RAIN-CLOUD.
(Nimbus.)
Plate 36.
RAIN-CLOUD.
(Nimbus.)
Nimbus, indeed, is not a type-form, but is merely a typical condition, and when used as a substantive is only a convenient way of expressing our ignorance as to the real form of the cloud we so describe.
The altitude of the base of a rain-cloud may vary considerably. It may be anything from sea-level up to heights which vary with the geographical conditions and with the conditions of temperature and pressure, but probably in this country never greater than 7000 or 8000 metres.
Rain, or snow, often falls from clouds at greater altitudes than these, but unless in its descent it passes through other lower clouds, the drops, as a rule, will dry up and disappear. The author has often seen quite heavy rain descending from a cloud, and disappearing completely within a thousand feet or so of the cloud-base. On rarer occasions a still more remarkable thing may be seen—namely, a shower falling from an upper cloud into a lower, and none between this lower cloud and the ground. This curious phenomenon can only be explained by supposing that the convection currents which make the lower cloud are strong enough to support the small raindrops.
Pure stratus is a level sheet of cloud with little variation of thickness, not ascending every here and there into rounded lumps. Its most typical form covers the whole sky with a uniform grey pall, which may or may not completely hide the sun. Such a cloud does not lend itself to pictorial representation. A frequent form, in which the sheet is more or less broken, is shown in Plate [37]. This is a variety which is frequent in the summer mornings, and generally breaks up and clears away before eleven o’clock. If, however, it appears in autumn and winter with layers of alto cloud above, it may grow denser, and turn into a stratiform nimbus, or it may go on drifting overhead for several days without sign of change.