Or more, annually) on the Westward or windward slope, and there they are heavily forested; but the rainfall is light on the eastward slope and the piedmont district is dry; hence the forests thin out on that side of the range and treeless lava plains follow next eastward.
In altitude, having among them narrow valleys in which the vegetation is scanty, with exceptional regions of greater fertility in the neighbourhood of the coasts, where the rainfall is greatest.
The average annual rainfall decreases quite regularly westward and south-westward from 47.6 in.
The Yeres, a tributary of the Seine, and the Grand Morin and Petit Morin, tributaries of the Marne, are the chief rivers, but the region is not abundantly watered and the rainfall is only between 20 and 24 in.
For Honolulu the mean annual rainfall (1884-1899) was 28.18 in.; the maximum 49.82; and the minimum 13.46.
On the varying strength of these two monsoons the rainfall of the district depends.
That great separation of positive and negative electricity sometimes takes place during rainfall is undoubted, and the charge brought to the ground seems preponderatingly negative.
Irrigation, which has not been used to any great extent, is needed in some parts of the country for the best results, but in others, as in the valleys and on the northern slopes of the Maritime Andes, the rainfall is sufficiently well distributed to meet most requirements.
In the Tehama occasional showers fall during the winter months; at Aden the average rainfall for the year is 2.97 in., but during 1904 only 0.5 in.
In the dry, saline regions of the west and north-west, where the rainfall is slight, there are large thickets of low-growing, thorny bushes, poor in foliage.