back to the wind the pressure decreases toward your left, and increases toward your right."

For navigators, this law is more generally expressed in the words of the Hydrographic Office on "Cyclonic Storms."

"Since the wind circulates counter-clockwise in the northern hemisphere, the rule in that hemisphere is to face the wind, and the storm centre will be at the right hand. If the wind traveled in exact circles, the centre would be eight points (90 degrees) to the right when looking directly in the wind's eye. But the wind follows a more or less spiral path inward which brings the centre from eight to twelve points (90 to 135 degrees), to the right of the wind. The centre will bear more nearly eight points from the direction of the lower clouds than from the surface wind."

Fig. 11. Skiron—The Northwest Wind

The law given on the preceding page is named after C. H. D. Buys Ballott, a Dutch meteorologist. It was announced in a paper published in the Comptes rendus in 1857. Two American writers on the Winds, J. H. Coffin and William Ferrell, had however earlier found the law to hold.

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While most of us study storms from a window at home and are not called upon to handle a ship in a storm, yet it may not be out of place to include here the diagram of the winds in an ideal storm and give the rules for maneuvering. See Figure 12. The Winds in an Idealized Storm. The rules apply only to storms in the northern hemisphere.

"Right or dangerous semicircle,—Steamers: Bring the wind on the starboard bow, make as much way as possible, and if obliged to heave-to, do so head to sea. Sailing vessels: Keep close-hauled on the starboard tack, make as much way as possible, and if obliged to heave-to, do so on the starboard tack.

Left or navigable semicircle,—Steam and sailing vessels: Bring the wind on the starboard quarter, note the course and hold it. If obliged to heave-to, steamers may do so stern to sea; sailing vessels on the port tack.

On the storm track in front of center,—Steam and sailing vessels: Bring the wind two points on the starboard quarter, note the course and hold it, and run for the left semicircle, and when in that semicircle manoeuvre as above.

On the storm track, in rear of center,—Avoid the center by the best practicable route, having due regard to the tendency of cyclones to recurve to the southward and eastward."

Fig. 12. The Winds in an Idealized Storm

WIND AND ALTITUDE

The law of the turning of the wind with altitude.

A casual observation of the lower clouds where no means of measuring small angles is available will not usually show any difference between the motion of the clouds and the surface wind; but with the upper clouds the case is different, and one readily detects a difference.

Several thousand observations with various agencies, such as kites and pilot balloons and more especially measurements made with theodolites and nephoscopes, show that there is a definite twist to the right with elevation. The amount of the deflection is shown in Figure 13. Turning of the Wind with Altitude. Here the average yearly values are given for directions and velocities. Thus if the mean wind direction at Blue Hill is from a point a little to the north of west, 306 grads or 275 degrees, and the mean velocity 7 metres per second; the clouds at 1000 metres elevation will move from 312 or 280 degrees and at a speed of approximately 11 metres per second (24 miles an hour).

These however, are average values. In individual cases the difference between surface winds and stratus clouds may be considerably greater. It may be as much as 180 degrees; that is, the cloud may move directly opposite to the wind. In general there will be a difference of 10 to 20 degrees.

WIND AND RAIN

The law of wind direction, approximate cooling and rain.

When the lower clouds are moving from the north or northwest, without sharply defined edges, the LOW is east or northeast of the observer; and rain or snow is not likely unless there is a rapidly falling temperature.

Fig. 13. Turning of Wind with Altitude

When a stream of warm air with a high absolute humidity flows north on the east side of a LOW, and a cold northwest wind follows quickly after the LOW, rain or snow may be expected.

Any rapid chilling of warm, moist air produces cloudiness and rain or snow; but a cold stream blowing into a warm area will not produce as much rain as a warm stream blowing into a cold area.

DURATION OF WIND

The average duration of wind from various directions is as follows:

From the north about 16 hours each week; from the northeast, the same; from the east, 11 hours; from the southeast, 10 hours; from the south, 24 hours; from the southwest, 27 hours; from the west, 33 hours; and from the northwest 31 hours.

During an individual disturbance lasting about 36 hours, we may have 8 hours of southwest wind; 4 hours of west wind, backing during the next 4 hours to south; 2 hours of south wind; 2 hours of southeast wind; 2 hours of east wind; 8 hours northeast wind and 4 hours north wind, 2 hours northwest, when it may be considered that a new pressure distribution prevails.

The above values hold only for a storm moving with normal velocity. LOWS are often blocked by slow moving HIGHS in advance. In such cases the duration of east winds is greater.

THE WINDS OF A YEAR

The following table shows the marked increase in the prevalence of northwest and west winds during winter months, the decrease in north winds during July, the increase in northeast winds in May, also in east winds; the increase of south and southwest winds in July; and the falling off of southeast winds in December. See Table, page 72.

In cities near the Atlantic Coast, a continuance of northeast wind, especially in the fall and winter months, results in frequent altho not necessarily heavy rains. On the other hand a period of continued northwest and west wind is a dry period.

In summer, southeast and east winds bring fog and cooler weather; while southwest winds are favorable for the development of thunderstorms.

WINDS OF A YEAR

TABLE I.—Number of Hours the Wind Blows from Different Directions.