Wind and Weather Part 1

Wind and Weather.

by Alexander McAdie.

THE TOWER OF THE WINDS

In Athens on the north side and near the base of the hill on which the upper city--the Acropolis--is built, there is a small temple still standing, altho its walls were completed twenty-two centuries ago. It is known as the Tower of the Winds; but as a matter of fact, the citizens of Athens used it to tell the hour of the day and the seasonal position of the sun. It was a public timepiece. It served as a huge sun dial.

Water from a spring on the hillside filled the basins of a water clock in the bas.e.m.e.nt of the Tower. And so, whether the day was clear or cloudy the measure of the outflow of water indicated the time elapsed.

Also there were markings or dials on each of the eight walls of the temple, and the position of the shadow of a marker indicated the seasonal advance or retreat of the sun as it moved north from the time of the winter solstice and then south after the summer solstice.

The sun is not an accurate time keeper and no one to-day runs his business or keeps engagements on sun time. But the old Athenians were quite content to do so; and their Tower served excellently for their needs. And they did what we moderns fail to do, namely, give distinctive names to the winds. They represented figuratively the characteristics of the weather as the wind blew from each of the eight cardinal directions.

[Ill.u.s.tration: FIG. 1. THE TOWER OF THE WINDS

Erected in Athens, on the north side of the Acropolis, B. C. 150]

The allegorical figures of the winds used in this little book are reproductions of the eight bas-reliefs in the library of the Blue Hill Observatory, placed there by the late Professor A. Lawrence Rotch. They are copied from the frieze of the Tower of the Winds at Athens.

THE NAMES OF THE WINDS

Boreas, the north wind, is perhaps the most important of all winds. At Athens this a cold, boisterous wind from the mountains of Thrace. The noise of the gusts is so loud that the Greek sculptor symbolized the tumult by placing a conch sh.e.l.l in the mouth of Boreas. His modern namesake, the Bora of the Adriatic, is the same noisy, bl.u.s.tering, cold wind-rush from the north.

The northeast wind Kaikias is a trifle more pleasant looking than Boreas, but still not much to brag about. Master of the squall and thunderstorm, he carries in his shield an ample supply of hailstones, ready to spill them on defenseless humanity. He might well serve as the patron saint of air raiders dropping their bombs on helpless humans below.

Apheliotes, the east wind, is a graceful youth, with arms full of flowers, fruit and wheat. Naturally this was a favorite wind, blowing in from the sea, with frequent light showers. Some of us who dwell on the Atlantic Coast, in more northern lat.i.tudes than Athens, do not always regard with favor the east wind, a.s.sociating it with chilly, damp and sombre weather. Yet it is the harbinger of good--tempering the cold of winter and the heat of summer. It is an angel of mercy in mid-summer when the temperature is above the nineties and there is no air stirring.

Then it is, that we all welcome the refreshing wind from the sea.

Euros, the southeast wind, and neighbor to Apheliotes, is a cross old fellow, intent on the business of cloud making. He alone of all the winds carries nothing in his hands. In the New Testament he becomes Euroclydon, wind of the waves. He is no friend of the sailor; and the seasick traveler prays to be rid of his company.

The figure on the south face of the tower, Notos, is the master of the warm rain. He carries with him a water jar which has just been emptied.

Compare his light flowing robes and half-clad neck and arms with the close fitting jacket of old Boreas. At his shrine, hydraulic engineers well might worship.

Next, the Mariner's wind, Lips, the southwest favoring breeze bringing the ships speedily into harbor; yes, into that Piraeus, famed in cla.s.sic history. Incidentally it is the southwest wind which differentiates the climate of Great Britain from that of Labrador. This wind makes Northwest Europe habitable; while on the other side of the Atlantic, in similar lat.i.tudes, but under the influence of prevailing northwest winds, we find Labrador--a section certainly misnamed, for it is not the abode of farmers, as the name implies--but barren and bleak. What a difference it would make thruout this region if the Gulf Stream continued north, close to the sh.o.r.e, and the prevailing winds were _from the east_. Our North Atlantic Coast would then be _the land of zephyrs_, using the word in the sense of pleasant, gentle winds.

[Ill.u.s.tration: FIG. 2. BOREAS--THE NORTH WIND]

Zephyros, the west wind, is represented as a graceful youth, scantily clad, with his arms filled with flowers. In Greece this wind traversed the Ionian Sea and the Gulf of Corinth before reaching Athens. It is quite unlike our west wind which blows across a continent, and is continuously robbed of its water vapor on the long pa.s.sage. The Ionian wind is pleasantly moist and refreshing.

Last of all, but by no means least important, is Skiron, lord of gusty northwest gales. Freezing in winter, parching in summer, he carries with him a brazen fire basket and spills a generous stream of hot air on all below. His husky Highness might not inappropriately adorn legislative halls and editorial sanctums. He would displace the blindfolded lady holding scales very much out of balance. Think of the deep significance of his presence.

In our country the northwest is of all winds, except the west, most persistent. For 1600 hours in a year, this wind is with us. Joining forces with the west wind, these directions prevail one third of the time. These northwest-west winds also have the greatest speed and gustiness. The climate of the United States is essentially determined by the prevalence of the north, northwest and west winds.

FORECASTING THE WEATHER

In old days, the _haruspices_ (for this is what the Romans called weather men in the days of Caesar) proclaimed the will of the G.o.ds by consulting the entrails of some freshly killed animal. Evidently these haruspices did not always make correct forecasts; for there were some Romans who openly questioned their worth. Cato, the Censor, is on record as saying "that he wondered how one haruspex could look another in the face without laughing!"

[Ill.u.s.tration: FIG. 3. KAIKIAS--THE NORTHEAST WIND]

The modern professional forecaster would scorn to consult the entrails.

There are however many amateur forecasters who foretell weather by their aches and rheumatic pains. Probably there is a high correlation factor between body sensations and dampness; and some individuals are quite sensitive to changes in both relative and absolute humidity. This, however, does not always mean that a storm is approaching. Humidity or dampness is only one factor and may be quite local, whereas most storms are wide-spread.

THE WEATHER MAP

The official forecaster consults a daily weather map and certain auxiliary maps which show changes in pressure and temperature for twelve hours or more. He examines closely the contours of pressure as shown on the map. The synoptic map, as it is called, because it is a glance at weather conditions over a large area at one and the same moment, is a map on which are plotted pressure, temperature, wind direction, velocity and rainfall. The lines of equal pressure or isobars generally curve and inclose what is known as a cyclonic centre, or depression or LOW. The arrows point in, but not exactly toward the centre of the depression.

On the map there will probably appear also an area of high pressure where the surface air flows leisurely outward and away from the place of highest pressure. Such an area is called an anticyclone, a word first used by Sir Francis Galton in 1863 to designate not only high pressure, but general flow of the air in a reversed or opposite direction to that of the low area or cyclone. The word cyclone was first used by Piddington in 1843 in describing the flow of the air in the typhoons of the East Indian Seas. It is from the Greek and literally means the coils of a serpent. The word cyclone must possess some special merit in the minds of journalists for it is quite commonly misused for tornado in descriptions of the smaller and more destructive storm.

THE LOW

Cyclone is simply the generic name for a large rotating air ma.s.s. It is a barometric depression or LOW and is characterized by a flow of air inward and around a moving centre. The air circulation is counter-clockwise in the northern hemisphere and clockwise in the south.

Perhaps if the earth stopped rotating and there was no planetary circulation, with the great west-moving trades and east-moving "westerlies," the arrows on the weather map would all point directly toward the centre of the LOW; but, as things are, there are some very good reasons why air can not move directly into a LOW, that is at right angles to the isobars.

Moreover, the weather map does not indicate the true flow of the air, for observations of the wind made at the ground tell only a part of the story of the balance which the flowing air must maintain under the action of various forces, such as gravitation, rotational deflection, centrifugal tendency, and the various expansion and compression forces.

The winds near the ground are modified both in velocity and direction by friction. The free flow is often interfered with by topography.

THE TRUE AIR FLOW

One must rise above the ground some distance to get the true air flow, or what is known as the gradient wind, the flow which balances the gradient, i.e. a flow along the isobars. The gradient velocity is found about 300 metres above the ground, and the gradient direction a little higher. The lower clouds as a rule indicate true wind values very well; and so, it is desirable in studying winds to use cloud directions and velocities rather than surface values. In cloud work a nephoscope is essential. The unaided eye, unless properly shielded, suffers from the glare of a sunlit sky; and moreover, there are no fixed points or references. A black mirror, with suitable sighting rods and measuring devices, enables an observer to follow the cloud, estimate its height and determine with accuracy the direction from which it is moving. There is an average difference of 30 degrees between the cloud direction and the surface wind; the upper direction being more to the right. At times the directions may be opposite.

[Ill.u.s.tration: FIG. 4. APHELIOTES--THE EAST WIND]

It may seem surprising but few of us, except at sunrise and sunset, really see what is going on in cloud land.

Some meteorologists hold that the circulation of air 3000 to 5000 metres above the ground controls the path and perhaps the intensity of storms. It is therefore important to know something of the flow at high levels if we would improve the forecasts.