The 18 year annual average for Crater for the whole period 1 April 1901 to 31 March 1919 was 1.17 inches. Ignoring the four driest and the four rainiest years the median is slightly less at 1.11 inches. From 1924 to Aden becoming a Colony in 1937 there appears to have been only one official rain gauge, and that was at the Hogg Tower on the ridge between Tawahi and Post Office Bay. I have not been able to locate the details for 1927-28 but for the other 12 years the average annual rainfall was 1.39 inches a year. Ignoring the three driest and the three wettest the median works out at 1.2 inches. For the seven driest years out of the 12 the average was less than half the 12-year average.

From the two example periods given above one might think that the long-term annual average rainfall for Aden was no more than one and a quarter inches a year. But let us look at a different 16 consecutive years from 1 April 1883 to 31 March 1899. The average for Crater was 3.81 inches – about treble the average for the other 30 years! For the first six years of this earlier period the average was 5.42 inches, with no less than 9.12 inches falling in 1889-90. In the early 1890s the Aden community was searching for a reason for such a drastic change in weather pattern – from the incomplete statistics available it would appear there had been a succession of ‘normal’ dry years immediately prior to 1883. With no global warming to blame it on there were many who seriously thought that the culprit was the long-term effect of constructing the Suez Canal!

One can safely say that in years when the annual rainfall exceeded more than an inch or so the total for the year was not achieved by numerous showers but by one or more torrential downpours. It was only after a succession of such storms that the ‘barren rocks of Aden’ turned green for a few days!

A rhetorical question I have often asked myself is what quantity of rain was required to fill the Tanks in Crater, assuming they were completely empty. I have found no written evidence on this but the photograph of the Large Tank full is one of a set of postcards issued around 1931. Not unexpectedly one finds that the only abnormal year for Crater in the period 1924-37 was 1929-30 when 3.93 inches fell. (0.2 inches fell in 1930-31 and 2.29 inches in 1928-29). The most likely month for heavy rain was March; one possibility is that around two inches fell at the end of March 1929, to be followed by three or more inches in another storm in early April (thus spanning two reporting years). The alternative is that the tanks were entirely filled by one storm, probably in March 1930. Another photograph of the main tank the main tank nearly full appears on postcards on sale over quite a long period, the earliest being on a ‘court’ sized card from around 1899. In 1898-99 some 3.59 inches of rain fell in Crater, the first abnormal year for a while.

Occasionally there would be a storm in the main monsoon season, which was normally from June to August, but in some years it began up to a month late. ‘Monsoon’ for the Gulf of Aden meant rough seas but seldom any rain in Aden itself. The storms would be in the mountains inland. Dthala, for example averages about 20 inches a year, most of which fall during the monsoon season.

Unfortunately I have not been able to find rainfall statistics for 1921-22 and therefore am unable to quantify the amount of rain that fell in the downpour on 20 March 1922!

I have mentioned that Crater had an official rain gauge for many years (at the Civil Hospital). For most of the period 1883 to 1919 there were also gauges in Steamer Point (at the European General Hospital) and at the dispensary at Sheikh Othman. Quite often more rain fell in Steamer Point than in Crater, or vice-versa, but over a long period one can say that Steamer Point averaged more than Crater. Annual figures for Sheikh Othman were usually a little higher than for Crater and Steamer Point, and in a couple of years significantly higher – all of which points to storms rather than showers being the provider.

So what is Aden’s average annual rainfall? Looking at the 51 years for which I have statistics between 1879 and 1937 (there being therefore seven gaps) the average for the whole period is 2.07 inches. Knocking off the five driest and the five wettest years (the latter all between April 1884 and March 1891), the median average falls to 1.70 inches a year. I would be interested to hear from anyone who has more modern statistics covering at least 25 years - from what you have read I hope you will agree that a period much shorter than that will not be sufficiently reliable.