_C._ Write out a short connected account of the wind conditions and changes ill.u.s.trated on the whole set of six maps.

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.

=Cold Waves in Other Countries.=--Cold waves in the United States come, as has been seen, from the northwest, that being the region of greatest winter cold. In Europe, cold waves come from the northeast. This is because northwest of Europe there is a large body of warm water supplied by the Gulf Stream drift, and therefore this is a source of warmth and not of cold. The cold region of Europe is to the northeast, over Russia and Siberia.

Cold waves have different names in different countries. In southern France the cold wind from the north and northeast is known as the _mistral_, derived from the Latin word _magister_, meaning _master_, on account of its strength and violence. In Russia the name _buran_ or _purga_ is given to the cold wave when it blows along with it the fine dry snow from the surface of the ground. This _buran_ is apt to cause the loss of many lives, both of men and cattle. In the Argentine Republic the coolest wind is from the southwest. It is known as a _pampero_, from the Spanish _pampa_, a _plain_.

=Cyclones and Anticyclones.=--A system of winds blowing towards a common center (such as is well shown over the Gulf States on the weather map for the second day, and over the middle Atlantic coast on the third day) is called by meteorologists a _cyclone_. The name was first suggested by Piddington early in this century. It is derived from the Greek word for _circle_, and hence it embodies the idea of a circular or spiral movement of the winds. A system of _outflowing_ winds, such as that over the northwestern United States shown on the maps for the first five days, and over the western Gulf States on the sixth day is called an _anticyclone_.

This name was proposed by Galton in 1863, and means the opposite of _cyclone_.

CHAPTER VII.

PRESSURE.

_A._ =Lines of Equal Pressure: Isobars.=--One of the most important weather elements is the _pressure_ of the atmosphere. This has already been briefly discussed in the sections on the mercurial barometer (Chapter II). It was there learned that atmospheric pressure is measured by the number of inches of mercury which the weight of the air will hold up in the gla.s.s tube of the barometer. Our sensation of heat or cold gives us some general idea as to the air temperature. We can tell the wind direction when we know the points of the compa.s.s, and can roughly estimate its velocity. No instrumental aid is necessary to enable us to decide whether a day is clear, fair or cloudy, or whether it is raining or snowing. Unlike the temperature, the wind, or the weather, the pressure cannot be determined by our own senses without instrumental aid. The next weather element that we shall study is pressure.

Proceed as in the case of the thermometer readings. Enter upon a blank map the barometer readings for the different stations given in the third column of the table in Chapter VIII. When this has been done you have before you the actual pressure distribution over the United States at 7 A.M., on the first day of the series. Describe the distribution of pressure in general terms. Where is the pressure highest? Where lowest?

What are the highest and the lowest readings of the barometer noted on the map? What is the difference (in inches and hundredths) between these readings?

[Ill.u.s.tration: FIG. 32.--Isobars. First Day.]

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 pa.s.sed through by some line of equal pressure. Philadelphia, Pa., with 29.90 must be pa.s.sed through by the 29.90 line; Wilmington, N. C., with 30.00, must have the 30.00 line pa.s.sing 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 (Fig. 32).

Lines of equal pressure are called _isobars_, a word derived from two Greek words meaning _equal pressure_.

Describe the distribution of pressure as shown by the arrangement of the isobars. Note the differences in form between the isotherms and the isobars. The words _high_ and _low_ are printed on weather maps to mark the regions where the pressure is highest and lowest.

Draw isobars for the other days, using the barometer readings given in the table in Chapter VIII. Figs. 33-38 show the arrangement of the isobars on these days.

The pressure charts may be colored, as indicated by the shading in these figures, in order to bring out more clearly the distribution of pressure, according to the same general scheme as that adopted in the temperature charts. Color _brown_ all parts of your six isobaric charts over which the pressures are below 29.50 inches; color _red_ all parts with pressure above 30.00 inches. Use a _faint shade of brown_ for pressures between 29.50 inches and 29.00 inches, and a _darker shade_ for pressures below 29.00 inches. In the case of pressures over 30.00 inches, use a _pale red_ for pressures between 30.00 and 30.50 inches, and a _darker shade of red_ for pressures above 30.50 inches. By means of these colors the pressure distribution will stand out very clearly. The scheme of color and shading may, of course, be varied to suit the individual fancy.

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 ill.u.s.trated on the whole series of six charts.

[Ill.u.s.tration: FIG. 33.--Pressure. First Day.]

[Ill.u.s.tration: FIG. 34.--Pressure. Second Day.]

[Ill.u.s.tration: FIG. 35.--Pressure. Third Day.]

[Ill.u.s.tration: FIG. 36.--Pressure. Fourth Day.]

[Ill.u.s.tration: FIG. 37.--Pressure. Fifth Day.]

[Ill.u.s.tration: FIG. 38.--Pressure. Sixth Day.]

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.

=Mean Annual and Mean Monthly Isobaric Charts.=--We have thus far been studying isobaric charts based on barometer readings made at a single moment of time. Just as there are mean annual and mean monthly isothermal charts, based on the mean annual and mean monthly temperatures, so there are mean annual and mean monthly isobaric charts for the different countries and for the whole world, based on the mean annual and mean monthly pressures. The mean annual and mean monthly isobaric charts of the world show the presence of great oval areas of low and high pressure covering a whole continent, or a whole ocean, and keeping about the same position for months at a time. Thus, on the isobaric chart showing the mean pressure over the world in January, there are seen immense areas of high pressure (anticyclones) over the two great continental ma.s.ses of the Northern Hemisphere. These anticyclonic areas, although vastly greater in extent than the small ones seen on the weather maps of the United States, have the same system of spirally _outflowing_ winds. Over the northeastern portion of the North Pacific and the North Atlantic, in January, are seen immense areas of low pressure (cyclones) with spirally _inflowing_ winds. In July the northern continents are covered by cyclonic areas, and the central portion of the northern oceans by anticyclonic areas.

_B._ =Direction and Rate of Pressure Decrease: Pressure Gradient.=--In Chapter V we studied the direction and rate of temperature decrease, or temperature gradient. We saw that the direction of this decrease varies in different parts of the map, and that the rate, which depends upon the closeness of the isotherms, also varies. An understanding of temperature gradients makes it easy to study the directions and rates of pressure decrease, or _pressure gradients_, as they are commonly called. Examine the series of isobaric charts to see how the lines of pressure decrease run. Draw lines of pressure decrease for the six isobaric charts, as you have already done on the isothermal charts. When the isobars are near together, the lines of pressure decrease may be drawn heavier, to indicate a more rapid rate of decrease of pressure. Fig. 39 shows lines of pressure decrease for the first day. Note how the arrangement and direction of these lines change from one map to the next. Compare these lines with the lines of temperature decrease.

[Ill.u.s.tration: FIG. 39.--Pressure Gradients. First Day.]

Next study the _rate_ of pressure decrease. This rate depends upon the closeness of the isobars, just as the rate of temperature decrease depends upon the closeness of the isotherms. Examine the rates of pressure decrease upon the series of isobaric charts. On which charts do you find the most rapid rate? Where? On which the slowest? Where? Do you discover any relation between rate of pressure decrease and the pressure itself?

What relation?

When expressed numerically, the barometric gradient is understood to mean the number of hundredths of an inch of change of pressure in one lat.i.tude degree. Prepare a scale of lat.i.tude degrees, and measure rates of pressure decrease, just as you have already done in the case of temperature. In this case, instead of dividing the difference in temperature between the isotherms (10 = _T_) by the distance between the isobars (_D_), we subst.i.tute for 10 of temperature .10 inch of pressure (_P_). Otherwise the operation is precisely the same as described in Chapter V. The rule may be stated as follows: Select the station for which you wish to know the rate of pressure decrease or the barometric gradient. Lay your scale through the station, and as nearly as possible at right angles to the adjacent isobars. If the station is exactly on an isobar, then measure the distance _from_ the station to the nearest isobar indicating a lower pressure. The scale must, however, be laid perpendicularly to the isobars, as before. Divide the number of hundredths of an inch of pressure difference between the isobars (always .10 inch) by the number expressing the distance (in lat.i.tude degrees) between the isobars; the quotient is the rate of pressure decrease per lat.i.tude degree. Or, to formulate the operation,

_R_ = _P_ / _D_,

in which _R_ = rate; _P_ = pressure difference between isobars (always .10 inch), and _D_ = distance between the isobars in lat.i.tude degrees.

Determine the rates of pressure decrease in the following cases:--

_A._ For a number of stations in different parts of the same map, as, _e.g._, Boston, New York, Washington, Charleston, New Orleans, St. Louis, St. Paul, Denver, and on the same day.

_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.

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

=Pressure Gradients on Isobaric Charts of the Globe.=--The change from low pressure to high pressure or _vice versa_ with the seasons, already noted as being clearly shown on the isobaric charts of the globe, evidently means that the directions of pressure decrease must also change from season to season. The rates of pressure decrease likewise do not remain the same all over the world throughout the year. If we examine isobaric charts for January and July, we shall find that these gradients are stronger or steeper over the Northern Hemisphere in the former month than in the latter.

CHAPTER VIII.

WEATHER.

Hitherto nothing has been said about the _weather_ itself, as shown on the series of maps we have been studying. By weather, in this connection, we mean the state of the sky, whether it is clear, fair, or cloudy, or whether it is raining or snowing at the time of the observation. While it makes not the slightest difference to our feelings whether the _pressure_ is high or low, the _character of the weather_ is of great importance.

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: [full moon] clear; [quarter moon] fair, or partly cloudy; [new moon] cloudy; [circle-R] rain; [circle-S] snow.

Enter on a blank map, at each station, the sign which indicates the weather conditions at that station at 7 A.M., on the first day, as given in the table. When you have completed this, you have before you on the map a bird's-eye view of the weather which prevailed over the United States at the moment of time at which the observations were taken. Describe in general terms the distribution of weather here shown, naming the districts or States over which similar conditions prevail. Following out the general scheme adopted in the case of the temperature and the pressure distribution, separate, by means of a line drawn on your map, the districts over which the weather is prevailingly cloudy from those over which the weather is partly cloudy or clear. In drawing this line, scattering observations which do not harmonize with the prevailing conditions around them may be disregarded, as the object is simply to emphasize the _general_ characteristics. Enclose also, by means of another line, the general area over which it was snowing at the time of observation, and shade or color the latter region differently from the cloudy one. Study the weather distribution shown on your chart. What general relation do you discover between the kinds of weather and the temperature, winds, and pressure?