Handbook of Meteorology/Local Winds
THE AIR: MAJOR CIRCULATION: LOCAL WINDS
The convectional or vertical movements of the air have been mentioned incidentally as affecting the diffusion of warmth. They are considered in detail in succeeding chapters. The horizontal movements constitute the winds. So far as human
General movements of the atmosphere.
activities are concerned, the general horizontal movements consist of a broad tropical belt of easterly winds flanked on the north and on the south by a broad belt of westerly winds. The three belts move northward with the apparent motion of the sun northward in June, and southward in December. The yearly oscillation covers about 120 degrees of latitude, a total of about 7200 nautical miles.
Trade Winds.—The broad belt of easterly winds within the tropics is popularly known as the Trade Winds. Their direction is southwesterly in the northern and northwesterly in the southern part of the belt. The heating of the air in equatorial regions causes a convectional updraught; and this is balanced by an inflow of air from higher latitudes. The rotation of the earth deflects the movement of the air, giving a westerly motion to the winds. Their force and direction are best studied from the monthly pilot charts published by the United States Hydrographic Office. Although the pilot charts refer specifically to ocean winds, the general information published, so far as wind direction is concerned, applies to land winds also.
The strength of the Trade Winds varies according to latitude and also according to season. The velocity is highest near the edges of the belt and lowest at its center; it varies from about 8 miles per hour in the fall months to about twice this rate in the spring and summer months. The southeast winds are materially stronger than the northeast winds. The easterly component is the important commercial factor—hence the popular name. For the year their average is from 11 to 14 miles per hour, or from 2 to 3, Beaufort scale.[1] On the Pacific Ocean the Trade Winds are neither so strong nor so regular as in the Atlantic; on the Indian Ocean only the southern part of the belt is observable.
Prevailing Westerlies.—The two broad belts which flank the Trade Winds are variously named “Counter Trades,” “Return Trades,” and “Anti-Trades.” Their direction varies—northeast, east, and southeast as shown on the pilot charts, and their strength is indicated by the arrows. In the southern hemisphere because of their strength, they are known, as the “Roaring Forties.” In the days of sailing vessels, a ship from a port of Europe to Australia could usually make the return trip more expeditiously by way of Cape Horn. The force of the Prevailing Westerlies is from 2 to 4, Beaufort scale.
The Prevailing Westerlies begin as an upper wind in Trade Wind latitudes, descending to sea level at the edges of the Trade Wind belt, approximately Lat. 30° N. and S. Over Cuba the airman may find them at the height of about 11,500 feet; over Jamaica about 19,500 feet; and over Trinidad about 26,000 feet. In each case the height of the Prevailing Westerlies is also the depth of the Trade Wind belt.
Winds of the United States.—The main body of the United States is situated in the belt of Prevailing Westerlies. The prevailing surface winds therefore are northwest, west, and southwest. East of the Missouri River northwest winds prevail except during the hottest part of summer when southwest winds are the rule. These include practically all the winds of various altitudes between sea level and the summit of Mount
Tracks of cyclonic storms preceded by easterly and followed by westerly winds.
Washington (6293 feet), one of the highest points east of the Mississippi River.
South of the thirty-first parallel the influence of the Trade Winds is very apparent, and the prevailing winds in summer vary from northeasterly to easterly. Along the coast this influence extends much higher than the thirty-first parallel, and northeasterly fair weather winds occur at times as far north as the Maine Coast.}}
West of the Mississippi River to the Rocky Mountains, the winds vary from southwesterly to northwesterly. They are steadier and stronger than those east of the Mississippi River. Southwesterly winds prevail much of the time.
From the Atlantic Coast to the Rocky Mountains the general westerly direction of the winds prevails pretty steadily, except as it is upset occasionally by cyclonic storm winds.
In the plateau region and the basin, the upper winds are westerly — southwest to northwest; but the surface winds in many instances are deflected by mountain ranges and become either southerly or northerly.
The surface winds of the Pacific Coast are westerly; but in the Sacramento and San Joaquin valleys they become northerly or southerly, being deflected by the mountain ranges. When the temperature of the interior is high, strong westerly winds are the rule along the coast. This is especially noticeable in the vicinity of San Francisco.
Monsoons.—The monsoons are seasonal winds which blow from the sea over the land during summer months and in an opposite direction during winter months. The name, meaning “season,” was first applied to the seasonal winds of the Indian Ocean Coast; subsequently it was applied to various seasonal winds of ocean coasts. In southern Asia the crop yield depends very largely on the rainfall which accompanies the southwest monsoon; hence its importance in the economic history of a considerable part of southern Asia.
The advance and recession of the Trade Wind belt along the Gulf Coast of the United States seems to emphasize a similar alternation of sea wind and land wind; but the monsoon characteristics extend as far north as Long Island Sound on the north and far into Mexico on the south. In the latitude of New York City, about eight weeks of southwesterly winds prevail in summer, while northwest winds prevail the rest of the year.
Calm Belts.—Along the narrow belt where the northeast and the southeast Trade Winds meet, the easterly components of the winds disappear, the only movement being an updraught. In the days of sailing vessels ships sometimes lay becalmed for many days—hence the expressive name, Doldrums. This calm belt lies north of the equator and practically covers the thermal equator. It is a region of low barometer, very moist air, cumulus clouds and excessive rains. It is practically coincident with the tropical rain-belt. Its detrimental effects on marine transportation ceased with the advent of steam navigation. Years ago it was a terror to sailing craft.
The Calms of Cancer and of Capricorn separate the belts of prevailing westerly winds from the Trade Wind belt. They are regions of high pressure and usually of cloudless skies. Vessels from ports of Europe and the United States crossed the Calms of Cancer when making West Indian ports. These calms were therefore a great drawback to commerce. Steam navigation has eliminated the waste of time and the loss of jettisoned cargoes;[2] but conditions more or less detrimental, which humanity cannot overcome, still exist. Much of the southwestern
Winds of the Atlantic.
part of the United States and northern Mexico are covered by the high-pressure Calms of Cancer, and, at a little distance from the ocean coasts, rain-bearing winds are infrequent. Similarly, the sparse rainfall of parts of South America is due to the Calms of Capricorn.
Local Winds.—The local winds of a region appeal to a community more forcibly than do the general movements. One may not appreciate the fact that the habitability of a region depends very largely upon the general movements of the air; but no one can fail to realize the importance of a hot blast, a blizzard, a tornado or a sand storm—or, indeed, of any occasional storm wind that may injure growing crops and destroy property.
Along coasts, the sea breeze and the alternating land breeze are the rule rather than the exception during a considerable part of the year. As a rule, the sea breeze extends rarely higher than 3000 feet. At such times it may be merely a crosswind, and the clouds at a height of a mile may be moving in an opposite direction. The succeeding land breeze which sets in is apt to be a much stronger wind.
Mountain Valley Winds are common in all mountain regions. During the day, when the air is growing warmer, the wind blows up the valley; at night, when it is losing its heat the flow is down the valley. In narrow canyons, the night winds may be very strong—a force of 6 to 7 of the Beaufort scale.
The Chinook, one of the most important local winds, derives its name from the jargon of a tribe of Indians living near the mouth of the Columbia River. According to tradition the name means “snow-eater,” from the fact that, with its appearance, the snow begins to melt first from the higher parts of the mountain slopes and, last of all, from flood plains and valley floors.
The Chinook was made known first by early settlers in Oregon. In time it was found to exist throughout much of the montane part of the northwest. The Chinook begins as a moist wind on the windward side of a high range. As it is pushed upward along the mountain slope it is chilled by expansion below the dew-point, and condensation takes place. This liberates a great deal of latent heat, materially warming the air. The air is warmed still further by compression as it rolls down the leeward slope of the range.
In Montana, Idaho and Alberta, the Chinook wind is far-reaching in its climatic effects. Both grazing and wheat-growing are made possible in regions that otherwise would be unproductive. The Chinook wind does not differ from the Foehn wind of Europe with which it is classed. In each case moist air drawn into a cyclone and pushed over a range, descends on the other side as a warm, dry wind.
The Hot Winds of the Plains, including the Summer Winds of Texas, and the Norther of the San Joaquin-Sacramento Valleys are classed among the “destroyers,” from the fact that, in many localities, two or three days of their duration is fatal to growing crops.
The Santa Ana of southern California is the outpouring of a hot, dust-laden desert wind through one or more of the mountain passes. In the past thirty-five years, irrigation and cultivation have been extended into the arid region, with a result that the Santa Ana is largely deprived of its dust content and its high temperature. The Santa Ana in its old time vigor was merely the edge of a desert simoon that intruded upon nearby fertile lands. The simoon itself occurs in every desert so far as is known. It is a sand storm because of its velocity. In the Colorado and Mohave deserts the simoon may have a velocity exceeding 75 miles an hour. The Washoe Zephyr of the Basin Region of the United States, and the Khamsin of Egypt are desert winds of the same kind. They are thought to be cyclonic in character, but practically they are dust-laden winds, either blowing into a desert, or out of a desert.
The Texas Northers are biting cold winds, common to the high western plains of the United States and northern Mexico. They usually follow warm and balmy winds of southerly direction. The onset may be very sudden. A fall of temperature of 50 degrees within a day is not uncommon. The Bora and the Mistral of the Mediterranean coast of Europe are similar in character; they are cold winds sliding down the steep mountain slopes because of increasing pressure to the northward. In the southern hemisphere, the Pampero is the counterpart. It is most noticeable in the pampas, or great plains east of the Andes, and in many instances it extends to the coast. Although a southwest wind, it is classed with Northers because of its origin.[3] The Blizzard is nominally a cold-wave wind which is sufficiently vigorous to pick up and carry loose snow; it is a northwesterly wind. Popular usage applies the name to any wind of gale force that accompanies a snowstorm.
Direction and Velocity.—The diagram of the major circulation of air shows that the normal movements of winds are northwest, southwest, northeast, and southeast and that winds from the north, south, east or west are the exception. Winds over the land, however, are apt to be modified by local topography; and it frequently happens that a surface wind differs materially in direction from the wind at low cloud heights.
Throughout the eastern half of the United States, about half the recorded mileage of the wind is from points between north and west. Along the Gulf Coast to a distance of about 300 miles inland, winds with a southerly element of movement prevail. As a rule, the winds are strongest during the winter months and mildest in summer.[4] For the greater part, the prevailing winds of the Pacific Coast region are northwesterly. At San Diego about two-thirds of the mileage is recorded by winds blowing between west and north.
The strongest winds are apt to occur along the coasts of the sea and the Great Lakes. The mean hourly velocity at Sandy Hook, Block Island, Delaware Breakwater and Cape Mendocino exceeds 14 miles. Throughout the plains west of the Missouri River, high winds prevail. The long downward slope over a smooth surface adds to the velocity of westerly winds. During winter months the cold-wave winds from Canada contribute a mass of air which, moving eastward, gives added velocity to the winds of this region.
The latitude of strongest winds in the United States is approximately along the forty-fifth parallel in the summer and a few degrees lower in winter. Winter months are the season of the strongest winds. The winds of greatest strength, however, are storm winds—winter cold-wave blasts, or the recurved portion of West Indian hurricanes which sweep northward along the Atlantic Coast.
Other Features of General Circulation.—The foregoing paragraphs present a very elementary view of the greater circulation of the air. As a matter of fact, not much is known, even of the surface winds over a very large part of the earth. Until within the past few years, knowledge of the upper winds was imperfect and fragmentary. Sounding balloons and kites furnished with recording apparatus are beginning to supply humanity with much-needed information concerning horizontal movements of the upper air; the airmen are furnishing knowledge of vertical movements.
Research in recent years shows that the updraught in equatorial regions is not uniform in force nor continuous at all times. Neither is the overflow of rising air toward polar regions uniform or regular. Sounding balloons occasionally have been carried toward the equator instead of away from it. Stiff west winds also have been observed in equatorial regions at the height of a few thousand feet, surmounted by easterly winds at a still greater elevation.
Sounding balloons do not find the decrease of temperature with increasing altitude to be regular; on the contrary, they encounter layers of air throughout which the temperature is practically unchanged. They also encounter other layers in which an inversion occurs—that is, the temperature rises with increasing altitude. In other words, instead of a uniform temperature gradient from ground level to stratosphere, the air consists of a succession of layers, differing in temperature, humidity and horizontal velocity of movement. Usually the planes of contact between adjacent layers are indicated by clouds.
The air of adjacent layers, or strata, does not readily mix one with the other. Smoke, dust, and cloud matter, rising to the top, or sinking to the bottom of a layer does not always penetrate the adjacent layer. In the absence of strong winds such matter is apt to spread out laterally. Moreover, the aviator, in passing from one layer to another, is apt to receive a sharp bump at the plane of contact. In meteorology the plane of contact is commonly known as a ceiling or lid.
The convectional layer of air—that is, from ground level to stratosphere—is marked by constant motion as noted, the movements consisting of general circulation, local winds and the turbulence connected with vertical movements. There seems to be no such complexity of movement in the stratosphere; indeed the knowledge of the movements of the air in the stratosphere is next to nothing. Tidal movements probably warp the shape of the shell of air composing it; but they may not cause a general circulation. The fact that the air of the stratosphere is warmer in high than in equatorial latitudes indicates that a circulation of some sort exists and that the general movement may be the reverse of that of the lower shell of air. The coldest air is over the warmest zone.
Winds Encountered by the Airman.—The marine pilot is concerned wholly with the horizontal movements of the surface air; he is not conscious of the updraughts or the downdraughts of convection. To the airman, on the other hand, the horizontal air movements are usually less of consequence than the vertical movements. Good air for flying must be free from holes and bumps.
An air hole is not a vacuous space, nor is it one in which the density of the air is abnormally low. Sometimes it is a downdraught; quite often it is convectionally still air. If the airman has been flying over hot, bare ground, where the updraught is strong, his plane takes a drop when he passes over a patch of greensward, where the updraught ceases; this is the airman's “hole.” In going from convectionally still air into an updraught, he gets a “bump.” The same result is apparent if, while traversing a downdraught, he strikes still air.
It has been noted that the air ranges itself in layers differing in density, temperature, and moisture content. In many cases an acquired sense born of experience enables the airman to discern these layers and to adjust the wings of his plane in encountering them. Frequently a sheet of smoke or dust separates two cloud layers and experience has taught how to avoid or how to penetrate it.
Air is either going up or coming down. Turbulent ascending currents are manifest in the rapid motion of cumulus clouds, both within the cloud and beneath it. Measurements have shown that ordinary updraughts may have a velocity of 10 feet per second; under a cumulus cloud of the thunderhead type the velocity may be as high as 40 feet per second. The updraughts that produce clouds give visible signs of their existence. Those caused during clear days by bosses of rock or by bare ground are not so easily detected. They are not apt to begin existence until the sun is high enough to heat the areas producing them; they rarely form during cloudy days.
Downdraughts are sometimes real and sometimes only apparent. In passing from an ascending current into still air the drop may be real, but the downdraught may be merely apparent. In flying from an adverse wind into a wind blowing in the direction in which the plane is moving, the drop is real but the downdraught is apparent merely.
There are actual downdraughts, however, which the airman is certain to encounter—because air going up must be balanced by air coming down. Just as water pours over a perpendicular ledge, forming thereby a cataract, so air is usually pouring over a steep scarp in a similar manner. The air over a plateau is apt to be colder than that several hundred feet below. More certainly it will be colder if it has traversed great fields of snow. When, therefore, it reaches a steep scarp it pours over the edge by virtue of its own gravity. Air-falls of this sort are common in mountainous regions, but they rarely occur in lowlands.
Billow-cloud levels may be a serious problem to the airman, not because they interfere with visibility, but because occasionally they do not do so. When billow clouds are in sight the airman may fly above them or below them. If the two wind layers have about the same degree of humidity there may be no clouds to indicate the position of the plane of contact. Once within this plane, the airman experiences a series of disconcerting bumps, due to the quick transition from one billow to another; sometimes he finds it difficult to rise to the upper layer of air.
Gusty winds, eddies and whirls occur most frequently near ground level; they rarely affect high flights. Even in low flights they are infrequent, unless the plane is within the influence of cumulus clouds. They are disconcerting in making sharp turns and they may be dangerous in making a landing.
- ↑ Table, page 240.
- ↑ On various occasions vessels whose cargoes consisted of horses were becalmed in this region. When the supply of water gave out the horses were thrown overboard—hence the name “horse latitudes.”
- ↑ The name is also applied to the “squall” type of descending wind accompanied by thunder and lightning, occasioned in the pampas of South America.
- ↑ The records of about twenty stations in the northeast quarter of the United States show the following mean velocities in miles per hour for the year:
Northwest
8.8 West4.6 Southwest5.2 Northeast5.3 East4.7 Southeast4.8