697 inches (pp. 33–34, Tables I–III).
  • Average number of wet days: 194 per year (1820–1825); 175 per year at the Prison (1869–1880); 209 per year at Mount Pleasant (1864–1880). The minimum recorded was 109 days in 1877 (Prison) and 144 in 1877 (Knight); the maximum was 244 days in 1879 (Knight) (pp. 34, 44–46).
  • The longest continuous drought recorded was 35 days, from 27 January to 2 February 1864, confirmed by both Vaughan’s and Knight’s registers; the greatest consecutive rainless interval within a single month was 17 days at the Prison and 23 days at Mount Pleasant (pp. 34–35, Tables VI–VII).
  • The first half of the North-East monsoon (November–January) averaged 31.58 inches over eleven years, compared with 22.15 inches for the second half (February–April); the total North-East monsoon averaged 53.73 inches (p. 49, Table VIII).
  • The heaviest single 24-hour rainfall recorded across all registers was 7.69 inches on 14 April 1866 (Vaughan’s register, River Valley Road) (p. 41, Table I).

Conclusion

Wheatley concludes that Singapore’s rainfall is a function of its latitude and monsoonal position, not of its forest cover, and that the data available over sixty years of observation show no trend of diminution. He nonetheless acknowledges the broader principle that deforestation can alter climate elsewhere, and redirects concern toward the more immediate practical consequence of clearing: the future depletion of firewood and timber supplies for the growing population and its steam engines. He urges the Society to continue systematic collection of meteorological records, expressing the hope that future generations may yet discover the underlying periodic laws governing rainfall.

Context

  • Wheatley was a civilian resident of Singapore writing within the colonial administrative milieu of the Straits Settlements; his access to Government Gazette returns, Blue Books, and the P.C.M.O.’s meteorological registers reflects his position within or adjacent to the colonial bureaucracy.
  • The article draws on the Singapore Observatory records (1841–1845), J.D. Vaughan’s private register (1862–1866), the Criminal Prison register (1869–1880), and A. Knight’s private register at Mount Pleasant (1864–1880), the last two being the principal empirical bases of the analysis.

References