Practical Exercises in Elementary Meteorology Part 5

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?

[Ill.u.s.tration: 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 pa.s.sed 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 lat.i.tude degree (about 70 miles). In order to make our measurements, we use a scale of _lat.i.tude 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 lat.i.tude lines cross the meridian line. These lat.i.tude lines are five (lat.i.tude) degrees apart. Therefore divide the s.p.a.ce between them on your paper into five divisions, and each of these will measure just one lat.i.tude 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 lat.i.tude 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 lat.i.tude degrees on your scale between the isotherms of 10 and 0, on a line running through Buffalo. There are, roughly, about two degrees of lat.i.tude 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 lat.i.tude degrees.

As our standard of measurement is the amount of change of temperature in one lat.i.tude degree, we divide the 10 (the number of degrees of temperature) by the 2 (the number of degrees of lat.i.tude), which gives us 5 as the rate of decrease of temperature per lat.i.tude 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 lat.i.tude 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 lat.i.tude degrees) between the isotherms, and the quotient is the _rate of temperature decrease per lat.i.tude 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 lat.i.tude degrees.

Thus, a distance of 10 lat.i.tude 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.

[Footnote 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 lat.i.tude 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 sh.o.r.e, 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 p.r.o.nounced 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 lat.i.tudes, where the sun is always high in the heavens, to higher lat.i.tudes, 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.

CHAPTER VI.

WINDS.

The observational work already done, whether non-instrumental or instrumental, has shown that there is a close relation between the _direction of the wind_ at any station and the _temperature_ at that station. Our second step in weather-map drawing is concerned with the winds on the same series of maps which we have thus far been studying from the point of view of temperature alone.

In the second column of the table in Chapter VIII are given the wind directions and the wind velocities (in miles per hour) recorded at the Weather Bureau stations at 7 A.M., on the first day of the series. Enter on a blank weather map, at each station for which a wind observation is given in the table, a small arrow flying _with_ the wind, _i.e._, pointing in the direction _towards_ which the wind is blowing. Make the lengths of the wind arrows roughly proportionate to the velocity of the wind, the winds of higher velocities being distinguished by longer arrows, and those of lower velocities by shorter arrows. The letters _Lt._ (= light) in the table denote wind velocities of 5 miles, or less, per hour.

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. (See solid arrows in Fig. 26.)

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. (See broken arrows, Fig. 26.)

[Ill.u.s.tration: FIG. 26.--Winds. First Day.]

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.

Enter on five other blank maps the wind directions given in the table in Chapter VIII for the other five days of the series, making, as before, the lengths of the arrows roughly proportionate to the velocity of the wind, and adding extra broken arrows as suggested. (See Figs. 27-31.)

_A._ Study the whole series of six maps. Describe the wind conditions on each map by itself, noting carefully any system in the wind circulation that you may discover. Examine the wind velocities also. Are there any districts in which the velocities are especially high? Have these velocities any relation to whatever wind systems you may have discovered?

If so, include in your description of these systems some consideration of the wind _velocities_ as well as of the wind _directions_.

[Ill.u.s.tration: FIG. 27.--Winds. Second Day.]

[Ill.u.s.tration: FIG. 28.--Winds. Third Day.]

[Ill.u.s.tration: FIG. 29.--Winds. Fourth Day.]

[Ill.u.s.tration: FIG. 30.--Winds. Fifth Day.]

[Ill.u.s.tration: FIG. 31.--Winds. Sixth Day.]

_B._ Compare each map of the series with the map preceding it. Note what changes in direction and velocity have taken place at individual stations. Group these changes as far as possible by the districts over which similar changes have occurred. Compare the wind systems on each map with those on the map for the preceding day. Has there been any alteration in the position or relation of these systems? Write for each day an account of the conditions on that map, and of the changes that have taken place in the preceding 24-hour interval.