Handbook of Meteorology/Storms
THE DAILY WEATHER MAP: STORMS
The Daily Weather Map
The daily weather map is a bird’s-eye view of the United States with respect to temperature, pressure, wind, and storm at 8 o’clock in the morning, seventy-fifth meridian, standard time. A few minutes before 8 o’clock more than two hundred observers are busy recording all weather conditions covering the various stations. These observations are completed by 8 o’clock, morning and evening, and are promptly telegraphed to the Weather Bureau at Washington in coded form. So carefully and thoroughly is the work done that a few code words from each station contain all the necessary information concerning temperature, pressure, direction of the wind, rain or snow, cloudiness, moisture, thunder-storms, fog, and other phenomena.
Making the Daily Weather Map.—At the central office, and also at certain other designated offices, the figures and other information are charted on a base map containing the name and position of each station, the boundaries of the states, and the outline of the United States. For the sake of clearness, all other features and names are omitted. Blue lines are drawn to indicate isotherms; red lines similarly indicate isobars. When the isobars are completed it will be found that some of them are roughly concentric, inclosing irregularly shaped ellipses. In some of these the pressure is highest at the center; in others, the center is the point of lowest pressure. These are the highs and the lows that indicate storm centers—that is, anticyclones and cyclones.
In order to give the forecaster additional information, the direction of the wind and the sky condition must be noted. These are shown in each case by a circle pierced with an arrow. The arrow points the direction of the wind. If the sky is clear, the circle of the arrow is left clear; if partly cloudy, half the circle is blackened; if cloudy, all the circle is blackened. An R in the circle indicates rain; S means snow; and M means that the report for that particular station is missing. All localities in which rain or snow is falling are shaded. The map thus finished is the daily weather map, and from it the forecasts of the following twenty-four to thirty-six hours are made. These also may include information discovered by the long- distance forecasts.
Distributing Weather Information.—The base maps on which the information is to appear are distributed to such stations as issue daily weather maps. As soon as the matter described in the preceding paragraph has been placed graphically on the map, it is reproduced by a quick process in the form of a printing plate. The matter for the plate is usually ready by half-past nine o’clock, and is printed on the base maps which are usually folded, wrapped, and addressed within a short time. At the New York City Station about 3000 maps are required for the daily issue. They are sent to shippers, railroad offices, merchants, newspapers, educational institutions, post offices, and public places of various sorts. Almost every daily paper publishes a résumé of the weather map; some reproduce the map itself. All told, the daily forecast is so widely published that it is within almost instant reach of everyone within the main body of the country.
Features of the Weather Map.—The chief desire of the public is to learn whether the weather during the succeeding few hours is likely to be pleasant or stormy, warmer or colder, clear or cloudy, quiet or windy. These are features that affect all people; and the daily weather map answers the questions correctly a little more than four times in five. The verification of rain or of snow practically is four times in five; of tempera- ture and wind direction, rather better than four times in five.[1]
A study of the weather map will show the area or areas in which rain or snow was falling at the time of observation; it will show where freezing temperatures may, or may not have
A winter cyclone (low) followed by a cold wave (high). The daily weather of Jan. 26, 1905.
existed; it will show the areas of clear, cloudy and windy conditions. The man in Chicago may learn at a glance the weather conditions at Los Angeles, New York City, Winnipeg, Key West, Bermuda, or Havana. A merchant who has shipped perishable goods may learn whether or not his consignments are threatened by washouts, snow blockades, or cold waves. In other words, the daily weather map is very much more than a mere bird’s-eye view of the air and its conditions; it is a survey—a topographic map—with measured values.
Storms
The Movement of Weather Conditions.—The ripples, whirl-pools, and waves of a river are carried along in its flow; so also the waves and whirlpools of the air are carried along with the great streams of the air. Throughout the greater part of the United States, this movement is from a westerly to an easterly quadrant—that is, the greater part of the main body is in the belt of Prevailing Westerlies. The Gulf Coast, together with the Florida Peninsula are in the Trade Wind belt in summer, but not in winter. Therefore, such movements as cyclonic areas or lows will move from a westerly to an easterly quadrant with the velocity of the general movement of the air.
The paths of the principal types of cyclonic storms which have already been described are shown on the accompanying map. They are discovered by means of the isobars. That is, somewhere in the middle western part of the United States an isobar of 30.00 inches will be found to inclose an elliptical area. Within this area isobars are drawn for every tenth of an inch of decreasing pressure. This area is a low, and probably an area of updraught; if the pressure is below 29.50 inches it is pretty certain to be a strong updraught, and the arrows which indicate wind direction are pointing toward the center of the low.
If the pressure at the central part of the low is only two- or three-tenths below 30.00 inches, the updraught is not very strong and the winds blowing into the low are light. Rain or snow may or may not be falling. On the other hand, if the pressure within the low is 29.50 inches or less, rain or snow, followed by heavy winds, is pretty certain to occur, mainly on the east and south sides of the cyclones; if the pressure falls below 29.00 inches a violent storm, with winds from whole gale to storm strength, is certain.
The various types of cyclonic storms differ but little in character, and their names apply to the locality where they originate or are first observed. Thus, they are variously known as “Alberta,” “North Pacific,” “Northern Rocky Mountain,” “Colorado,” “Texas,” “Central,” and “West Indian.” Other names occasionally are used in designating the storms. The
Wind, cloud, and precipitation in a cyclonic storm.
system employed by the Weather Bureau is one of convenience rather than of scientific value. About a third of the storms that cross the continent are of the Alberta and North Pacific type.
The map, p. 148, shows the isobars of a cyclonic storm. The low pressure at the center indicates a storm of unusual intensity; this is indicated also by the closeness of the isobars. In other words, the pressure gradient is steep, when the isobars are close, and this also indicates the degree of violence of the storm.
Observations covering more than twenty years show that winter storms of the United States advance at the rate of a little more than 700 miles per twenty-four hours; summer storms cover about 500 miles. These figures differ from the values obtained by the British Meteorological Office, 576 miles and 474 miles per day respectively. The progress of the cyclone is merely the velocity of the general drift of air, and this varies in different latitudes, and at different times.
Inasmuch as the storm tracks of the different types are fairly regular in position, and the velocity of progress is known, it is not difficult to forecast the position of a storm from day to day; that is, a storm center which is over Cincinnati may be expected to reach Philadelphia or New York at about the same hour on the following day. Fast express trains run at a rate of speed that rarely varies; the cyclonic storm moves also at a fairly uniform speed. The express train does not ordinarily leave its steel-bound track; in this respect it differs from the cyclonic storm which occasionally does swerve from its expected track to the confounding of the forecaster. This is likely to happen about once in five times.
Let us suppose that a storm of the Alberta type, after reaching the Great Lakes, takes a dip southward and passes off the coast somewhere near Cape May, instead of following a predicted course across New York. In the eastern part of the United States practically all forecasts north of Cape Hatteras will be upset. Instead of rain, central New York and Massachusetts will have clear or partly cloudy weather. Baltimore and Washington will have cloudiness, easterly winds and rain, instead of clear or partly cloudy skies.
Not only may a cyclonic storm swerve from its predicted track; it also may fail to produce the rain or the snow which, according to popular tradition, constitutes the storm. As a matter of fact, the rain and the snow are merely an incident in a cyclonic movement. The essential feature of cyclone mechanics is the updraught. Now, in its progress if the cyclone invades an area of very dry air, the updraught may not be cooled to the temperature of condensation; in such a case there will be no precipitation. All lows are not rain storms or snow storms in the ordinary meaning; but practically all the rain and snow that fall on large areas accompany winter lows.
Let us take a low which is central in Illinois. The wind is blowing into it from all quadrants, to fill the updraught. The storm is preceded by a wind from an easterly quadrant and clears with one from a westerly quadrant, which is apt to settle in the northwest. Within the storm area the winds acquire a spiral motion, whirling upward contra-clockwise as they approach the updraught. The whirl brings warm and moist air from a southerly region to the easterly side of the low, where the air is colder and the temperature nearer to the dew-point—that is, to condensation. For this reason, most of the precipitation is on the east and south sides of the low. On the west side colder and drier air is blowing from the west and the northwest and, being colder, is drawn into the updraught to a less extent or perhaps, not at all. Westerly and northwesterly winds, therefore, usually are clearing winds.
Just as the trough of a wave is followed by a crest, so a low is pretty apt to be followed by a high; and cyclonic storms of the Alberta type are frequently followed by crests or waves of cold air from high latitudes. If a winter high pressure area lies over the northwestern part of North America, and a low forms anywhere in the vicinity of this area, a flow from the high to the low will naturally follow. This means that, in order to fill the low, the clearing northwest winds must also be descending currents; and, as a matter of fact, they flow along the surface, lifting the warm air above them. In their flow into lower latitudes and their descent, they, too, acquire a whirl. But the whirl is clockwise, or the reverse of the whirl of the cyclone; hence it is known in Weather Bureau cant as the “anticyclone.” The winter anticyclone, therefore, is usually a cold wave.
The high of the winter cold wave is an area of considerable pressure. Usually the barometer stands above 30.50 inches; occasionally it mounts nearly to 31.00 inches. For this reason the cold air spreads far south—sometimes carrying freezing weather far into Florida, to the detriment of the semitropical orchards. The southern part of Florida is the only part of the United States which escapes freezing weather.
In many respects, the cold wave is one of the most valuable health assets of the United States. Should the ground be covered with snow, so that gale winds pick up no dust, it brings the purest air that mortals on land ever breathe. Even if the 
A winter barogram; record of the Meteorological Laboratory at Mount Vernon, N. Y.
ground is bare, the high pressure invades the nooks and crannies where foul air and putrefaction lurk, and drives them out. The cold wave with its stinging wind is the greatest scavenger in existence.
West Indian Hurricanes.—The West Indian hurricanes do not differ materially from other cyclonic storms in general principles, and they differ from the typhoons of the China Sea in name and place only. They are cyclonic storms of very great violence and, with the exception of tornadoes, they are the most destructive storms that reach any part of the United States. The wave that covered Galveston, the floods that many times have swept the Sunderbunds of India, and the storm that caused Isle Dernier to melt away were hurricanes of the cyclonic type—whirling up draughts toward which the surface wind blew from every direction.
Storm tracks of the United States: Heavy arrows, cyclones; broken arrows, anticyclones.
The West Indian hurricanes originate in tropical latitudes, somewhere north of the equator. They move in a northwesterly direction until they reach the latitude of westerly winds; then they recurve and move in a northeasterly course. In some instances a hurricane recurves before reaching the Florida coast; in others it advances until the recurve crosses the Gulf of Mexico. In the first instance it is not likely to cover anything more than
Storm cards showing movement of wind in West Indian hurricane.
These storms are called West Indian hurricanes from the fact that they are first noted at a West Indian weather station, frequently at Barbados. They sometimes originate far to the eastward of the West Indies, sometimes in the Caribbean Sea. Since vessels are now fitted with radio-telegraphic apparatus, hurricanes are commonly reported before reaching a land weather station. Once discovered, their movements are closely watched and are made known to shipping until they disappear in the North Atlantic.
The dead calm of tropical seas is the real beginning of the West Indian hurricane. The air, moist almost to the dewpoint, is heated next the surface until it becomes more buoyant than the colder air above it. Finally the unstable equilibrium is overcome and an updraught occurs. The warm air of the updraught is chilled by its expansion and its moisture is condensed. The latent heat thus set free adds to the strength of the draught, and the cyclonic movement quickly develops into a hurricane of tremendous energy. Hurricane winds at Galveston were estimated to have a velocity of 125 miles per hour; 100 miles per hour was registered before the anemometer was blown away.
According to Chief Forecaster E. H. Bowie, U. S. Weather Bureau, if a West Indian hurricane, moving westward in the longitude of eastern Cuba, is north of the island, it will recurve east of Florida, provided an area of high pressure covers the northwestern states. But if the hurricane is moving westward over Cuba or the western Caribbean Sea when an area of low pressure occupies the northwest, and the pressure is high in the eastern states, the storm will probably move to the Gulf of Mexico and reach the Gulf Coast after recurving.
Form and Dimension of Cyclonic Storms.—Extended measurements of the areas of low and of high pressure, made by Loomis and based on the isobars of the daily weather map, showed them to be elliptical in form, the longer axis usually pointing a little east of northeast. The average dimensions were found to be 1600 miles on the long axis by about one-half of that extent along the short axis. The average dimension of anticyclones is about the same. These values apply pretty closely to the dimensions of the cyclonic storms of western Europe.
The low of the West Indian hurricanes is very much smaller in area. Even after its existence has been discovered it may not be more than 100 miles in diameter; and by the time it passes a West Indian weather station it may not be more than 200 or 300 miles across. After it recurves and enters the United States, its area is much less than that of the ordinary cyclonic storm; the isobars are usually regular and more nearly of circular shape than those of ordinary storms.[2]
Storm Probabilities.—Before storm forecasts were sent to vessels by radio-telegraphy, the sailing master of the vessel was obliged to rely upon himself for weather predictions. He based his forecasts on his barometer, clouds, and the wind. A close study of these enabled him to make forecasts that were marvelously good. With intelligent study of wind, clouds, and moisture, one should be able to forecast most ordinary weather changes from eight to twelve hours in advance, without the aid of barometer or weather map. This does not apply to such local disturbances as tornadoes, thunder-storms and hail, nor to such conditions as ice storms and sleet.
Throughout the greater part of the United States easterly winds indicate the approach of a cyclonic storm. If the wind is from the south or the southeast, the storm is probably approaching along a path to the north of the observer; if the wind has settled to a quarter between east and northeast, the track is somewhere south of the observer; if the wind is due east, the observer is probably in or near the track of the storm center.
If the sky remains clear with an easterly wind the rain area is likely to pass some distance from the observer; but if the sky becomes gray, and then white, and the air perceptibly damper, rain is not likely to be far away. When cirro-stratus clouds appear in the easterly sky, rain or snow is pretty certain at hand within a few hours.
The position of the storm center may be determined by watching the wind closely and noting any change that may occur. Standing with the back to the wind the area of low pressure is on the left hand, and the area of high pressure on the right hand. During the passage of the storm, if the wind shifts from the east through north to northwest—that is, if it “backs in”—the cyclone center is passing to the south of the observer. If it veers through the south to the west or the northwest, the storm center is passing north of the observer.
The cooperative observer can do much to aid in establishing definite facts on which predictions may be made. Among them and of first importance is establishment of the direction of rain-winds. These, as has been shown, are easterly winds, but conditions of topography may change the real direction to one that is apparent. The apparent direction should be established for each month in the year. In every community there are weather-wise people who possess valuable information that they have not recorded. Such information should be considered carefully and accepted or rejected as the case may be.
The number of days in each month on which o.oi inch or more of rain has fallen should be. noted, tabulated, and compared with the map of rain frequency published by the Weather Bureau. From this table a coefficient of the probability of rainfall for the particular station may be deduced by dividing the number of rainy days by 365, or 366, as the case may be. In a similar manner, the probability of rain for each month may be established. It is pertinent to add, however, that forecasts made from such coefficients are by no means certain; often they are disappointing.
The average duration of rainfall may be deduced by dividing the total number of hours during which rain has fallen for the month by the number of rainy days. If the duration is to be based on the average length of storms, the number of storms may be taken as the divisor. In the northeastern part of the United States the average duration of rainstorms is five hours; in the southeastern part, four hours; in the western highland region, including the plains, about three hours; and in the basin region probably not more than one hour. The intensity, or rate of rainfall per hour, is a matter of great importance. It is tabulated at regular intervals at Weather Bureau stations.
It is well to bear in mind that the artificial production of rain is a delusion. No appreciable fall of rain will occur unless a continued updraught of air is produced, and neither cannonading nor explosions at a considerable height has accomplished this. Possibly the conjunction of planets may affect the movement and the formation of storms; if so, however, the connection has not been established.
Secondary Storms; Tornadoes.—When a whirlpool forms in a stream, smaller whirlpools almost always occur near its edge. These secondary whirls result from the formation of the larger whirl. Similarly, secondary whirls of the air are very apt to accompany the cyclonic storms which pass over the Great Lakes and down the St. Lawrence Valley. In the winter the secondary storms thus formed appear along the Virginia coast, or perhaps to the north of it, and move north or northeast with heavy snow squalls and high, gusty winds. In the summer they are attended by hailstorms, thunder-storms and tornadoes. These occur usually on the south or the southeast side of the low.[3]
Tornadoes are less frequent than thunder-storms, but they are the most violent and destructive storms that come into the experience of humanity. The term is loosely applied to almost every violent wind; it is incorrectly applied to any secondary storm that is not a true whirlwind, or “twister.” There are no definite conditions known by which tornadoes may be forecast; but when the path of a northerly storm dips southward and increases in intensity, tornadoes are likely to occur.
The tornado is a whirling storm, and the whirl becomes so rapid that the vortex develops into a black funnel-cloud. The funnel is usually observed first in the air. As the whirl increases, the funnel gradually extends downward to the ground. No measured velocity of the whirl is known to have been made; but calculations based on the weight and the surface of bodies moved by the wind show that the velocity, in various instances, has exceeded 500 miles per hour.[4]
The first visible warning of the tornado is the gathering of a bank of very dense cloud, usually in a westerly quadrant—southwest, west or northwest. The color of the cloud bank varies. Not infrequently it appears much like the smoke from a burning hay barn, or a strawstack; quite frequently it is a dark greenish gray. The color depends on the position of the observer with reference to the sun. The cloud bank is always in tumultuous commotion within itself.
It is in this cloud bank that the funnel of the tornado forms. In some instances, as the tip of the funnel approaches the ground, an inverted funnel is formed at the ground, quickly joining the funnel hanging from the cloud. The funnel is the destructive part of the tornado. It uproots trees, or twists their trunks to the breaking point. Wherever the tornado passes through woodlands its path is marked by uprooted, shattered and twisted trunks of trees. When the funnel strikes a building the latter bursts outwardly. In various instances a roof has been carried in fragments a distance several miles away. Wooden railway bridges have been dismembered and splintered beyond repair, and steel bridges have been torn from their abutments and crumpled into shapeless heaps. Chickens have been almost completely plucked; straw and twigs have been driven
Lake Gervais tornado, July 1, 1890, as seen from St. Paul. Note the mass of air as shown by the upper cloud overrunning the warm surface air.
endways into boards; and large animals have been lifted and carried considerable distances. In one case a cow was lifted out of a high corral and deposited, not seriously injured, several hundred feet away.[5]
The cause of the tornado cannot always be determined; in a few instances it has been assumed by reason of strong circumstantial evidence. During the passage of a northerly
The graphic story of a tornado.
The air movement of tornadoes is three-fold—the updraught, the whirl, and the progressive movement. The destructive path of the tornado is as wide as the funnel-cloud, rarely more than a few rods. The entire whirl is not much more than half a mile in diameter; the extent of the path varies from a few miles to about 200 miles. The tornado progresses along its track at a rate varying from 10 or 12 miles an hour to express-train speed. The funnel-cloud is formed at a height of about half a mile.
From the nature of the case, the best values concerning the dynamic force of the tornado are only approximate, but even these are instructive. Normal air pressure is at the rate of 2117 pounds per square foot. Now, if the air pressure within the funnel is only three-fourths normal when the funnel involves a building, the air pressure inside the building will be 530 pounds per square foot greater than on the outside. Such a difference in pressure is sufficient to burst the walls of almost any building.
Tornadoes are most prevalent in May, June and July; the average of these months exceeds that of the rest of the year. They are more common in the United States than in Europe. The regions of greatest frequency are the lower Ohio and Missouri valleys and the Central Mississippi region. Very few occur in the arid region west of the one-hundredth meridian and fewer still are reported north of the fiftieth parallel.[6]
Hail and electrical discharges frequently accompany tornadoes, but they have nothing to do with the cause of them; and although the updraught occurs in thunder-storms—and probably a cyclonic movement of the air within it—one is hardly warranted in considering the tornado as an exaggerated thunderstorm.
Desert Whirlwinds.—Dust spouts are common in desert regions. A little after sunrise during warm weather, the still air next to the ground becomes very much warmer than the air at the distance of a few hundred feet above the ground. In time the unstable equilibrium is upset and chimneys of updraught are formed here and there, carrying columns of fine dust to a height of several hundred feet. At a distance the dust columns are strongly outlined. When the cold air has settled to the ground the whirl and its dust column ceases. Later in the day, the setting in of a steady wind puts an end to the unequal warming of the air.
Apache Indians have made use of the desert whirls as signals, creating them by setting fire to the spines of a columnar cactus that is common in the southwestern states. The burning of the spines at the right moment made enough heat to start the up- draught. When the warm air at the surface has been pressed upward the descending air is perceptibly colder at times.
Waterspouts.—If the whirl of the updraught over water increases to a velocity whereby the skin friction of the wind overcomes the cohesion of the water, a waterspout is formed. The whirl of the updraught is strong enough to whisk the water into the air, at the same time whirling it into a mist. Undoubtedly some of the water drawn into the air is vaporized. When the spout breaks, a considerable part of the water in the air drops in a torrential deluge. Popular tradition has it that sea water drawn into a spout falls as fresh water—a tradition that is contrary to the facts of the case.
White squalls are fair-weather whirlwinds over the water. In many instances there is not enough condensation in the air to form a cloud; occasionally, however, a bit of misty cloud, the “bull’s eye,” is visible. At the surface, the wind is strong enough to whisk the water into white spray, but the whirl is not strong enough to draw it into the updraught.
- ↑ The percentage of verification varies with locality. In California, where the rain and temperature conditions are seasonal, the percentage of verification is high.
- ↑ It is not unlikely that the eccentricity of the ellipse of the cyclonic storm depends on the velocity of the whirl—the higher the wind-velocity, the more nearly it approaches a circular form.
- ↑ In the southern hemisphere they form on a northerly quadrant.
- ↑ This was computed by Bigelow in the case of the Missouri tornado of May 27, 1896.
- ↑ The accompanying diagram illustrates a gruesome story. When the funnel cloud approached the house, the family fled. At first they ran northward, a direction of safety. Then, one after another, they turned eastward and ran into the whirl. The younger of two girls ran directly into the tornado path and was instantly killed. The mother had reached a place of safety; then she turned into the tornado path and was crushed to death against a tree trunk. The older girl and a boy also turned toward the storm track; their clothing was stripped from them and they were torn and bruised. The father, with the baby in his arms, had reached a place of safety; then, in fright, he too ran back into the storm track where both were killed. This is one story that illustrates many. As a rule, the path of safety is toward the northwest or the southeast if the direction of the tornado track can be determined.
- ↑ Sir Napier Shaw, of the Meteorological Office, London, does not even mention tornadoes in his “Forecasting Weather.” They are unknown in the British Isles, the line squall being its nearest approach.