2. The force of the wind is estimated according to Beaufort's Scale (0-12). After a little exercise, very satisfactory results may be attained by mere estimation, but the observer must not be deceived by gusts of wind. He should note the average direction and force of the wind prevailing from about ten minutes before to about ten minutes after the time for which it is registered. In case of a typhoon, the extreme force of the wind since the last observation should also be registered.
BEAUFORT SCALE.
Velocity.Miles per hour. Force. 0 Calm,.... Light air, 1-2 Light breeze, 3-4 Gentle breeze, just sufficient to give steerage way, in which a well-conditioned man-of-war, with all sails set, and clean full would go in 1-2 knots 5-6 Moderate breeze, 7 Fresh breeze, 13 Strong breeze, or that in which she could just carry royals, &c., in chase, full and by smooth water. 18 Strong breeze, Top gallant sails, 23 Fresh gale, Topsails, jib, &c., 27 Strong gale, Reefed upper topsails and courses, 28 Strong gale, 34 Whole gale, Lower topsails and courses, in which she could scarcely bear lower maintopsail and reefed foresail, 40 Whole gale, 48 Storm, which would reduce her to storm-stay-sails, 56 Storm, 65 Storm, 75 Typhoon, which no canvas could withstand, above 80 Typhoon
VIII.-Clouds.
1. Water vapour when condensed forms into minute drops of water, which if at the level of the ground appear as fog, but if at a higher level are called cloud. Clouds and fog have the property of reflecting light, but the same cloud that dazzles the eyes of the observer in one place, may at the same time, if thick and heavy enough, obscure the light and present a gloomy and threatening appearance at another place. Observations of clouds are of great interest, as they show us what takes place in the upper regions of the atmosphere.
2. The nebulosity, i.e., the amount of clouds, is expressed in parts (0-10) of the whole sky. 0 means no clouds, while 10 indicates that the whole sky is covered. In noting the nebulosity, be careful not to be led astray by a certain optical delusion. The sky does not look to us like a sphere. Overhead, it seems nearer to us, so that objects, such as, for instance, the moon, look much larger when near the horizon than they do when high in the sky. In registering the average amount of clouds during a past period, and also for weather telegraphy, a less exact notation is adopted: b, blue sky, indicates that at most 2 tenths are covered, e, detached clouds, that between 3 and 7 tenths are covered, i.e., that it is roughly speaking half overcast, while o, overcast, indicates that at least three-quarters of the sky are covered.
3. The direction whence the clouds come should be carefully observed. It is of special importance to note the direction from which the upper clouds (clouds of the cirrus type) come. If the clouds are high in the sky, this is easily noted. The observer should so place himself as to have some fixed object, such as a projecting corner of a house, a chimney, or a flagstaff between his eye and a recognizable portion of the cloud. If a window is situated exactly to the north, south, east, or west, the cloud may be observed through it with advantage. But if no part of the cloud is high in the sky, or is traveling directly towards or from the observer, the effects of perspective render it in the beginning somewhat difficult to estimate the direction, but an observer who takes an interest in the subject soon learns to estimate it properly.
If clouds appear to be moving very fast, a note to that effect should be entered in the register.
4. The form of clouds should be entered, as they are seen at the time of observing. Clouds exist at very different levels in the atmosphere, but certain forms, which consist of ice crystals, are usually found at a very high level, while others, consisting of minute particles of water, are nearer to the surface of the earth. The different varieties of cirrus are upper clouds, all the others lower. The lower clouds usually move in the same direction as the wind, but the upper clouds often come from a quite different quarter. It is of very great importance that the direction whence the upper clouds are coming should be registered.