Practical Exercises in Elementary Meteorology

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.

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 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,

R = P / D,

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?

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: ○ clear; ◑ fair, or partly cloudy; ● cloudy; Ⓡ rain; Ⓢ 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?

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.

Fig. 40.—Weather. First Day.

Fig. 41.—Weather. Second Day.

Fig. 42.—Weather. Third Day.

Fig. 43.—Weather. Fourth Day.

Fig. 44.—Weather. Fifth Day.

Fig. 45.—Weather. Sixth Day.

The Weather of Temperate and Torrid Zones.—The facts of the presence of clear weather in one region while snow is falling in another, and of the variability of our weather from day to day in different parts of the United States, are emphasized by these charts of weather conditions. This changeableness of weather is a marked characteristic of the greater portion of the Temperate Zones, especially in winter. The weather maps for successive days do not, as a rule, show a repetition of the same conditions over extended regions. In the Torrid Zone it is different. Over the greater part of that zone the regularity of the weather conditions is such that, day after day, for weeks and months, the same features are repeated. There monotony, here variety, is the dominant characteristic of the weather.

Part IV.—The Correlations of the Weather Elements and Weather Forecasting

CHAPTER IX.

CORRELATION OF THE DIRECTION OF THE WIND AND THE PRESSURE

The study of the series of weather maps in Chapters V-VIII has made it clear that some fairly definite relation exists between the general flow of the winds and the distribution of pressure. We now wish to obtain some more definite result as to the relation of the direction of the wind and the pressure. In doing this it is convenient to refer the wind direction to the barometric or pressure gradient at the station at which the observation is made. The barometric gradient, it will be remembered, is the line along which there is the most rapid change of pressure, and lies at right angles to the isobars (Chapter VII).

Fig. 46.

Take a small piece of tracing paper, about 3 inches square, and draw upon it a diagram similar to the one here shown. Select the station (between two isobars on any weather map) at which you intend to make your observation. Place the center of the tracing paper diagram over the station, with the dotted line along the barometric gradient, the minus end of the line being towards the area of low pressure. Observe into which of the four sectors (marked right, left, with, against) the wind arrow at the station points. Keep a record of the observation. Repeat the observation at least 100 times, using different stations, on the same map or on different maps. Tabulate your results according to the following scheme, noting in the first column the date of the map, in the second, third, fourth, and fifth columns the number of winds found blowing with, to the right or left of, and against, the gradient.

Table I.—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.

The Deflection of the Wind from the Gradient: Ferrel’s Law.—The law of the deflection of the wind prevailingly to the right of the gradient is known as Ferrel’s Law, after William Ferrel, a noted American meteorologist, who died in 1891. The operation of this law has already been seen in the spiral circulation of the winds around the cyclone and the anticyclone, as shown on the maps of our series. In the case of the cyclone the gradient is directed inward towards the center; in the case of the anticyclone the gradient is directed outward from the center. In both cases the right-handed deflection results in a spiral whirl, inward in the cyclone, outward in the anticyclone. The operation of this law is further seen in the case of the Northeast Trade Winds. These winds blow from about Lat. 30° N. towards the equator, with wonderful regularity, especially over the oceans. Instead of following the gradient and blowing as north winds, these trades turn to the right of the gradient and become northeast winds, whence their name. From about Lat. 30° N. towards the North Pole there is another great flow of winds over the earth’s surface. These winds do not flow due north, as south winds. They turn to the right, as do the trades, and become southwest or west-southwest winds, being known as the Prevailing Westerlies. Ferrel’s Law thus operates in the larger case of the general circulation of the earth’s atmosphere, as well as in the smaller case of the local winds on our weather maps.

Fig. 47.

CHAPTER X.

CORRELATION OF THE VELOCITY OF THE WIND AND THE PRESSURE

Prepare a scale of latitude degrees, as explained in Chapter V. Select some station on the weather map at which there is a wind arrow, and at which you wish to study the relation of wind velocity and pressure. Find the rate of pressure change per degree as explained in Chapter VII. Note also the velocity, in miles per hour, of the wind at the station. Repeat the operation 100 or more times, selecting stations in different parts of the United States. It is well, however, to include in one investigation either interior stations alone (i.e., more than 100 miles from the coast) or coast stations alone, as the wind velocities are often considerably affected by proximity to the ocean. And, if coast stations are selected, either onshore or offshore winds should alone be included in one exercise. The investigation may, therefore, be carried out so as to embrace the following different sets of operations:—

A. Interior stations.

B. Coast stations with onshore winds.

C. Coast stations with offshore winds.

Enter your results in a table similar to the one here given:—

Table II.—Correlation of Wind Velocity and Barometric

Gradient.

For interior (or coast) stations, with onshore (or offshore) winds, in the United States during the month (or months) of

The wind velocity for each station is to be entered in the column at whose top is the rate of pressure change found for that station. Thus, if for any station the rate of pressure change is 3

⁄

(i.e., .03 inch in one latitude degree), and the wind velocity at that station is 17 miles an hour, enter the 17 in the fourth and fifth columns of the table. When you find that the rate of pressure change for any station falls into two columns of the table, as, e.g., 10, or 5, or 3

⁄