Read Ebook: Practical Exercises in Elementary Meteorology by Ward Robert DeCourcy

Font size: 10 px 15 px 20 px 25 px 30 px 35 px

Background color: BlackWhiteGrayLight GrayDark GrayDim Gray

Text color: BlackWhiteGrayLight GrayDark GrayDim Gray

Apply/Save settings

Ebook has 292 lines and 120433 words, and 6 pages

The observations are received at the central office of the Weather Bureau in Washington by special wires, and are usually all there within an hour after the readings were made. As the messages are received in the forecast room, they are translated from the cipher back again into the original form, and the data are entered upon blank maps. The official charged with making the forecasts then draws upon the maps lines of equal temperature, lines of equal pressure, lines of equal pressure-change and temperature-change during the past 24 hours. These several sets of lines, together with those showing the regions of precipitation during the past 24 hours, furnish the necessary data on which the forecasts can be based. In other words, the forecast official has before him, on the several maps, a bird's-eye view of the weather conditions over the United States as they were an hour before, and also of the changes that have taken place in these conditions during the preceding 24 hours. Thus, by knowing the general laws which govern the movements of areas of high and low temperature, of fair and stormy weather, across the country, he can make a prediction as to the probable conditions which any state or section of the country will experience in 12, 24, or 36 hours.

In a later chapter some suggestions will be given for studies of forecasting.

The forecasts made in Washington, and printed on the Washington daily weather map, relate to all sections of the United States, and include predictions of cold waves, killing frosts, storm winds, river floods, and the like, besides the ordinary changes in weather conditions. These forecasts, as soon as made, are at once given to the local newspapers and to the press associations. They are also sent by telegraph to all regular stations of the Weather Bureau, and to all stations at which cautionary or storm signals are to be displayed, along the Atlantic or Gulf coasts, and on the Great Lakes.

The Washington weather map is about 24 by 16 inches in size, and is newly lithographed each day. The total number of maps issued from the central office during the fiscal year ending June 30, 1898, was 310,250. In addition to these, there are now 84 stations of the Weather Bureau in different parts of the country, at which daily weather maps are issued and local forecasts made. These latter forecasts are made by a corps of local forecast officials, each of whom has to make the weather prediction for his own district. At first, and until within a few years, one predicting officer in Washington made all the forecasts for the country, but it was found better to have the country divided into geographical sections, over each one of which the meteorological conditions are fairly similar, and to have a local forecast official in charge of each section. These local forecast officials have the double advantage of being able to study the weather conditions over the whole country, as sent them by telegraph each morning, and also of knowing the special peculiarities of their own regions. This enables them to make more accurate predictions than can be made by an official who may be one or two thousand miles distant, in Washington.

The greater portion of the maps issued at the map stations outside of Washington are prepared by what is known as the chalk-plate process, suggested by Mr. J. W. Smith, local forecast official at Boston. This process is as follows: A thin covering of specially prepared chalk, 1/8 of an inch in thickness, is spread upon a steel plate of the size of the prospective weather map. On this chalk are engraved, by means of suitable instruments, the various weather symbols, the lines of equal pressure and of equal temperature, and the wind arrows. The plate is then stereotyped in the ordinary way, and printed on a sheet prepared for the purpose, which has a blank outline map of the United States at the top, and space in the lower half for the forecasts, summary, and tables.

The size of the chalk-plate map itself is 10 by 6-1/2 inches; the size of the whole sheet, which includes also the text and tables, 16 by 11 inches. Weather maps prepared by the chalk-plate process are now issued from 28 of the 84 stations which publish daily maps. At the remaining stations the maps are prepared by a stencil process, the size of the map being 13-1/2 by 22 inches. The total number of weather maps issued at the various stations during the fiscal year 1897-1898 was 5,239,300.

Besides recording the usual meteorological data, and publishing weather maps and forecasts, the various stations of the Weather Bureau serve as distributing centers for cold wave, frost, flood, and storm warnings. These warnings are promptly sent out by telegraph, telephone, and mail. Besides these usual methods of distributing forecasts, other means have also been adopted. In some places factory whistles are employed to inform those within hearing as to the coming weather; railway trains are provided with flags, whose various colors announce to those who are near the train fair or stormy weather, rising or falling temperature; and at numerous so-called "display stations," scattered all over the country, the forecasts are widely disseminated by means of flags.

TEMPERATURE.

Notice that the warmest districts on the map are in Florida, along the Gulf Coast, and along the coast of California. The marked contrasts in temperature between the Northwest and the Pacific and Gulf Coasts at once suggest a reason why Florida and Southern California are favorite winter resorts. To these favored districts great numbers of people who wish to escape the severe cold of winter in the Northern States travel every year, and here they enjoy mild temperature and prevailingly sunny weather. To the cold Northwest, on the other hand, far from the warm waters of the Pacific, where the days are short and the sun stands low in the sky, no seekers after health travel. This annual winter migration from the cities of the North to Florida and Southern California has led to the building of great hotels in favored locations in these States, and during the winter and spring fast express trains, splendidly equipped, are run from north to south and from south to north along the Atlantic Coast to accommodate the great numbers of travelers between New York, Philadelphia, Boston, Chicago, and other large northern cities, and the Florida winter resorts. Southern California also is rapidly developing as a winter resort, and rivals the far-famed Riviera of Southern Europe as a mild and sunny retreat from the severe climates of the more northern latitudes. The control which meteorological conditions exercise over travel and over habitability is thus clearly shown. Florida and Southern California are also regions in which, owing to the mildness of their winter climates, certain fruits, such as oranges and lemons, which are not found elsewhere in the country, can be grown out of doors, and these are shipped to all parts of the United States.

Carry this process a step further by drawing the line which shall pass through all places with a temperature of 40?. This line begins at Jacksonville, Fla. , and runs west, passing between Montgomery, Ala. , and Pensacola, Fla. . Thence it turns to the northwest, passing between Vicksburg, Miss. , and New Orleans, La. , and through Shreveport, La. . From Shreveport it turns to the southwest, passing to the north and west of Palestine, Tex. , and down through San Antonio, Tex. . Its further exact location cannot be determined in Mexico, because there are no observations from Mexican stations, but the readings at Yuma, Ariz. , and at San Diego , Los Angeles , San Francisco , Red Bluff , and Cape Mendocino , all in California, show that the 40? isotherm may be started again just north of Yuma, and may be carried up through California, nearly parallel with the Pacific Coast, ending between Cape Mendocino, Cal. , and Roseburg, Ore. . You have now drawn the isotherms of 30? and of 40?, and in order to avoid confusion, mark the ends of the first line 30? and the ends of the second line 40?.

The dotted lines in Fig. 18 show the positions of the isotherms when drawn. Notice how clearly the temperature distribution now stands out, and how simple the description of that distribution has become. Observe that the isotherms, although more or less irregular, show a good deal of uniformity in their general courses, and this uniformity is a great assistance in drawing them. Study the distribution of temperature on this map, and the positions of the isotherms, very carefully.

In order to bring out the temperature distribution on the maps more clearly, color all that portion of each map which lies within the -20? isotherm a dark blue; that portion which is between the 0? isotherm and the -20? isotherm a somewhat lighter shade of blue, and those districts which are between 0? and +30? a still lighter blue. The portion of the map above 30? and below 40? may be left uncolored, while the districts having temperatures over 40? may be colored red. In the map for the third day the district which has temperatures below -50? should be colored darker blue than any shade used on the other maps, or black, in order to emphasize the extremely low temperatures there found. Figs. 19-24, on which the isotherms are shown, also illustrate the appearance of these maps when the different temperature areas are colored, as has been suggested.

Study the maps individually at first. Describe the temperature distribution on each map. Ask yourself the following questions in each case: Where is it coldest? Where warmest? What is the lowest temperature on the map? What is the highest? At what stations were these readings made?

Then compare the successive maps and answer these questions: What changes have taken place in the intervening 24 hours? In what districts has the temperature risen? What is the greatest rise that has occurred? Where? In what districts has the temperature fallen? What was the greatest fall in temperature and where did it occur? Has the temperature remained nearly stationary in any districts? In which? You will find it a help in answering such questions to make out a table of all the stations, and to indicate in columns, after the names of the stations, the number of degrees of rise or fall in temperature at each place during the 24-hour interval between the successive maps. When the temperature is higher at any station than it was on the preceding day, note this by writing a plus sign before the number of degrees of rise in temperature. When the temperature has fallen, put a minus sign before the number of degrees of fall. Thus, New Orleans, La., had a temperature of 48? on the first day. On the second it had 33?. Therefore the change at New Orleans was -15? in the 24 hours. At Key West, Fla., the change was +11? in the same time.

Write a brief account of the temperature distribution on each day of the series, and of the changes which took place between that day and the one preceding, naming the districts and States over which the most marked falls and rises in temperature occurred, with some indication of the amount of these changes. Note especially the changes in position, and the extent, of the districts with temperatures below -20?; between 0? and -20?, and between 30? and 0?. Write out a clear, concise statement of the temperature distribution and changes shown on the whole set of six maps.

Notice that the region from which the greatest cold came in this cold wave is Canada. In that northern country, with its short days and little sunshine, and its long, cold nights, everything is favorable to the production of very low temperatures.

Draw similar lines on the other isothermal charts, for the same stations. Are the directions of temperature decrease the same on these charts as on the chart for the first day, for Kansas City, Seattle, Salt Lake City, Denver, St. Paul, Cleveland, New York? Draw lines of decrease of temperature from the following additional stations: Key West, Fla.; New Orleans, La.; Charleston, S. C.; El Paso, Tex.; San Diego, Cal.; Hatteras, N. C.

Compare the directions of these lines on the different days. How do they change from one day to the next?

And, using the school file of weather maps,

The determination of the rates of temperature decrease under these different conditions over the United States prepares us for an appreciation of the larger facts, of a similar kind, to be found on the mean annual and mean monthly isothermal charts of various countries, and also of the whole world.

In our winter months the contrasts of temperature in the United States are, as a rule, violent, there being great differences between the cold of the Northwest and the mild air of Florida and the Gulf States. In the summer, on the other hand, the distribution of temperature is relatively equable, the isotherms being, as a rule, far apart. In summer, therefore, we approach the conditions characteristic of the Torrid Zone. These are uniformly high temperatures over large areas. The same thing, on a larger scale, is seen over the whole Northern Hemisphere. During our winter months the isotherms are a good deal closer together than they are during the summer, or, in more technical language, the temperature gradient between the equator and the North Pole is steeper in winter than in summer.

WINDS.

When you have finished drawing these arrows, you will have before you a picture of the wind directions and velocities all over the United States at the time of the morning observation on this day.

The wind arrows on your map show the wind directions at only a few scattered points as compared with the vast extent of the United States. We must remember that the whole lower portion of the atmosphere is moving, and not merely the winds at these scattered stations. It will help you to get a clearer picture of this actual movement of the atmosphere as a whole, if you draw some additional wind arrows between the stations of observation, but in sympathy with the observed wind directions given in the table and already entered on your map. These new arrows may be drawn in broken lines, and may be curved to accord in direction with the surrounding wind arrows. Heavier or longer lines may be used to indicate faster winds.

It is clear that the general winds must move in broad sweeping paths, changing their directions gradually, rather than in narrow belts, with sudden changes in direction. Therefore long curving arrows give a better picture of the actual drift of the atmospheric currents than do short, straight, disconnected arrows.

Study the winds on this chart with care. Describe the conditions of wind distribution in a general way. Can you discover any apparent relation between the different wind directions in any part of the map? Is there any system whatever in the winds? Write out a brief and concise description of the results of the study of this map.

In the last chapter we studied the progression of the cold wave of low temperatures in an easterly direction across the United States. Notice now the relation of the winds on the successive maps of our series to the movement of the cold wave. Place your wind charts and isothermal charts for the six days side by side, and study them together. The temperature distribution on the second day differs from that on the first. What are the chief differences? Examine the wind charts for these two days. Do you detect any differences in the wind directions or systems on these days? Do these differences help to explain some of the changes in temperature?

Compare the temperature distribution on the second day with that on the third. What are the most marked changes in the distribution? What changes in the winds on the corresponding wind maps seem to offer an explanation of these variations?

Proceed similarly with each map of the series. Formulate, in writing, the general relation between winds and cold waves, discovered through your study of these charts.

PRESSURE.

Draw lines of equal pressure, following the same principles as were adopted in the case of the isotherms. The latter were drawn for every even 10? of temperature. The former are to be drawn for every even .10 inch of pressure. Every station which has a barometer reading of an even .10 inch will be passed through by some line of equal pressure. Philadelphia, Pa., with 29.90 must be passed through by the 29.90 line; Wilmington, N. C., with 30.00, must have the 30.00 line passing through it, etc. Chicago, with 30.17 inches, must lie between the lines of 30.10 and 30.20 inches, and nearer the latter than the former. Denver, Col., with 30.35 inches, must lie midway between the 30.30 and 30.40 lines .

Study the isobaric chart of each day of the series by itself at first. Describe the pressure distribution on each chart.

Then compare the successive charts. Note what changes have taken place in the interval between each chart and the one preceding; where the pressures have risen; where they have fallen, and where they have remained stationary. Write a brief account of the facts of pressure change illustrated on the whole series of six charts.

Compare the charts of temperature and of pressure, first individually, then collectively. What relations do you discover between temperature distribution and pressure distribution on the isothermal and the isobaric charts for the same day? What relations can you make out between the changes in temperature and pressure distribution on successive days? On the whole series of maps? Write out the results of your study concisely and clearly.

Compare the wind charts and the pressure charts for the six days. Is there any relation between the direction and velocity of the winds and the pressure? Observe carefully the changes in the winds from day to day on these charts, and the changes in pressure distribution. Formulate and write out a brief general statement of all the relations that you have discovered.

Have these gradients at the different stations any relation to the proximity of low or high pressure? To the velocity of the wind?

WEATHER.

The character of the weather on each of the days whose temperature, wind, and pressure conditions we have been studying is noted in the table in this chapter. The symbols used by the Weather Bureau to indicate the different kinds of weather on the daily weather maps are as follows: clear; fair, or partly cloudy; cloudy; rain; snow.

Proceed similarly with the weather on the five remaining days, as noted in the table. Enter the weather symbols for each day on a separate blank map, enclosing and shading or coloring the areas of cloud and of snow as above suggested. In Figs. 40-45 the cloudy areas are indicated by single-line shading, and the snowy areas by double-line shading.

Now study carefully each weather chart with its corresponding temperature, wind, and pressure charts. Note whatever relations you can discover among the various meteorological elements on each day. Then compare the weather conditions on the successive maps. What changes do you note? How are these changes related to the changes of temperature; of wind; of pressure? Write a summary of the results derived from your study of these four sets of charts.

CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE.

At the bottom of each column write down the number of cases in that column, and then determine the percentages which these cases are of the total number of observations. This is done by dividing the number of cases in each column by the sum-total of all the observations. When you have obtained the percentage of each kind of wind direction, you have a numerical result.

A graphical presentation of the results may be made by laying off radii corresponding in position to those which divide the sectors in Fig. 46, and whose lengths are proportionate to the percentages of the different wind directions in the table. Thus, for a percentage of 20, the radii may be made 1 inch long, for 40%, 2 inches, etc. When completed, the relative sizes of the sectors will show the relative frequencies of winds blowing in the four different directions with reference to the gradient, as is indicated in Fig. 47.

CORRELATION OF THE VELOCITY OF THE WIND AND THE PRESSURE.

For interior stations, with onshore winds, in the United States during the month of

Study the results of your table carefully. Deduce from your own results a general rule for wind velocities as related to barometric gradients.

FORM AND DIMENSIONS OF CYCLONES AND ANTICYCLONES.

CORRELATION OF CYCLONES AND ANTICYCLONES WITH THEIR WIND CIRCULATION.

Deduce a general rule for the circulation and velocity of the wind in a cyclonic area, as shown on your tracing, and write it out.

Deduce a general rule for the circulation and velocity of the wind in an anticyclonic area, as shown on your tracing, and write it out.

CORRELATION OF THE DIRECTION OF THE WIND AND THE TEMPERATURE.

Share on: WhatsApp @Hash Facebook Tweet