The apparent difference in character between the lower and central clouds is dependent chiefly on proximity.

The clouds which I wish to consider as characteristic of the lower, or rainy region, differ not so much in their real nature from those of the central and uppermost regions, as in appearance, owing to their greater nearness. For the central clouds, and perhaps even the high cirri, deposit moisture, if not distinctly rain, as is sufficiently proved by the existence of snow on the highest peaks of the Himalaya; and when, on any such mountains, we are brought into close contact with the central clouds, we find them little differing from the ordinary rain-cloud of the plains, except by being slightly less dense and dark. But the apparent differences, dependent on proximity, are most marked and important.

Their marked differences in colour.

In the first place the clouds of the central region have, as has been before observed, pure and aërial greys for their dark sides, owing to their necessary distance from the observer: and as this distance permits a multitude of local phenomena capable of influencing colour, such as accidental sunbeams, refractions, transparencies, or local mists and showers, to be collected into a space apparently small, the colours of these clouds are always changeful and palpitating; and whatever degree of grey or of gloom may be mixed with them is invariably pure and aërial.