CHAPTER XIII.

ON CIRRUS CLOUD OBSERVATIONS.

THE following Instructions, devised by the Rev. W. Clement Ley, were adopted by the Meteorological Council in 1885, and have been issued to such of the observers connected with the Office as are willing to undertake the reporting of cirrus movements:

The attention of the observer is especially directed to clouds of the 'cirrus' type, including what are heie defined as true cirrus, sheet-cirrus, high cirro-cumulus, and cirrus-haze. They are the highest kinds of cloud, and it is to these types only that the following Instructions refer.

True Cirrus.-When in moderate quantities it is commonly white, though when seen through haze it is usually somewhat cream-coloured, and when the sun's rays have reached it through a long stratum of hazy atmosphere, is often of either an orange or rosy tint. In all cases it has a very delicate appearance. It is sometimes arranged like bunches of fine hair; and such tufts of cirrus are often called 'mare's tails.' times it resembles small curled feathers.

At other Quite as

commonly, however, it lies in thin light strands, like pale gossamer threads.

Sheet-Cirrus.-When cirrus overspreads as a continuous sheet a large portion of the sky it becomes what is here termed 'shect-cirrus.' In this state, when not too thick, it produces 'halos' or large rings round the sun and moon. The sheet-cirrus sometimes appears fibrous, sometimes reticulated. When the veil of this cloud becomes thick it assumes some neutral or muddy tint, except in those cases when it is so disposed that the rays of the rising or setting sun are reflected to us from its under surface, which then appears of either an orange or rosy colour.

'High cirro-cumulus' differs from simple cirrus in consisting of small detached masses somewhat rounded in form, a great flotilla of which is often seen in the sky, especially in fine summer weather. Clouds at lower levels are frequently disposed in the same manner, but the observer must be very careful to distinguish between these and the high cirro-cumulus, which latter is either white, or changes its colour under the same circumstances as true cirrus, no part of any of the cloudlets ever seeming to be decidedly thrown into shadow by another part. The high cirro-cumulus also possesses the same faintness and delicacy of outline which we observe in cirrus,

Cirrus-Haze.-Sometimes cirrus is only visible as either a milky or an oily-looking haze, which is here termed cirrus-haze.

I.

Amount. Whenever on looking round the sky the observer notices clouds of the kinds above described, he should in the first place endeavour to ascertain their amount. In doing this he should observe whether they cover, (1) only a trifling portion; (2) about a quarter ; (3) about a half; (4) about three-quarters; or finally, the whole of the sky. The extent, not the density, of the cloud is here referred to. If there be any considerable quantity of lower cloud in the sky, so that the amount of upper cloud cannot be accurately ascertained, it should be invariably entered as doubtful, by a '?'

II.

Direction of Movement.-The next thing to be attended to is the direction from which the upper clouds move. Considerable difficulty will here be encountered. If some of the clouds happen to be nearly overhead, attention should at first be confined to such; and if possible the observer should so place himself as to have the projecting corner of a roof or chimney, the summit of a steeple, flagstaff, or other stationary object, very close to the line between his eye and the portion of cloud which he is about to watch.

A convenient plan is to set up a pole reaching 5 feet 71 inches above the level of the observer's eye; a mark or ring should be placed round the pole at the

latter level.

The summit of the pole must carry two thin rods, fixed crosswise, and set truly to the four cardinal points. Through or near the extremities of these rods should pass a thin circular iron ring of 3 feet diameter, the use of which will be mentioned presently. The observer should, when opportunity offers, so station himself that some recognisable part of a cloud appears to move vertically either upwards from the top of the pole, or vertically downwards towards it. The direction of the pole from the observer's position, which may be judged of by the cross rods, is then, in the first case, the direction of the upper current; in the second, its opposite. In watching the movements of the clouds, the observer may conveniently use a staff, one end of which is sharpened so as to stick in the ground, while the other is fitted with a crescent-shaped top in which to rest the arm, in order to support and steady the head. If the clouds be observed from a window it is some assistance to have a few wires-some horizontal, others vertical-fastened across the window to the wall on the outside.

When, however, no part of the cloud is moving directly either towards or away from the observer, the effects of the perspective render it difficult to estimate the direction of movement exactly. To perfect oneself in the art, it is advisable as often as possible to endeavour to estimate the direction of movement of some portion of the cloud which is inconveniently placed, and then to find the actual direction of

movement of another portion of the same cloud, or same sort of cloud, which happens at the time to be conveniently placed. By patiently practising the eye in this process the observer eventually learns to make, automatically, tolerably exact allowance for the errors arising from perspective. In no case (should he have the least doubt as to the direction of movement) must any conjecture be telegraphed, but the figure for 'motion doubtful' should be inserted in the telegram.

It is always well, when observing the movements of the upper clouds, either to rest the head against a wall or tree, or to support it on a staff, or by some other means, so as to be quite motionless while taking an observation.

III.

Apparent Velocity.-The next particular to be attended to is the apparent velocity of motion in the clouds-i.e., the force of the current which carries them. The observer is requested to notice whether the cloud be actually motionless, or whether its motion is very slight, moderate, rapid, or very rapid. Clouds of the cirrus type are seldom really motionless, but owing to their great distance they commonly, though not always, appear to move more slowly than the lower clouds. estimate, even according to the very rough scale proposed, the apparent velocity of movements, we have to encounter precisely the same difficulties which have presented themselves in finding the direction, and the

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