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 glass 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 compass, 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?

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 (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 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.

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.

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 latitude degree. Prepare a scale of latitude 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 substitute 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 latitude degrees) between the isobars; the quotient is the rate of pressure decrease per latitude degree. Or, to formulate the operation,

in which R = rate; P = pressure difference between isobars (always .10 inch), and D = distance between the isobars in latitude 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?

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