CHAPTER VIII

WINDS AND STORMS

The study of the movements of the atmosphere constitutes a rather formidable branch of science known as _dynamic meteorology_. This subject has engaged the attention of a number of able physicists--though far too few--and has begun to assume the character of an exact science, but is still fruitful of unverified hypotheses.

We shall have only a little to say here about the theories and hypotheses relating to atmospheric circulation. They are at present, to a notable degree, in process of revision. Important modifications in them have resulted from the revelations of upper-air research, as well as from progress in other fields of inquiry. There are, however, a few fundamental matters that we must not ignore. We shall start with the solar heat that keeps the atmospheric machinery in motion.

Of the heat that comes to us from the sun, it is estimated that more than one-third is reflected by clouds and the earth, or scattered by dust and air molecules, and thus passes back into space without having had any effect in heating the atmosphere. Part of the remainder heats the atmosphere directly, and the rest indirectly, after first heating the underlying land and water. In both cases, certain atmospheric gases--notably water vapor--absorb a great deal more heat than others.

The first step in the production of a wind is a difference in temperature between two parts of the earth’s surface, and hence of the overlying air. Such contrasts of temperature always exist, both locally and on a large scale. The high sun of the equatorial regions heats the earth much more strongly than the low sun of high latitudes; a water surface has a more equable temperature than an adjacent land surface; a stretch of bare earth is warmer by day and colder by night than a neighboring tract covered with vegetation; and so on. Differences in atmospheric temperature produce differences in pressure, which gravity tends to adjust by setting up a circulation.

The exact manner in which this circulation is begun and maintained is not yet perfectly clear, and current ideas on the subject are difficult to put into brief language. Meteorological writers now lay less stress than formerly upon the lateral spreading, at high levels, of air that has been heated and expanded at the earth’s surface, and the inward flowing of the lower air toward the heated area. There is, we know, an initial impulse that tends to drive air from a region of high pressure toward a region of low pressure; but the actual movement of the air is another matter. The “life history” of an air current is found to be a very devious affair.

The important fact, for practical purposes, is that air does not flow in a straight line from the place where the pressure is high to that where it is low. As soon as it begins to flow it curves from the straight path, in accordance with Ferrel’s Law, which is thus stated:

“In whatever direction a body moves on the surface of the earth, there is a force arising from the earth’s rotation that deflects it to the right in the northern hemisphere, and to the left in the southern hemisphere.”

This law applies to all bodies moving freely over the earth, and not merely to the winds.

At the earth’s surface, if the atmospheric pressure is measured simultaneously at various places by means of barometers, we can get a clear picture of the horizontal distribution of pressure by drawing on a map lines, called _isobars_, through places at which the pressure is identical. The isobars reveal the presence of extensive areas over which the pressure is above the average and of others over which it is below the average. If, at the same time, we chart the flow of air by indicating the direction of the winds at various points, we shall notice that the air shows a strong tendency to travel _around_ these areas; and if we could observe its course a thousand feet or so above the earth we should find the tendency even more pronounced at that level.

Another important law, springing in part from Ferrel’s Law and describing the movements of the air around areas of high and low pressure, is called Buys Ballot’s Law. One way of stating this law is as follows:

“If you stand with your back to the wind, in the northern hemisphere, the barometer will be lower on your left than on your right. The reverse is true in the southern hemisphere.”

The reader, whether he lives in the United States or any other civilized country, will have no difficulty in securing documentary evidence of the correctness of Buys Ballot’s Law in the shape of the daily weather maps issued by the various meteorological services. On a weather map showing conditions anywhere in the northern hemisphere it will be found that the winds (which are indicated by little arrows), though subject to a good many local variations, have a general tendency to blow in the direction followed by the hands of a clock (“clockwise”) around an area of high pressure, and in the opposite direction (“counterclockwise”) around an area of low pressure. It will likewise be noticed that, in general, the winds, instead of blowing along the isobars, are strongly inclined inward in the case of a low-pressure area and outward in the case of a high-pressure area. Lastly, if the map indicates the force of the winds at different places, it will be seen that winds are strongest where the isobars are close together and weakest where they are far apart.

It is a matter of much interest to aeronauts that the force of the wind generally increases with altitude, and that, in the lower flying levels, the winds are little, if any, inclined to the isobars.

The spacing of the isobars is called the _barometric gradient_. One of the conventional ways of expressing a gradient numerically is to state the horizontal difference of pressure, in hundredths of an inch, for an interval of fifteen nautical miles; but meteorologists as a rule merely describe gradients as “steep,” “gentle,” “moderate,” etc., without indicating their numerical values.

If the great difference in temperature between the equatorial and polar regions were the only factor in the control of atmospheric circulation, there would be a strong barometric gradient between these regions and there would result a simple circulation of winds, blowing poleward from the equator aloft and equatorward from the poles at the earth’s surface. The former tendency of writers on meteorology and physical geography was to regard such a circulation as a substantial fact, though modified by the effects of the earth’s rotation and various local causes. Thus the idea has prevailed of a wholesale, direct exchange of air between the poles and the equator. Nowadays we can hardly maintain this idea, because we see that, on account of the great deflections they undergo, the main drifts of air are approximately along parallels of latitude and not along meridians of longitude. Within the tropics the general drift of the lower air is from the east (and near the equator this drift prevails up to a great height); in middle latitudes it is from the west; and in the circumpolar regions it is again from the east. Air from the equator presumably does find its way to high latitudes, and _vice versa_, but neither rapidly nor directly.

Perhaps the dominant feature of the whole circulation is the banking up of the air in so-called high-pressure belts at about latitude 30° North and South. From these “belts”--which are really broken up into separate areas of high pressure, and which shift north and south to a certain extent with the seasons--blow the northeast and southeast _trade winds_, in the full development of which, found only over the Atlantic and the eastern Pacific, we have the most remarkable “permanent winds” of the globe.

Between the trade wind belts lies the equatorial region of low pressure, known as the “doldrums.” This is, in general, a region of light and variable winds, heavy rains, and thunderstorms.

In the temperate zones of both hemispheres, on the poleward side of the high-pressure belts above mentioned, the general drift of the atmosphere near the earth’s surface is from west to east. In the south temperate zone there is a very strong preponderance of west winds, especially over the vast oceanic tract of the “roaring forties,” where blow the boisterous “brave west winds,” well known to sailors. The corresponding belt of the northern hemisphere, which includes all but the southernmost part of the United States, is described as a region of “prevailing westerly winds,” but it is also a region of storm tracks, and hence, as local episodes in the general movement of the atmosphere from west to east, there are constant shifts of the wind to all points of the compass, for reasons that will presently be explained.

On the poleward borders of the two belts of “prevailing westerlies,” a little outside the Arctic and Antarctic Circles, there are zones of low pressure. That of the southern hemisphere is a continuous girdle around the earth, and has the lowest pressures found anywhere in the world. The corresponding subarctic zone, while fairly continuous in summer, is broken up in winter by the formation of high-pressure areas over Siberia and northern Canada.

The permanent ice sheets of Greenland and Antarctica are regions of high pressure, with calm air in the interior and strong outblowing winds at the borders. Furious blizzards prevail at the margin of Antarctica.

The table on the opposite page will serve as a recapitulation of the facts above stated.

+-------------------------------------------------------------------+ | | | NORTH POLE | | | | Calms and high pressure over the interior of Greenland. | | Out-blowing winds at the border. | | | | More or less broken subarctic belt of low pressure. | | | | Prevailing westerlies (much interrupted by moving cyclones and | | anticyclones). | | | | Belt of high pressure at about lat. 30° N. “Horse latitudes,” or | | “calms of Cancer.” | | | | Northeast trade winds. | | | | Equatorial belt of low pressure, calms, and variable winds. The | | “doldrums.” | | | | Southeast trade winds. | | | | Belt of high pressure at about lat. 30° S. “Calms of Capricorn.” | | | | Prevailing westerlies, more constant than in the northern | | hemisphere. “Brave west winds.” | | | | Subantarctic belt of very low pressure. | | | | Calms and high pressure over the interior of Antarctica. Violent | | outblowing winds at its border. | | | | SOUTH POLE | | | +-------------------------------------------------------------------+ WIND AND PRESSURE BELTS OF THE GLOBE.

The great wind and pressure belts of the globe are much more constant and sharply defined over the oceans than over the land, and it was upon the high seas that mankind first distinguished them and gave them their names. The northeast trade winds of the Atlantic wafted Columbus to the New World and aroused the misgivings of his sailors, who wondered how they should ever sail homeward against them. The high-pressure belt north of these trade winds is a region of calms, which Maury called the “calms of Cancer.” This region, or a part of it, is likewise known as the “horse latitudes,” the story being that, in the old sailing days, vessels laden with horses were often becalmed here so long that the cargoes had to be thrown overboard. As a matter of course, the prosaic modern etymologist declines to accept this origin of the name and has proposed others less picturesque. The so-called equatorial calms, which lie mostly a little north of the equator, are often nicknamed the “doldrums,” or sometimes the “equatorial doldrums,” to distinguish them from other regions of dolorous, baffling calms. The doldrums vary a great deal in width and the masters of sailing ships try to cross them where they are narrowest.

The name of the trade winds implies that, according to the old nautical phrase, they “blow trade,” or constantly in one direction. Strictly speaking, they vary considerably in direction, at any one spot, though they are nearly always from an easterly quadrant, and they are even more variable in force. The average speed of the Atlantic trades is about eleven miles an hour. In view of the prospective requirements of aeronauts, it is a fact of much interest that the trades are rather shallow winds. Their vertical thickness has been found, by observations with pilot balloons and otherwise, to range from less than a mile to two or three miles. Some distance above the trades there are winds blowing more or less in the opposite direction, known as the _counter-trades_. Aircraft will probably use the trade winds in flying from southern Europe to the Caribbean, and the countertrades on the return voyage to Europe.

In contrast to the permanent or quasi-permanent winds just described, there are certain important winds of the “periodic” type, which reverse their directions in the course of the year or from day to night. Some of these, also, first became generally known through the reports of mariners. The ancient Greek navigators utilized the monsoons in trading with India; while we owe to the voyages of William Dampier, in the seventeenth century, one of the earliest and best descriptions of land and sea breezes.

A monsoon is a wind that blows from a continent toward the sea in winter, when the land is colder than the water, and in the opposite direction in summer, when the reverse conditions of temperature prevail. The pressure gradient is reversed with the seasons, and the wind varies accordingly. The most striking example of monsoon winds is found in southern Asia--where these winds are of special economic importance because they control the rainfall of India--but well-developed monsoons also occur in Australia and West Africa, over the Caspian Sea, on the coast of Texas, and elsewhere.

An analogous reversal of gradients, due to the change of temperature over the land from day to night, is of common occurrence on the shores of large bodies of water, resulting in land and sea breezes (or land and lake breezes). The breeze blows from the land to the water by night and in the opposite direction by day. These breezes are generally best developed and most regular within the tropics, and particularly on shores adjacent to mountains. East Indian fishermen put out to sea with the land breeze in the early morning and come home with the sea breeze in the afternoon. The refreshing and health-giving character of the sea breeze of tropical climates has earned it the sobriquet of “the doctor.”

Mountain and valley breezes furnish another example of diurnally reversed winds. Relatively cold and heavy air drains down from the upper slopes by night, constituting the mountain breeze. By day the air in the valley is warmed and expanded, and as it is confined laterally by the sides of the valley it flows up the slopes, constituting the valley breeze. Long before meteorologists undertook to classify the winds of the globe, these mountain air currents attracted attention and acquired local names. Among the Alps, alone, we find scores of such names. In many cases, too, the breezes have acquired a legendry of their own. Thus the _pontias_, a cold, nocturnal wind that blows out of a narrow valley opening upon the plains of the Rhône near the town of Nyons, is said to have been brought thither in a glove by St. Cæsarius, Archbishop of Aries, for the purpose of improving the fertility of the valley! There is a quaint little book about the pontias, published by Gabriel Boule in 1647, in which the author sets forth at length the arguments for and against the miraculous origin of this wind. The Italian lakes are especially rich in locally named mountain and valley breezes.

Parenthetically it may be remarked that wind nomenclature in general is a vast subject, owing to the habit that prevailed in prescientific times, and still prevails to some extent among nonscientific people, of giving individual names to the winds characteristic of different localities. As a matter of curious interest, we set down here some of these names (a small fraction of the total number):

_Khamsin_, _leveche_, _leste_, _levanter_, _pampero_, _zonda_, _papagayo_, _buran_, _purga_, _brickfielder_, _southerly burster_, _williwaw_, _willy-willy_, _pontias_, _vésine_, _solore_, _joran_, _morget_, _rebat_, _vaudaire_, _breva_, _tivano_, _ora_, _Wisperwind_, _Erlerwind_, _Rotenturmwind_, _vent du Mont Blanc_, _vent d’aloup_, _autan_, _tramontana_, _gregale_, _imbat_, _kite and junk winds_, _bad-i-sad-o-bist roz_ (the furious “wind of 120 days” of Persia and Afghanistan).

The present writer has collected several hundred local wind names--and is constantly adding to the list.

There are several other types of wind peculiar to mountains besides the alternating mountain and valley breezes. Most of these are descending winds, or “fall winds,” which may blow by day as well as by night. Thus a strong daytime wind sometimes blows down from lofty snowfields and glaciers. A _foehn_ (pronounced like “fern,” but without the _r_) is a wind that has been robbed of most of its moisture through precipitation on the windward slope of mountains and which is further dried, and also strongly heated by compression, in descending the leeward slope. Winds of this type are common in the Alps, where they were first described and named, and their heat and aridity led to the belief that they came by way of the upper atmosphere from the distant deserts of Africa. Now that their origin is better understood, we find that foehns prevail in many other mountainous countries throughout the world, including the western United States, where they are called _chinooks_. When the foehn blows in winter, it causes snow to disappear with amazing rapidity--not only melting it, but speedily drying the ground--whence it has earned the name of “snow eater” in America, and “_Schneefresser_,” which means the same thing and a little more, in Switzerland.

The _bora_ of the Adriatic and the _mistral_ of southern France are winds that blow from a cold, mountainous interior down to a warm coastal region, where they arrive as relatively cold winds, in spite of the dynamic heating they have undergone in their descent. The bora is sometimes moderate (_borina_) and at other times a tremendous gale (_boraccia_), while the mistral has been known to blow a railway train from the track in the valley of the Rhône.

The _blizzard_ is a wind of which Americans once thought they had almost a monopoly until Sir Douglas Mawson located the “home of the blizzard” on the shores of the Antarctic continent. The true blizzard, whether American or Antarctic, is a violent, intensely cold wind, heavily charged with snow. Such winds are a characteristic feature of the winter climate of our Middle Western States. Although the name of this wind first became current as recently as the seventies of the last century, nobody knows its origin. Nowadays the name is often loosely applied to big snowstorms that are not really blizzardlike.

The dry “hot winds” that sometimes wither the crops of our western plains are the American antithesis of the blizzard. These winds belong to the great sirocco family--the name “sirocco” having become, in recent scientific usage, a generic designation for extensive hot winds, whether dry or moist, as distinguished from local hot winds, such as the foehn.

The _harmattan_ of West Africa is a dry, dusty wind from the Sahara, and one that feels relatively cool; perhaps on account of causing rapid evaporation from the skin. The _simoom_ (with a final _m_), especially the variety blowing in southern Asia, is perhaps the hottest and most parching of all winds--judging from its effects on animal life.

The great majority of the winds above enumerated are merely minor features of what are called _cyclonic_ and _anticyclonic_ wind systems. Reverting to what has been said about weather maps and Buys Ballot’s Law--if the reader will examine a series of maps for successive days, he will notice that the areas of high pressure and low pressure are not stationary, but show a more or less rapid displacement, the general direction of which, in our latitudes, is from west to east. The fact that there are great traveling vortices or swirls in the atmosphere, which, in whatever regions they occur, partake of the general drift of the air around the globe, has been known for about a century, and constitutes the corner stone of practical meteorology. In the temperate zone, where these swirls are sometimes of moderate force and sometimes very stormy, they are the chief factor in controlling changes of weather from day to day, and their observation is the basis of weather forecasting. Within the tropics, where they are much less frequent and are confined to a few restricted regions, they are always violent storms.

An area of high pressure, with its attendant system of winds, is called an _anticyclone_ or _high_. An area of low pressure, with its winds, is called a _cyclone_--sometimes. The word “cyclone” was invented by Henry Piddington in the year 1848. Nearly all the early studies of cyclones were made chiefly for the benefit of mariners, and related to the severe revolving storms encountered at sea. Hence the word “cyclone” passed into the general vocabulary with a connotation of violence, which, in everyday speech, it still retains. Perhaps the early “cyclonologists” themselves hardly realized that a “gentle cyclone” was not a contradiction in terms.

Meteorologists are still so much under the influence of the popular idea of a “cyclone” that they hesitate to apply this term to a disturbance of moderate force, except in a few special phrases (such as “extra-tropical cyclone”), though the adjective “cyclonic” is used freely without reference to the force of the wind. British meteorologists speak mostly of “depressions,” while American meteorologists speak of “lows.” The status of the latter term, as well as that of the term “high,” is, however, paradoxical. Though both words have been used for years, they are nearly always printed with quotation marks around them, as if they had not yet been assimilated in the vocabulary. The Weather Bureau has lately taken to printing these words in capital letters. Neither of these practices will be followed in the present book.

Tropical cyclones are called “hurricanes” in the West Indies and the South Pacific, “typhoons” off the east coast of Asia, “baguios” in the Philippines, and “cyclones” in the Indian seas. They form in the doldrums, and generally take a long, sweeping course, curving westward and poleward, and sometimes passing into the temperate zones, where they either die out or increase in size, diminish in violence, and become similar to the storms originating in the higher latitudes. One of the curious features they often exhibit within the tropics is the calm center at the “eye of the storm,” to which Tennyson alludes when he writes of the blast (unknown to meteorologists) that drove a ship

Across the whirlwind’s heart of peace, And to and thro’ the counter-gale.

These cyclones are the worst of all storms found at sea, and also exercise their destructive effects over islands and along continental coasts. The greatest disasters attending them have been due to the inundation of low-lying shores by the huge waves they generate, as in the Galveston hurricanes of 1900 and 1915 and in the far worse catastrophes that have occasionally visited the coast of India. Hurricanes of the West Indies occur chiefly from August to October, inclusive. The number varies from none to a dozen a year (with four as an average).

Over the large land areas of the north temperate zone highs and lows show a tendency to travel over typical tracks, the locations of which vary a good deal with the season. One of the most remarkable facts about the lows of North America is that, wherever they come from, whether from the Canadian northwest, the western United States, or the West Indies, they nearly always leave the continent by way of the Gulf of St. Lawrence or the northeastern corner of this country. Our North American lows travel at an average speed of 600 miles a day. Highs travel somewhat more slowly; about 540 miles a day is the average in this country.

A _tornado_ is a small vortex in the atmosphere, occurring generally in the southeastern part of a cyclonic area, where, in some cases, several separate tornadoes develop at the same time. The tornado, for some reason that is not altogether clear, is far more common in the interior of North America, east of the Rocky Mountains, than anywhere else in the world, though true tornadoes do occur in other countries. The West African storms bearing this name are merely thundersqualls, quite different from the American tornado. The chief visible feature of a tornado is the so-called funnel-shaped cloud (sometimes balloon-shaped or, again, like a great coiling serpent), which is always in contact with the ground when destruction is in progress. The passage of the storm is attended by a loud roaring or rumbling. The path of a tornado varies in width from a few rods to half a mile or (rarely) more. Within its borders buildings are blown to pieces, trees are uprooted and human beings only find safety underground; while even at a distance of a few yards outside the path no damage is done. The tornado travels at an average speed of about twenty-five miles an hour. Its speed of rotation has been estimated, from the effects produced, to amount to 500 miles an hour in some cases; a wind force far exceeding that of any other type of storm.

_Waterspouts_, which occur on the ocean and other large bodies of water, are similar in character to tornadoes, though much less violent. They range in height from 100 to 1,000 yards, or more. One measured recently from the British steamer _War Hermit_, near Cape Comorin, was 4,600 feet high to the base of the overlying cloud. The column tapered from 500 feet wide at the junction with the cloud to 150 feet wide at the sea. Spray was thrown up to a height of more than 800 feet over a region 250 feet in diameter.

_Thunderstorms_ occur chiefly in warm climates and during the warm season in temperate climates, but they are by no means unknown in the polar regions. They are characterized by rapidly rising air currents, which may be either incidental to the circulation of a low, or due to local overheating of the lower atmosphere. In the former case they are called “cyclonic thunderstorms,” and in the latter “heat thunderstorms.” This is only a rough classification, however. Some thunderstorms partake of the features of both these types, and, on the other hand, additional classes are distinguished by many authorities. It is a common occurrence for thunderstorm conditions, starting in some small area, to travel across country at a speed of perhaps thirty or forty miles an hour, at the same time spreading out fanwise until the front of the storm is hundreds of miles in length. This front constitutes a “line squall” (so called from the long line or apparent arch of dark cloud that marks its location), and is attended by more or less thunder and lightning, but is not necessarily a continuous thunderstorm. The characteristic wind of a thunderstorm is the squall that rushes out in front of the storm when close at hand. This blast of wind, lightning, hail and torrential rain are all agencies of destruction in severe thunderstorms.

Concerning the winds of the globe in general and the remarkable atmospheric interchanges that they involve, Sir Napier Shaw writes:

“Of the millions of tons of air which form the atmosphere nearly the whole is moving. The regions of calm at the surface at any one time, taken all together, do not form a large part of the earth’s surface, and above the surface calm regions are still rarer. Let us remember that the motion of the air is always ‘circulation’; air cannot move forward or backward or upward or downward without displacing other air in front of it and being replaced by other air behind it, though the circulation may be quite local and limited in extent, as is frequently the case when warm air rises or cold air sinks. In the course of investigations into the life history of surface air currents in the Meteorological Office we have traced air over long stretches of the surface of the Atlantic. We have found, on one occasion, the shores of Greenland to be fed with air that left the middle of the Atlantic four days previously, while in the course of six days air traveled from Spitsbergen to join the northeast trade wind off the west coast of Africa. On another occasion the air that formed the wind off the south of Ireland was traced back to the north of Africa, but that which blew at the opening of the Channel two days later came from Hudson Bay, via the Azores.”

Such are the ever-shifting currents of the ocean of air.