Wind

( Originally Published Early 1900's )

WIND is an odd phenomenon. It blows good and blows evil. At times we delight in it and again we fear it. Among the fables credited to Aesop we find popular recognition of the power of the wind in the fable entitled "The Wind and the Sun." The North Wind engages in a contest with the Sun to determine which can the sooner force a traveler to remove his coat. Cold blasts out of the north force the the wayfarer to pull his coat tightly about him, but the subsequent hot sun succeeds in making him remove it. No doubt the north wind was selected for this contest because in Greece, where the story supposedly originated, the occasional breezes from the north bring such cold as to chill the Mediterraneans to the marrow.

The wind has its virtues. People who live along lakes or seashores in regions characterized by warm weather for all or part of the year find relief in the cool winds blowing off the water. Sportsmen and others seeking the thrill of sailing or ice-boating enjoy brisk breezes. Farmers look to the northeast or east winds to bring them much-needed rain. In India people pray for the rain-bearing southwesterlies. In China the coolie pushing his heavily laden wheelbarrow with a lone sail attached hopes for breezes strong enough to fill the sail and aid him in pushing the load. The wind does mechanical work for us when it turns the great wheel of the windmill. Many persons prefer the windy regions as places of residence, because they feel that alternating strong and weak winds are stimulating. In fact, studies of the effects of the weather upon factory workers, students, and others seem to show that these persons work most efficiently where winds are variable and blow with moderate to strong force. The most progressive nations are located in regions of frequent winds and where temperatures are neither excessively high nor extremely low.

The adverse effects of the wind are known to us in terms of the wind's destructive force both on land and at sea. On most of our plains the wind blows hard almost continuously, and when accompanied by high temperatures may in a few hours wither crops which represent an entire season's labor. On plains such as the pampas of Argentina or in parts of Central Asia continuously strong winds seem to disturb the nervous systems of the natives. Explorers who have visited the hot deserts as well as polar areas tell us that the howling winds are among the most nerve-racking experiences they encounter. Winds at sea develop tremendous waves which destroy much shipping. They force waves against the coasts to erode them, and in places the erosion is so destructive as to necessitate the building of sea-walls such as that at Galveston, Texas, to prevent towns from slumping into the sea.

Unwelcome as winds may be at times, yet when considered from all points of view their contributions to mankind more than offset their opposition. Of course, we have no means of measuring the quantities of good and bad, but as optimists we may well believe that they are not only balanced but that the good exceeds the bad.

Vast areas enjoy many hours of calm practically every day, as in the lowlands along the equator or, during certain seasons, in Mediterranean districts. Some people think calms are not wind. True, they are not if by wind we mean the pressure of the air which we can feel against our bodies. On the other hand, if we adopt the technical definition previously cited namely, that wind is merely air in motion then, since calms represent air moving either upward or downward, they must be wind.

Why does the wind blow? The reasons are many, some perfectly evident and others not so easily appreciated. Differences in temperature at successive points will produce differences in pressure and this in turn produces an unstable condition of the atmosphere. In other words, where the temperature rises at a given point the air expands and becomes less dense than it was. A given volume actually weighs less than before it expanded and less than the air around it. In consequence the heavier air moves toward the lighter, producing wind. If the difference in density is great within short distances the wind will be strong, while if the distance itself is considerable the wind will be weak.

Although we say pressure differences cause wind, we do not really explain wind by this statement. But the causes of pressure contrasts are numerous and complex. For our immediate understanding of weather phenomena it is hardly necessary for us to delve into this subject. We shall, however, note some of the earth's most interesting special types of winds where they play an important part in people's activities.

The strong north wind which 2E sop observed may have been the bora. The bora characteristic of many parts of the Mediterranean is perhaps most striking along the eastern shores of the Adriatic, particularly in the neighborhood of Trieste. Here the mountains rise, abruptly from the sea to heights of 3,000 to 5,000 feet, spreading at their summits into a broad plateau. Upon a winter's day the cold air accumulating in the high levels develops a marked contrast in density compared with the air at the sunny mountain base, where intense heating has been in progress throughout the day. Soon an unstable condition arises and the mass of cold air slides down the mountainside with a velocity sometimes reaching forty miles an hour. The natives, not prepared for this sudden chilly blast, shiver, and only with difficulty do they walk around the corners of buildings at street intersections where this wind attains great force. Iron hand-rails along the walks at these corners often come to one's rescue. Ships in the local harbors not forewarned often puffer damage from being buffeted against a dock or into other craft. Along the Mediterranean coast of France, the famous Riviera, a similar type of cold wind blows occasionally, but here it is called the mistral.

Winds originating in mountain districts, as do the bora and the mistral, but whose effects are quite different, are the chinook of our western mountains and the föhn of Switzerland. As these winds descend the mountain slopes they become sufficiently heated by compression to evaporate moisture in the valleys below. In Montana the chinook has a reputation for melting a snow layer one foot thick in a few hours' time. Various tales of its evaporating capacity sometimes pro-duce a little skepticism among those who hear them. However, the chinook has considerable economic importance in Montana, Wyoming, and other Western States, where it interrupts cold waves and clears the snow from the valley floors or bordering plains, enabling cattle to graze even in midwinter.

Northern Africa gives rise to a withering hot blast known as the sirocco. In southern Europe it often is the source of heat spells. Southern Asia experiences the terrible simoom, one of the hottest of the earth's winds. In the United States similar though not such severe winds are associated with summer hot waves.

Among our cool or cold winds the Texas norther is one of the most noted. Texans who have suffered from this sudden importation of winter air out of the cold north emphasize its severity in strange tales. They relate how the "flying-fish" in their rivers leaping into the air and attempting to return to the water strike an icy surface and become stranded, for during the moment they left the river's surface the deadly norther arose and froze all the waters ! Many of the winds just described are prevalent in other countries but under different names.

Among all of the special winds possessing the greatest economic influence is the monsoon. This name originated in India and means wind. It has been accepted as a standard term applicable to wind systems similar to the Indian, wherever they may prevail. In the United States we have a rather poorly organized monsoon along our South Pacific coast and along the southeastern Atlantic coast. Spain and northwestern Africa also have an acquaintance with a modified mon-soon. However, no region on earth is so dominated by a nearly perfect monsoon as north-western India.

Characteristic of the monsoon is the fact that winds blow in diametrically opposite directions in opposite seasons. Over India they shift from the southwest in summer to the northeast in winter. The southwesterlies "bring rain." Should they fail or set in late, drought is certain, crop failures ensue, and thousands of people in this congested country starve. Thanks to the investigations of the Indian Meteorological Service, the coming of the southwest monsoon can be forecast well in advance. If it is likely to arrive late the people may be duly warned and accordingly save themselves. Further relief will be afforded from the uncertainty of this wind when a vast project for the storage of water in immense reservoirs is completed within the next five or ten years. Then injury from a late monsoon will be practically a thing of the past.

Each one of us has some acquaintance with local winds and appreciates their characteristics and peculiarities, because they have a fairly definite direct effect upon us or upon the activities with which we are associated. But, after all, these are not always the winds which play a major part in our climates the year round. Such winds share in the make-up of the earth's major wind system, sometimes referred to technically as the planetary wind system or the winds of the general atmospheric circulation. Most of the readers of this book probably will be inhabitants of the regions in which the prevailing westerlies are dominant and where cyclones and anti-cyclones control the weather. However, before considering the westerlies let us note just where they fit into the whole system so that we may the better appreciate our own weather.

The major wind system is, in a sense, balanced upon the equator that is, the arrangement of these winds in sequence from the equator toward the North and South Poles respectively is identical. At the equator, and extending for a distance of approximately eight degrees north and south of it, are the doldrums or equatorial calms. Here, where the sun's rays are vertical the year through, excessive heating results in setting up vertical air currents. This upward motion of the air gives the apparent effect of no motion, hence receives the name calms. North of the calms is a zone of winds which move toward them, but because the earth rotates, these winds in the northern hemisphere are deflected to the right, becoming northeast, and take the name northeast trades. In the southern hemisphere a corresponding movement of air is effective, except that the deflection is toward the left, giving rise to the southeast trades. At the poleward limits of the trade-winds in lat. 30° N. and 30° S. the air descends from considerable heights and follows behind the trades. These descending winds are called "Horse Latitudes," a name at once so striking and in such great contrast with the other names that a word of explanation is in order. The story goes that in the days when trade was active between New England and the West Indies, horses were an important commodity included in the cargoes headed for the southern islands. The motive power for the sailing ships, of course, was wind, and frequently when the small craft of those times reached the thirtieth parallel they would have difficulty in finding enough wind to carry them across the calms. With only limited stores of fresh water and fodder, the supply for all the horses would often give out, and then, to prevent these faithful animals from dying of thirst and hunger, they threw some of them overboard in order to save others. Hence, these calms, always feared by the sailors, were referred to as the horse latitudes, a name that has remained undisturbed in spite of the subsequent replacement of most sailing vessels by steam-powered ships. All of these wind belts shift northward a few degrees as our summer season approaches, to return southward again as summer comes on in the southern hemisphere.

Bordering upon the trade-wind regions are the prevailing westerlies. They extend roughly from lat. 32 ° N. to the Arctic Circle in the northern hemisphere, and in the southern hemisphere from a corresponding latitude to the Ant-arctic Circle. These winds are interrupted to a large degree by pressure areas known as cyclones and anti-cyclones, which produce not only variable winds in many localities but even reverse the theoretical westerlies, converting them into easterlies. Over the oceans the westerlies are somewhat less disturbed than over the land. Along the west coast of extreme southern Chile they blow so strongly and steadily that sailors have appropriately dubbed them the "roaring forties," the number referring to lat. 40° S.

Beyond the westerlies and extending to the poles. the winds are often referred to as circumpolar. We know little about them. Explorations in the vicinity of both poles are steadily adding to our knowledge. In fact, studies in Greenland during the past several years and current investigations in Antarctica are being focused upon the winds and weather of these regions in the hope that their relationship to the weather in other latitudes, particularly those of the westerlies, may be established as an aid to long-range forecasting.

We have cited the deflection of the trade-winds from north winds to northeast and from south winds to southeast. Deflection of winds occurs not only in these latitudes but everywhere except along the equator. It is due to the rotation of the earth, and was first announced by William Ferrel shortly after 1856. He found that in the northern hemisphere all winds are deflected to-ward the right and in the southern hemisphere toward the left. This statement is usually referred to as Ferrel's Law. Here is an important principle to which we shall refer again in our discussion of weather forecasting.

When we described tornadoes we said that those fearful storms are frequently misnamed "cyclones," and that cyclones pass across the United States on an average of every three or four days, occasionally at slightly longer or shorter intervals. Cyclones are not to be feared but rather looked upon as a form of atmospheric activity quite normal and harmless. Cyclones and their counterparts, anti-cyclones, account for the tremendous variability in the weather of the eastern half or two thirds of the United States and in consequence share a goodly portion of the responsibility for the progress of the nation. Let us examine the structure of a cyclone first and then of an anti-cyclone.

Every day, except Sundays and national holidays, about sixty-five stations of the United States Weather Bureau print and distribute a map which shows the weather conditions through-out the country. All of these centers draw a map everyday for the forecast, but only the main office located at Washington, District of Columbia, prints and distributes a map on Sundays and holidays as well as on the other days. The actual preparation of the map we shall consider later. If we look upon any of these weather maps we shall notice solid curved lines called isobars (lines passing through all points having the same pressure), which tend to make circles or ellipses and which show an inclination to-ward parallelism that is, toward being concentric. Under ideal conditions isobars would make perfect concentric circles. A further study of the map reveals arrows showing wind directions, and if these are traced on a sheet of transparent paper laid upon the map they will show a tendency to point toward a common center. Under ideal conditions they would point just enough off center so that a line connecting them would form a spiral. Further observation will bring to light broken lines, isotherms that is, lines passing through all points of the same temperature. They show that the center of the spiral is some-what warmer than its northern and western areas and not so warm as the more southerly portions.

Now we have the essentials of the cyclone, which may be defined as a pressure area in which the winds blow spirally upward, inward, and counter-clockwise. Cyclones move generally in an easterly direction, as do anti-cyclones. However, one important exception is the tropical cyclone, sometimes of hurricane force, such as the occasional West Indian hurricane, which moves westward first from its place of origin over the Atlantic and subsequently turns into the usual easterly storm course upon approaching or reaching the continental land mass. These storms are few and, as previously noted, occur largely in the months of September to November.

The anti-cyclone, as one might guess, involves a circulation of air the reverse of that in the cyclone. The winds move spirally downward, out-ward, and in a clockwise direction. Since the air descends, quite naturally it is cold, in contrast with the warm air we find at the center of a cyclone ; but the temperatures are usually lower to the rear of the pressure area than at its center, and somewhat higher along its front. These storms move in a general easterly direction across the United States, altering with the cyclones, two anti-cyclones never following in succession. It is this sequence which enables the United States Weather Bureau, and others who have studied the behavior of the atmosphere, to fore-cast the weather with a fair degree of accuracy. The details of forecasting will be noted in the next chapter, in the hope of making possible forecasting by any persons who are willing to devote just a little of their time to this fascinating and profitable enterprise.

We have been discussing wind directions and velocities but have said nothing relative to the manner in which these factors are measured. The cardinal points, north, south, west, and east, are known to most persons, although at times even the most intelligent must stop a moment to get their bearings before being absolutely certain of their directions. Some persons never can tell directions easily. Smoke issuing from a tall stack, clouds, flags, and other objects readily movable by the wind are frequently used to tell wind directions. In many regions where the wind has a decided tendency to blow from. one direction more than any other, trees oftentimes indicate this by their shapes, being permanently bent away from the wind, or having much longer branches on the side of the trunk away from the wind than toward it. The Weather Bureau and a few individuals wishing more accurate indications utilize the wind-vane, whose sole function is to point out directions.

The wind-vane, occasionally referred to as the weather-vane, is in its simplest form merely a broad-tailed arrow supported about midway between head and tail upon a swivel which allows it to turn freely. The more sensitively balanced, the more readily and accurately it responds to every puff of the wind. One might, offhand, suppose that the arrow should point in the direction toward which the wind is blowing, but just the contrary is the fact. It heads right into the wind, pointing in the direction from which it blows. The finest wind-vanes have attachments which record on paper the wind direction at the end of each minute.

Observing directions is a relatively simple matter compared with velocities. Much experimental work has been done in the measurement of wind velocity, until to-day an instrument called an anemometer has been evolved which serves excellently. This device consists of either three or four hemispherical non-rusting metal cups, each about three inches in diameter, mounted on arms extending from a central rod, which in turn is mounted so delicately that the cups and rod may be made to turn by just blowing on them quite gently. The harder the wind blows against these cups the faster the central supporting rod rotates. By attaching a speedometer to the lower end of the rod the velocity of the wind can be read, or if desired a recording device can be attached which will print the wind velocity every minute, just as the wind-vane prints directions. Wind velocity as published by the Weather Bureau represents the average for five minutes rather than for a single minute, be-cause often the wind is gusty, and neither the figure for a gust nor a calm would tell the correct story. The average may not be accurate but it does give a better representation.

The readings of wind velocity are in miles per hour and when recorded instrumentally are fairly accurate. A descriptive scale, however, was worked out by Francis Beaufort, who served as a rear-admiral in the British Navy in the early part of the nineteenth century. He invented what is now known in scientific circles as the Beaufort scale. It is built upon a sequence of numbers ranging from 0 to 12, the lower figure representing a calm and the higher a wind velocity of seventy-five miles an hour or more. Navigators use it regularly, judging the strength of the wind by comparing the movement of smoke issuing from their ships' funnels with the known speed of the ship, or observing the size of the waves, or the nature of the spray blown from the crests of waves. The scale can be used on land too, smoke, leaves of trees, and other elements in the landscape serving as approximate indicators of wind force. The preceding table, with specifications for use on land, indicates the meaning of each number as officially interpreted by the United States Weather Bureau.