The characteristic of meteorology at Zanzibar, as generally the case in the narrow equatorial zone, is the extreme irregularity of its phenomena. Here weather seems to be all in confusion; hardly two consequent years resemble each other. In 1853-4, for instance, the seasons, if they may so be called, were apparently inverted; heavy showers fell during the dries, and a drought occupied the place of the wet monsoon. Sometimes the rains will begin with, this year (1857) they ended with, a heavy burst. Now April is a fine month, then the downfall will last through June.
I may also remark one great difference of climate between the eastern and western coasts of intertropical Africa. Whilst Zanzibar is supersatured with moisture, Angola, on the same parallel, is a comparatively dry, sandy, and sunburnt region. Kilwa, upon the eastern coast, and in S. lat. 8° 57′, is damp and steamy. S. Paulo de Loanda, upon the opposite shore (S. lat. 8° 48′), suffers from want of water. We find the same contrast in the South American continent. The middle Brazil is emphatically a land of rains, whilst Peru and Chili require artificial irrigation supplied by melted snow. Evidently the winds charged with moisture, the N. E. and S. E. trades and their modifications, discharge themselves upon the windward sides of continents, especially when these are fringed with cold sierras, which condense the vapour and render the interior a lee land.
In 1847 the Geographical Society of Bombay sent a barometer to Zanzibar, and requested that a meteorological register might be kept. Their wishes were not immediately carried into effect; but after a time the Eurasian apothecary in charge of the Consulate filled up in a rude way during nine months a weather-book, with observations of the barometer, of two thermometers attached and unattached, of wet and dry bulbs, of evaporation and of rainfall. In the Journal of the Royal Geographical Society (xxiii. of 1853), Colonel Sykes published a ‘record, kept during eleven months in 1850, of the indications of several intertropical instruments at Zanzibar,’ unhappily without those of pressure.[[38]]
The result of nine months’ observations is that the thermometer shows a remarkably limited range of temperature and an extreme variation of only 18°-19°. A storm, however, will make the mercury fall rapidly through 6°-7°. The climate is far more temperate than the inexperienced expect to find so near the equator. It is within the limits of the true Trades. The land and sea breezes laden with cool moisture blow regularly, and the excessive humidity spreads a heat-absorbing steam-cloud between sun and earth. The medium temperature of January is 83° 30′; of February, the hottest month, 85° 86′ (according to Colonel Sykes 83° 40′); and of March, 82° 50′. This high and little-varying mean then gradually declines till July, the coolest month (77° 10′). The mean average of the year is 79° 15′-90′. In September and October the climate has been compared with that of southern Europe. On the other hand, the atmosphere supports an amount of moisture unknown to the dampest parts of India.
The barometer, so near the equator, is almost uniformly sluggish and quiescent. Its range diurnal and annual is here at a min. It seldom, except under varying pressure of storms or tornadoes, rises or falls above or below 30 inches at sea level, and a few tenths represent the max. variation. It must be observed, however, on both coasts of Africa, within 6°-7° of the Line, this instrument requires especial study for nautical purposes. Here it is an imperfect indicator, because, affected from great distances, it rises without fine weather and it falls without foul. At Zanzibar the case of a whaling captain is quoted for wasting in vain precautions nearly two months. Moreover, sufficient observations have not yet been accumulated in the southern hemisphere. Where there is so little expansion in the mercurial column the convexity and concavity of the column-head must be carefully examined with a magnifying-glass, and by a reflecting instrument the smallest change could be correctly measured. The trembling of the aneroid needle, sometimes ranging through a whole inch during the gusts of the highly electrical tornado, also calls for observation. The sympiesometer is held to be even more sensitive than the mercurial barometer, especially before storms, and ignorance of its peculiarity has often ‘frightened a reef in’ at unseasonable times. The same was found to be the case, in high latitudes, by Lieut. Robertson, R.N., when sailing under Captain Ross (1818), between N. lat. 51° 39′ and 76° 50′.
Observations with the altitude and azimuth determined the variation of the needle in 1857 to be between 9°-10° (W.). If this be correct, it is gradually easting. In 1823 Captain Owen found it to be 11° 7′ (W.).[[39]] So, upon the opposite coast, the variation laid down in our charts of 1846 as 20° (W.) has gradually declined to between 18° 30′ and 19° (W.).
Of exceptional meteoric phenomena I can speak only from hearsay, no written records existing upon the island. A single earthquake is remembered. In the early rains of 1846, at about 4 P.M., a shock, accompanied by a loud rumbling sound, ran along the city sea-front, splitting the Sayyid’s palace, the adjacent mosque, and the side-walls of the British Consulate, in. a direction perpendicular to the town. It was probably the result of igneous disturbance below the coralline, and it tends to prove that the island was originally an atoll: some, however, have explained it by a land-slip. Three meteors are known since 1843. In December of that year a ball of fire was visible from windows facing the north; it disappeared without a report. The most remarkable was a bolis, which, about 6 P.M. on October 25, 1855, took a N.W. by W. path, burned during ten or eleven minutes, and frightened the superstitious burghers into fits. Water-spouts commonly appear during the month of April, and in the direction of the mainland: the people disperse them by firing guns.
Frost and snow are of course unheard of at Zanzibar, and hail, not uncommon in the interior, never (?) falls upon the island or the coast. During the wet season generally, especially when the heats are greatest, the hills of Terrafirma are veiled with clouds, and sheet-lightning plays over the horizon. The islanders assure the stranger that storms of thunder and lightning are rare, and that few accidents happen from the electric fluid. M. Alfred May, for instance, declares that thunder is heard only three or four times a year. The same is said in West African Yoruba, in parts of the Brazil, and even in Northern Syria—Damascus, for instance. It would be curious to inquire what produces this uniform immunity under climatic conditions so different. At Zanzibar, however, the phenomenon is irregular as the seasons. I was told of several deaths by the ‘thunderbolt,’ and in the year 1857 the S.W. monsoon was ushered in almost daily by a tempest. Lieutenant-Colonel Hamerton, when sailing about the island, lost by lightning his Baloch Sarhang (boatswain); he himself felt a blow upon the shoulder like that of a falling block. No blood appeared upon the side, but it was livid to the hip, and for some days the patient was decidedly ‘shaky.’ Some explained his escape by his wearing flannel; others by his standing near the davits of a longboat, which were twisted like wax by the electric fluid.
The mainlands of Zanzibar and of Mozambique are subject, as might be expected, to tornados, which much resemble those of the West African coast. Accompanying the formation and the dispersion of the nimbus, they are often violent enough to wreck small craft. Caught in a fine specimen, I was able to observe all the normal phenomena,—the building up of the warning arch, the white eye or gleam under the soffit, the wind blowing off shore, the apparent periodicity of throbs, and the frantic rage of the short-lived squall. The cyclones and hurricanes of the East Indian Islands rarely extend to Zanzibar. During 14 years there was but one tourbillon strong enough to uproot a cocoa-tree. It passed over the city about midnight, overthrowing the Mábandani or roof-sheds, and it was followed by a burst of rain. Colonel Sykes (loco cit.) remarks, philosophically explaining the why, ‘Another peculiar feature in the climatology of Zanzibar is that there is seldom any dew experienced.’ The reverse is the case, as might be known by the strength of the nightly radiation. Captain Guillain (i. 2, 72) declares that the rosées which accompany the rains are sufficient for watering the ground, and observes (p. 94), I presume concerning those who remain in the open air, ‘Rester à terre entre huit heurs du soir et le lever du soleil c’est s’exposer à une mort très probable, sinon certaine.’ The sunset, never followed by twilight, is accompanied by a sudden coolness which, as in equatorial, and even sub-tropical regions generally, causes a rapid precipitation of vapour. The dews are cold and clammy, and the morning shows large beads in horizontal streaks of moisture on perpendicular surfaces. I often remarked the deposition of dew when light winds were blowing; of course it did not stand in drops, but it wetted the clothing. This I believe is an exception to the general rule. At sunset the old stager will not sit or walk in the open air, although, as in Syria, he will expose himself to it at nine or ten p. m., when the night has acquired its normal temperature. As in the west coast squadron, so here, there is an order that all men on deck after sunset must wear their blanket-coats and trowsers, and many an unfortunate sailor has lost his life by sleeping in the streets, thus allowing the dew to condense upon his body while under the influence of liquor. Experienced travellers have taught themselves, even in the hottest seasons of the hottest equinoctial regions, to air the hut with a ‘bit of fire’ before sundown and sunrise, and it is doubtless an excellent precaution against ‘chills.’
Zanzibar Island, lying in S. lat. 6°, has the sun in zenith twice a year: the epochs being early March and October; more exactly, March 4 and October 9. Hence it has two distinct summers; the first in February, the second in September. It has double rains; the ‘Great Masika’ in April to June, and the ‘Little Masika’ in October to November. It has two winters; the shorter in December, and in July the longer, which is much more marked than the former. There are only three months of N.E. trade (Azyab)[[40]] to nine of S.E. and S.W. (Kausi). The regularity of these seasons is broken by a variety of local causes, and there is ever, I repeat, the normal instability of equinoctial climates. Theory appears often at fault upon these matters. A fair instance is Mr Cooley’s assertion, that about Kilima-njaro the ‘rainy season is also the hot season.’ Theoretically, of course, the period of the sun’s northing and of the great rains should be, north of the equator, the hot season; but where tropical downfalls are heavy, the excessive humidity intercepting the solar rays, and the valleys and swamps refrigerated by the torrents, make the rainy season the cold weather. From June to September the natives of Fernando Po (N. lat. 4°) die, like those of eastern intertropical Africa, of catarrh, quinsey, and rheumatism. Even in India the Goanese call the rains ‘o inverno,’ and Abba Gregorius makes the wet weather the winter of Abyssinia. About Kilima-njaro the hot and dry season opens with the end and closes with the beginning of the hot monsoon.