Perim Island
PERIM RAINFALL
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A rain gauge was installed on Perim at the beginning of June 1891 and readings are available of all rainfall during the next 247 months until the station was closed down early in 1912 and the equipment sent back to Calcutta. One might ask why 1891? Perhaps because in the previous few years there had been three or four times the long-term annual rainfall in Aden .These monthly statistics give one a fairly good idea of when major storms are likely and which are the driest months. September and October are normally virtually rainless. Although it only rained twice in June in 21 years one can imagine that the cyclone in the first days of June 1885, which passed closer to Perim than Aden, would have brought several inches of rain to Perim.
As in Aden March is consistently one of the wettest months. Outside the period under study, a war diary from 1915 records that there were two heavy rainfalls in March that year, the first of which brought an estimated three inches of rain.
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This quantity was only once exceeded in the 247 months under review. The main South West monsoon brings the possibility of a storm in July or August, as well as very occasionally in June. The main rainy season, if there can be said to be one, is between January and March. Long-term these three months account of around 60% of annual rainfall.
In the 247 months there were five periods of eight months or more (including one of 12) when no rain fell. Two were in consecutive years, 1903-04 and 1904-05. This lack of rain must have extended to the Aden hinterland and the Yemen and which caused such hardship and famine in 1905, resulting in a flood of refugees into Aden itself.
Time for a summary of statistics:
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A. Number of years (out of 20 for January to May, 21 June to December)when less than 0.10 inches or rain fell in the month
B. Number of years when at least one inch of rain fell in the month
C. Largest amount falling in any one year in that month (in inches)
D. Number of years in which at least some rain fell
E. Total rainfall in the month in the 20/21 year period (in inches)
F. Average rainfall per year in that month (in inches)
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The least meaningful of these statistics is the monthly average rainfall (F) especially for monsoon months. Looking at C, where the figure is above an inch it is very probable that most if not all fell at one go. Although October comes out as almost the driest month, a very heavy downpour in October is not unknown. In 1860, during the building of the lighthouse, during the last week of October Perim had two days of exceptionally heavy rain. The Aden Annual Report for 1860-61 mentions that following very heavy rainfall in Aden on 26 October all reservoirs there were filled to overflowing. On Perim the officer commanding the engineer detachment reported that if had rained much longer the lines would have been in ruins. He himself had spent the whole of the first day’s rain in an open buggalow and he thought it was not an exaggeration to estimate that between four and a half and five inches of rain had fallen in the two days.
This uncertainty about when it might rain, especially regarding downpours, is highlighted by some additional statistics for this 20 year period 1892-1911. In only two of the 20 years, 1902 and 1904, was the annual total within 10% of the annual average of 2.43 inches; in four years the amount was more than 50% over the average and in a further four years it was more than 50% under. The wettest year, 1900, was 140% above average whilst the driest year, 1895, only 25% of the average.
One might have expected the main monsoon season to have been wet. It was in Dthala, more or less the same distance from Perim as Aden, but crucially at 6,000 feet above sea level. Unfortunately there was no rain gauge at Dthala but the Medical Officer serving with the Boundary Commission maintained a record of rainfall for the period March 1902 to June 1903.
The table below shows the rainfall (in inches) from March to December 1902 in both locations:
Other sources mention the annual rainfall at Dthala as being in the region of 20 inches a year, more or less on a par with some areas of the United Kingdom. But in Dthala nearly all of this falls during the monsoon. In 1903, the start of two poor harvest years due to a weak monsoon, the monsoon was over a month late, although in August 1903 the Boundary Commission was reporting rain in late afternoon on more or less a daily basis.
No monthly rainfall figures are to hand for Aden for 1902-03, but the figures for the financial year (1 Apr 1902 to 31 Mar 1903)are as follows for the three locations with rain gauges, plus for the one at the Perim Government Hospital:
Crater Civil hospital: 2.26 ins
European General Hospital Steamer Point: 4.47 ins
Sheikh Othman Dispensary: 1.18 ins
Perim Government Hospital: 2.91 ins
Looking at the rainfall figures for Aden over a much longer period of just over 50 years, although the annual average works out at 2.14 inches a year, this hides the fact that for one 9-year period (1882-9) the average was 4.33 inches and at the other extreme the average of the 10 years from 1911 was a measly 0.99 inches a year. One can be sure that in the long run there would be similar extremes for Perim – and almost certainly there were abnormal highs in the mid-1880s. One only has to look at October 1860 to appreciate that one downpour can wash away what would otherwise be convincing statistics!