Wind and Weather Part 2

A surge of cold air from Alberta or farther north reached the international boundary January 21st and spread slowly eastward, reaching the Great Lakes on the 24th and the St. Lawrence Valley two days later.

Then seemingly it halted or moved slowly westward, retrograding. In three days, that is, on the 29th, the centre of the HIGH was apparently 500 miles _west_ of where it had been on the 27th. After the 29th it followed a normal track, moving slowly southeast, reaching the Atlantic near Long Island.

Meanwhile a depression on the south coast of Texas on the 25th, moved across the Gulf of Mexico, pa.s.sing over Southern Florida on the 27th and advanced steadily northeast, reaching Cape Hatteras in 24 hours. Owing to the presence of the anticyclone referred to above, the depression recurved off Hatteras. The result was a memorable snow storm in Northern Virginia and Maryland. At 8 p.m. January 27th, there had been a fall of 5 cms. (2 inches). Within the following twenty hours the average depth in the city of Washington was 66 cms. (26 inches). The weight of the snow caused the collapse of the roof of the Knickerbocker Theatre and the death of 97 persons.

The total snowfall in various coast cities was:

Raleigh 24 cms.*

Richmond 48 "

Washington 71 "

Baltimore 67 "

Wilmington 46 "

Philadelphia 31 "

Trenton 27 "

New York 18 "

New Haven 8 "

Boston 1 "

*Note: To convert to inches multiply by 0.4.

The table shows clearly how the snow was formed. On the east side of the LOW a stream of air, relatively warm, carried a load of water vapor, approximately 13 grams in each cubic metre.

[Ill.u.s.tration: BASE MAP BY GOODE

FIG. 10. PATHS OF HIGH AND LOW, GREAT SNOW STORM OF JANUARY 27-28, 1922]

This current was steered around the north side of the LOW and met the north-northeast wind. Under the new conditions the air saturated could hold only 2 or 3 grams; and so condensation and heavy precipitation resulted. The region of maximum snowfall was near Washington, and it will be seen that there is a proportional decrease north and south. The snowfall at Washington was the heaviest ever known at that city.

Unlike most storms, there was no strong cold northwest wind blowing into the depression. The temperature rose slowly. It was less a contrast of winds than a steady slow outward push of the anticyclone, and the consequent turning of the path of the cyclone eastward.

LAWS OF FORECASTING

Buys Ballot's Law.

"If you stand with your 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."

[Ill.u.s.tration: 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.

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."

[Ill.u.s.tration: FROM HYDROGRAPHIC OFFICE

FIG. 12. THE WINDS IN AN IDEALIZED STORM]

WIND AND ALt.i.tUDE

The law of the turning of the wind with alt.i.tude.

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 Alt.i.tude. 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.

[Ill.u.s.tration: TURNING OF WIND WITH ALt.i.tUDE, BLUE HILL

FIG. 13. TURNING OF WIND WITH ALt.i.tUDE]

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.