CIRRO-NEBULA CHANGING TO CIRRO-CUMULUS.
These changes of form continued until the whole was hidden from view by a veil of much lower stratiform cloud, one advance portion of which is shown. Plate [4] does not represent a type or a distinct variety of cloud. It is an intermediate form, or a temporary condition, showing cirro-nebula in the act of changing into cirro-cumulus, or possibly cirro-stratus.
Cirro-nebula itself, in its simpler form, is, however, a distinct type. It is true that it never persists over one locality for more than an hour or two without passing into some denser form, but while it lasts its features are so distinctive, and the optical phenomena to which it gives rise are so striking and significant, that it is a matter for surprise that it should in the International system have been relegated to the position of a subordinate variety of cirrus. It is more nearly related to cirro-stratus, but is sufficiently distinct from that to deserve at least specific rank.
True typical cirrus must have a plainly shown fibrous structure. The fibres may cross and interlace, they may radiate in fan-like manner, or they may curl and twist like a well-trimmed ostrich feather. The clouds so formed must not be arranged in a continuous level sheet, or they at once become cirro-stratus, and it is impossible to invent a definition which will mark the exact limits of either type. Typical cirrus consists of detached clouds. They cast no shadows on the landscape, for the simple reasons that they are semi-transparent and their component parts too narrow. If the sun is shining down obliquely through the naked boughs of a tall tree, it will be seen that the lowest twigs cast fairly sharp shadows on the ground, but that even these are bordered by a fading rim; the twigs further up cast no sharp shadows, but broader faint bands of shade; while the topmost boughs cast no shadows which can clearly be identified. In other words, the more distant the narrow twig is from the ground the narrower the real shadow or umbra, and the broader the penumbra becomes, until when the distance is sufficient the shadow is all penumbra. Cirrus filaments throw nothing but a faint penumbra. Indeed, it is only when they lie in the earth’s shadow, and stand against the background of a faintly lighted sky, that they show any sign of shadow even on themselves.
There is no doubt that they are composed of particles of ice. They are formed at altitudes where the thermometer must be many degrees below freezing-point, and not a few of the thinner examples show fragmentary halos like those of cirro-nebula.
Their actual altitudes are very variable, being greater in summer than in winter, and reaching a maximum for any given station after a long spell of hot weather. Exact measurements have not yet been made in tropical latitudes or in polar regions, but there is every reason to expect that the upper limit of cirrus for equatorial districts will be found to be much higher than in the temperate zones where actual observations have been made. In places nearer to the Arctic Circle it is also almost certain that the altitudes will be less.
In the New England states, as shown by the Blue Hill observations, the maximum altitude for summer was found to be little under 15,000 metres. At Upsala, in Sweden, it was 13,300 metres. The average altitudes at the same observatories were, respectively, about 9900 and 8800 metres. At Exeter the writer’s own measurements give an average for the summer months of 10,200 metres, with a minimum rather lower than was the case in America or Sweden, and with a maximum far above the foreign values. In winter cirrus certainly comes much lower down, but the number of observations is fewer.