The average mean annual temperature is about 82.5°, the maximum in the shade between 93° and 94° and the minimum 73°. March and April are the hottest months, with means of 82° and 83° and a maximum of 89°, while August to December—when the mean temperature is 79°—is the coolest part of the year. The mean annual temperature at Nankauri is 80°, and while the highest reading recorded is 99°, the lowest is 70°. The mean diurnal range there varies between 9° and 11° only.
Although the seasons of the monsoons are the same, they are not so well defined among the Nicobars as on the coast of the Bay of Bengal generally; but heavy rains occur in May, June and July—when the south-west monsoon is at its height—and rains rarely cease until December. March is the driest month, and while from May to December there is an average monthly rainfall of 12 inches, with twenty wet days per month, for the rest of the year the monthly average is only 2.9 inches, with showers on twenty-six days only.
At Nankauri the mean humidity is 79 per cent, and the annual rainfall 110 inches; while, as regards the southern group of islands, there is good ground for the belief that much more rain falls, probably not less than an average of 150 inches annually; this is doubtless attributable to the forest-clad mountains of Great and Little Nicobar.
The prevailing winds are the monsoons—the south-west from the beginning of May till mid-October, followed by variable winds to the end of the year; the north-east monsoon from January until April, with an interval of more variable winds before the other sets in. Hurricanes seldom visit the islands, but in March 1892 the central group was subjected to a cyclone which caused much destruction in the forest. During the south-west monsoon frequent thunderstorms and gales of wind occur, especially in the vicinity of Great Nicobar. The north-east monsoon brings fine weather, but sometimes blows with considerable strength.
A remarkable feature of the Nicobars is the manner in which the general botanical appearance of the islands coincides with the geological division, for, while the southern group (Great and Little Nicobar with Kachal) are wooded from beach to summit, the forests of the other islands are restricted to the plutonic rocks and the slopes and dells of the older alluvium, while the hilly plateaux and ridges are covered with park-like grass heaths.
The most prominent features of the flora are, perhaps, the quantities of Barringtonia speciosa, which, with their large shiny leaves and beautiful crimson-tipped tassel-like blossoms, grow all along the coasts; the tall screw-pines (Pandanus larum), bearing the immense fruits that provide the main food of the natives; and the graceful Nicobar palms (Ptychoraphis augusta), which occur in all the forests. Giant bamboos are extremely scarce, but the climbing species (Dinachloa) is common everywhere in the jungle, and beautiful tree ferns (Alsophila albo-setacea) grow in the forest and along the river banks of the south.
A mangosteen (Garcinia, sp.) and a cinnamon (Cinnamomum obtusifolium) grow wild, as do the pepper vine (Piper betel) that supplies the sireh leaf, and the betel palm (Areca catechu). These two are also cultivated, and it is said that the latter is not indigenous.
The large numbers of milky climbers leads to the hope that some rubber-yielding varieties may be discovered capable of supplying a sufficient quantity of raw material for export. The vanilla orchid occurs, and the southern forests produce quantities of rattan, both as a small variety that is exported, and a large cane two inches or so in diameter, which the natives use for the horizontal rafters in the circular framework of their houses.
Semecarpus heterophyllus, Morinda citrifolia, Artocarpus lakoocha, and A. chaplasha, Cordia mixa, Mallotus philipenensis, and Amomum fenzlii, may be mentioned specially as species capable of yielding commercial products; but their sparseness, coupled with the fact that it is easier and cheaper to cover the soil with coconuts and areca palms, puts out of the question the possibility of utilising the species to any profit.
The Nicobars produce few trees of any commercial value as timber, and those probably not in large quantities: the best of these are Myristica irya and Terminalia bialata, and of secondary value in this respect are Mimusops littoralis, Hopea odorata, Artocarpus chaplasha and lakoocha, Calophyllum spectabile, Terminalia procera and species of Garcinias.