Practical Exercises in Elementary Meteorology

CHAPTER V.

TEMPERATURE.

A. Lines of Equal Temperature.—Temperature is the most important of all the weather elements. It is therefore with a study of the distribution of temperature over the United States, and of the manner of representing that distribution, that we begin our exercises in map drawing. In carrying out the work we shall proceed in a way similar to that adopted by the officials of the Weather Bureau in Washington and at the other map-publishing stations over the country.

Enter on a blank weather map the temperature readings found in the first column of the table in Chapter VIII. These readings are given in degrees of the ordinary Fahrenheit scale [those which are preceded by the minus sign (-) being below zero], and were made at the same time (7 A.M., “Eastern Standard Time”) all over the United States. Make your figures small but distinct, and place them close to the different stations to which they belong. This is done every morning at the Weather Bureau in Washington, when the telegraphic reports of weather conditions come in from all over the country. When all the temperature readings have been entered on the outline map, you have before you a view of the actual temperature distribution over the United States at 7 A.M., on the first day of the series. Describe the distribution of temperature in general terms, comparing and contrasting the different sections of the country in respect to their temperature conditions. Where are the lowest temperatures? Where are the highest? What was the lowest thermometer reading recorded anywhere on the morning of this day? At what station was this reading made? What was the highest temperature recorded? And at what station was this reading made?

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.

Let us take another step in order to emphasize more clearly the distribution of temperature over the United States on the first day of our series. Draw a line which shall separate all places having a temperature above 30° from those having temperatures below 30°, 30° being nearly the freezing point and, therefore, a critical temperature. Evidently this will help us to make our description of the temperature distribution more detailed. If this line is to separate places having temperatures above 30° from those having temperatures below 30°, it must evidently pass through all places whose temperature is exactly 30°. Examine the thermometer readings entered on your map to see whether there are any which indicate exactly 30°. You will find this reading at Norfolk, Va., Wilmington, S. C., Atlanta, Ga., Chattanooga, Tenn., Ft. Smith, Ark., and Portland, Ore. Through all these stations the line of 30° must be drawn. Begin the line on the Atlantic Coast at Norfolk, Va., and draw it wherever you find a thermometer reading of 30°. It is best to trace the line faintly with pencil at first, so that any mistakes can be easily rectified, and it should be drawn in smooth curves, not in angles. From Norfolk the line must run southwest through Wilmington, and then westward through Atlanta, passing just north of Augusta, which has 31°. From Atlanta the line goes northwest through Chattanooga, and thence westward, curving south of Memphis (28°) and Little Rock (26°), and then northwestward again through Ft. Smith.

In fixing the exact position of the 30° line south of Memphis and Little Rock, the following considerations must be our guide: Memphis has 28°; Vicksburg has 35°. Neither of these stations has 30°. Suppose, however, that you had started from Memphis, with a thermometer, and had traveled very rapidly to Vicksburg. The thermometer reading at starting in Memphis would have been 28°, and at the end of your journey in Vicksburg it would have been 35°, presuming that no change in temperature at either station took place during the journey. Evidently the mercury rose during the journey, and in rising from 28° to 35° it must, somewhere on the way, have stood at exactly 30°. Now this place, where the temperature was exactly 30°, is the point through which our 30° line ought to pass. How are we to determine its location? Assume, as is always done in such cases, that the temperature increased at a uniform rate between Memphis and Vicksburg. The total rise was from 28° to 35° = 7°. In order to find a temperature 7° higher than at Memphis, you had to travel the whole distance from Memphis to Vicksburg. Suppose you had only wished to find a temperature 5° higher. Then, assuming a uniform rate of increase between the two stations, you would have had to travel only ⁵/₇ of the distance, and your thermometer at that place would have read 28° + 5° = 33°. But assume you had wanted to find the place where the thermometer stood at 30°. In this case you would have been obliged to go but ²/₇ of the total distance from Memphis to Vicksburg, and at that point your thermometer reading would have been 28° + 2° = 30°, which is the point we wish to find. In this way, then, when we do not find the exact temperature we are looking for on the map, we can calculate where that temperature prevails by noting places which have temperatures somewhat higher and somewhat lower, and proceeding as in the case just described. Take another example. Little Rock, Ark., has 26°; Shreveport, La., has 40°. 40° - 26° = 14°, which is the total difference. From 26° to 30° is 4°. Therefore a point ⁴/₁₄ or ²/₇ of the distance from Little Rock to Shreveport should have a temperature of 26° + 4° = 30°, which is the point we wish to find, and through which our 30° line must pass.

From Ft. Smith the line cannot go north or northwest or west, because the temperatures there are all below 30°. To the south the temperatures are all above 30°. Evidently there is only one direction in which you can prolong the line, and that is to the southwest. Temperatures of 30° cannot be found north of El Paso (28°), because there the temperature distinctly falls, Santa FÉ having 4°, Denver, -14°, and Cheyenne, -23°. Therefore temperatures above 28° must be found south of El Paso. From Ft. Smith you may, therefore, continue the 30° line southwest and west, passing close to El Paso, but to the south of it. In determining the further course of the 30° line, note that Yuma and all the California stations have temperatures above 30°, while Winnemucca, Nev., has 13°, and Portland, Ore., has exactly 30°. From El Paso you may, therefore, continue the line to the northwest, passing up through Central California parallel with the coast line, and to the east of all the California stations and of Roseburg, Ore., and thence running through Portland, Ore., ending just west of Seattle, Wash. Notice that the 30° line should be nearer to Sacramento, Cal., with 36°, than to Red Bluff with 44°.

Thus you have drawn the line which passes through all places that have a temperature of 30° on the map under discussion. This may be called a line of equal temperature. Isotherm, a compound of two Greek words meaning equal temperature, is the name given in meteorology to such lines as this. You have drawn the isotherm of 30°. All parts of the United States north and east of this line are below 30°, while all districts south and west of it are above 30°. You see, therefore, how much easier the drawing of this one line has made the description of the temperature distribution over 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. (40°), and runs west, passing between Montgomery, Ala. (33°), and Pensacola, Fla. (46°). Thence it turns to the northwest, passing between Vicksburg, Miss. (35°), and New Orleans, La. (48°), and through Shreveport, La. (40°). From Shreveport it turns to the southwest, passing to the north and west of Palestine, Tex. (46°), and down through San Antonio, Tex. (40°). Its further exact location cannot be determined in Mexico, because there are no observations from Mexican stations, but the readings at Yuma, Ariz. (41°), and at San Diego (42°), Los Angeles (44°), San Francisco (45°), Red Bluff (44°), and Cape Mendocino (43°), 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. (43°), and Roseburg, Ore. (37°). 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°.

Isotherms on weather maps are drawn for every even 10° of temperature. They are drawn in smooth curves and not in angular sections. Two isotherms cannot cross one another, for if they did you would have two temperatures, differing by 10°, at the point of crossing, which is obviously impossible. Complete the chart for this day by drawing the remaining isotherms, i.e., those for 50°, 20°, 10°, 0°, -10°, -20°, and -30°, bearing in mind what has been said in regard to the determination of the positions of isotherms when the exact temperature you are seeking is not given on the map.

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.

Construct isothermal charts for the remaining days of the series. Use a new blank map for each day, and take the temperature observations from the table in Chapter VIII. Proceed as in the case of the first day. Draw the isotherms for every even 10° of temperature, taking care to study the course of each line before you begin to draw the line. The charts when completed form a series in which the temperature distribution over the United States is shown at successive intervals of 24 hours.

Fig. 18.—Isotherms. First day.

In order to bring out the temperature distribution on the maps more clearly, color (with colored pencils or water colors) 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.

Fig. 19.—Temperature. First Day.

Fig. 20.—Temperature. Second Day.

Fig. 21.—Temperature. Third Day.

Fig. 22.—Temperature. Fourth Day.

Fig. 23.—Temperature. Fifth Day.

Fig. 24.—Temperature. Sixth Day.

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.

Cold Waves.—The series of charts for these six days furnishes an excellent illustration of a severe cold wave.

A cold wave, as the term is now used by the Weather Bureau, means, during December, January, and February, a fall in temperature of from 20° to 16° in 24 hours, with a resulting reduction of temperature to between 0° and 32°, and, during the months from March to November inclusive, a fall of from 20° to 16° in 24 hours, with a reduction of temperature from 16° to 36°. During December, January, and February a cold wave means the following falls and reductions of temperature. Over the Northwestern States, from western Wisconsin to Montana, including Wyoming, Nebraska, and western Iowa, and over northeastern New York and northern New Hampshire, northern Vermont and northern Maine, a fall of 20° or more to zero or below; over southern New England and adjoining districts, the Lake region, the central valleys and west to Colorado, including northern New Mexico and northwestern Texas, a fall of 20° or more to 10° or below; over southern New Jersey, Delaware, eastern Maryland, Virginia, western North Carolina, northwestern South Carolina, northern Georgia, northern Alabama, northern Mississippi, Tennessee, southern Kentucky, Arkansas, Oklahoma, and southern New Mexico, a fall of 20° or more to 20° or below; over eastern North Carolina, central South Carolina, central Georgia, central Alabama, central Mississippi, central and northern Louisiana and central and interior Texas, a fall of 18° or more to 25° or below; along the Gulf coasts of Texas, Louisiana, Mississippi, and Alabama, over all of Florida, and over the coasts of Georgia and South Carolina, a fall of 16° or more to 32° or below. From March to November inclusive a cold wave means falls of temperature of the same amounts over the same districts, with resulting temperatures of 16°, 24°, 28°, 32°, and 36° respectively.

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.

Cold waves occur only in winter. In the summer cool spells, with similar characteristics, may be called cool waves.

Cold-Wave Forecasts.—A severe cold wave in winter does much damage to fruit and crops growing out of doors in our Southern States, and to perishable food products in cars, on the way from the South to supply the great cities of the North. Therefore it is important that warnings should be issued giving early information of the coming cold, so that farmers and fruit growers and shippers may take every precaution to protect their crops and produce. Our Weather Bureau takes special pains to study the movements of cold waves and to make forecasts of them, and so well are the warnings distributed over the country that the fruit growers and the transportation companies, and the dealers in farm produce, are able every winter to save thousands of dollars’ worth of fruit and vegetables which would otherwise be lost. Cold-wave warnings are heeded by many persons besides those who are directly interested in fruits and farm products. The ranchmen in the West, with thousands of cattle under their charge; the trainmen in charge of cattle trains; the engineers of large buildings, such as hotels, stores, and office buildings, who must have their fires hotter in cold weather,—these and many more watch, and are governed by, the cold-wave forecasts of our Weather Bureau.

Mean Annual and Mean Monthly Isothermal Charts.—We have thus far considered isothermal charts for the United States only, based on the temperature observations made at a single moment of time. It is, of course, possible to draw isothermal charts, the data for which are not the temperatures at a given moment, but are the mean or average temperatures for a month or a year. Such charts have been constructed for other countries besides our own, as well as for the whole world. An isothermal chart based on the mean annual temperatures is known as a mean annual isothermal chart. These charts show at once the average distribution of temperature for the month or for the year, just as the ones we have drawn show the distribution of temperature over the United States at a single moment.

B. Direction and Rate of Temperature Decrease. Temperature Gradient.—Take your isothermal map for the first day and imagine yourself at Kansas City, Mo. In what direction must you go from Kansas City in order to enter most rapidly into colder weather? In what direction must you go from Kansas City in order to enter most rapidly into warmer weather? Take the case of Salt Lake City. In what direction must you go from that station in order to enter most rapidly into colder weather? Into warmer weather? What are the corresponding directions in the case of Spokane, Wash.? Of Bismarck, N. Dak.? Of Buffalo, N. Y.? Of Montreal, Que.? Of Portland, Me.? Of Sacramento, Cal.?

Draw a line from Kansas City to the nearest point at which there is a temperature 10° lower than at Kansas City. Evidently this point is on the isotherm of 0°, and will be found if a line be drawn from Kansas City towards, and at right angles to, the isotherm of 0°. Continue the line beyond the 0° isotherm in the direction of still lower temperatures, i.e., to the isotherms of -10°, -20°, and -30°. Beyond the isotherm of -30° the line must stop. Draw similar lines from Seattle, Wash.; Salt Lake City, Utah; Denver, Col.; St. Paul, Minn.; Cleveland, O.; and New York, N. Y. Prolong these lines all across the map, so that they will extend from the regions of highest temperature to those of the lowest. A number of intermediate lines may also be added. Note that the various directions followed by these lines are square to, or at right angles to, the successive isotherms, and that although the lines all run from higher to lower temperatures, they do not all trend in the same direction. These lines may be called lines of decrease of temperature. Fig. 25 shows a few of these lines of decrease of temperature drawn for the first day.

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?

Fig. 25.—Temperature Gradients. First Day.

Next select some line of decrease of temperature on the map for the first day which begins in Texas, and follow it northward. Where, along this line, is the decrease of temperature most rapid? Evidently this must be where the isotherms are closest together, because every isotherm that is crossed means a change of temperature of 10°, and the more isotherms there are in a given distance, the more rapidly the temperature is changing. Where the isotherms are closest together, a given decrease of temperature is passed over in the least distance, or, conversely, a greater decrease of temperature is experienced in a given distance. Study this question of rapidity or slowness of temperature decrease on the whole series of charts. On which of the charts, and where, do you find the most rapid decrease? The slowest decrease? Is there any regularity in these rates of temperature decrease either on one map or in the whole series of maps?

The term temperature gradient is used by meteorologists to describe the direction and rate of temperature decrease which we have been studying.

If we are to compare these rates of temperature change, we must have some definite scale of measurement. Thus, for example, in speaking of the wind velocity we say the velocity of the wind is so many miles per hour; in describing the grade of a railroad we say it is so many feet in a mile. In dealing with these temperature changes, we adopt a similar scheme. We say: The rate of temperature decrease is so many degrees Fahrenheit in a distance of one latitude degree (about 70 miles). In order to make our measurements, we use a scale of latitude degrees, just as, in calculating railroad grades, we must have a way to measure the miles of track in which the ascent or descent of the roadbed is so many feet. Take a strip of paper 6 inches long, with a straight edge, and lay this edge north and south at the middle of the weather map, along a longitudinal or meridian line. Mark off on the strip of paper the points where any two latitude lines cross the meridian line. These latitude lines are five (latitude) degrees apart. Therefore divide the space between them on your paper into five divisions, and each of these will measure just one latitude degree. Continue making divisions of the same size until you have ten altogether on the strip of paper. Select, on any weather map, some station lying between two isotherms at which you wish to measure the rate of temperature decrease. Take, for instance, Buffalo, N. Y., on the first day. What you want to find is this: What is the rate of temperature decrease, or the temperature gradient, at Buffalo? Lay your paper scale of latitude degrees through Buffalo, from the isotherm of 10° to the isotherm of 0°, and as nearly as possible at right angles to the isotherms.[3] Count the number of latitude degrees on your scale between the isotherms of 10° and 0°, on a line running through Buffalo. There are, roughly, about two degrees of latitude in this distance. That is, in the district in which Buffalo lies, the temperature is changing at the rate of 10° Fahrenheit (between isotherms 10° and 0°) in two latitude degrees. As our standard of measurement is the amount of change of temperature in one latitude degree, we divide the 10 (the number of degrees of temperature) by the 2 (the number of degrees of latitude), which gives us 5 as the rate of decrease of temperature per latitude degree at Buffalo, N. Y., at 7 A.M., on the first day of the series. The temperature gradient at Buffalo is therefore 5. The rule may be stated as follows: Select the station for which you wish to know the rate of temperature decrease or temperature gradient. Lay a scale of latitude degrees through the station, and as nearly as possible at right angles to the adjacent isotherms. If the station is exactly on an isotherm then measure the distance from the station to the nearest isotherm indicating a temperature 10° lower. The scale must, however, be laid perpendicularly to the isotherm, as before. Divide the number of degrees of difference of temperature between the isotherms (always 10°) by the distance (in latitude degrees) between the isotherms, and the quotient is the rate of temperature decrease per latitude degree. Or, to formulate the operation:

R = T / D,

in which R = rate; T = temperature difference between isotherms (always 10°), and D = distance between isotherms in latitude degrees. Thus, a distance of 10 latitude degrees gives a rate of 1; a distance of 5 gives a rate of 2; a distance of 2 gives a rate of 5; a distance of 4 gives a rate of 2.5, etc.

[3] Unless the isotherms are exactly parallel, the scale cannot be at right angles to both of them. It should, however, be placed as nearly as possible in that position.

Determine the rates of temperature decrease in the following cases:—

A. For a considerable number of stations in different parts of the same map, as for each of the six days of the series.

And, using the school file of weather maps,

B. For one station during a winter month and during a summer month, measuring the rate on each map throughout the month and obtaining an average rate for the month.

C. For a station on the Pacific Coast, and one on the Atlantic Coast during the same months.

D. For a station on the Gulf of Mexico, or in Florida, and one in the Northwest during a winter month.

E. For a station in the central United States, and one on the Pacific Coast, the Gulf Coast, and the Atlantic Coast, respectively, during different months of the winter and summer.

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.

Temperature Gradients on Isothermal Charts of the Globe.—The mean annual isothermal charts of the globe (see page 63) bring out some very marked contrasts in rates of temperature decrease. Thus, along the eastern side of the North American continent the isotherms are crowded close together, while on the western coast of Europe they are spread far apart. Between southern Florida and Maine there is the same change in mean annual temperature as is found between the Atlantic coast of the Sahara and central England. The latter is a considerably longer distance, and this means that the decrease of temperature is much slower on the European Atlantic coast than on the North American Atlantic coast. In fact, the rate of temperature decrease with latitude in the latter case is the most rapid anywhere in the world, in the same distance. These great contrasts in temperature which occur within short distances along the eastern coast of North America have had great influence upon the development of this region, as has been pointed out by Woeikof, an eminent Russian meteorologist. The products of the tropics and of the Arctic are here brought very near together; and at the same time intercommunication between these two regions of widely differing climates is very easy. Labrador is climatically an Arctic land, and man is there forced to seek his food chiefly in the sea, for nature supplies him with little on shore, while southern Florida is quite tropical in its temperature conditions and in the abundance of its vegetation. Between the Pacific coasts of Asia and of North America there is a similar but less pronounced contrast, the isotherms being crowded together on the eastern coast of China and Siberia, and being spread apart as they cross the Pacific Ocean and reach our Pacific Coast.

In general, we naturally expect to find that the temperature decreases as one goes poleward from the equator; from lower latitudes, where the sun is always high in the heavens, to higher latitudes, where it is near the horizon, and its warming effect is less. But there are some curious exceptions to this general rule. The lowest temperatures on the January isothermal chart (-60°) are found in northeastern Siberia, and not, so far as our observations go, near the North Pole. If you find yourself at this “cold pole,” as it is called, in Siberia in January, you can reach higher temperatures by traveling north, south, east, or west. In other words, here is a case of increase of temperature in a northerly direction, as well as east, south, and west. Again, there is a district of high temperature (90°) over southern Asia in July, from which you can travel south towards the equator and yet reach lower temperatures.

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.

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