Lesson CCXXIV

How to Read Weather Maps

Leading thought—Weather maps are made with great care by the Weather Bureau experts. Each map is the result of many telegraphic communications from all parts of the country. Every intelligent person should be able to understand the weather maps.

Method—Get several weather maps of the nearest Weather Bureau Station. They should be maps for successive days, and there should be enough so that each pupil can have three maps, showing the weather conditions for three successive days.

Observations—1. Take the map of the earliest date of the three. Where was your map used? What is its date? How many kinds of lines are there on your map? Are there explanatory notes on the lower left-hand corner of your map? Explain what the continuous lines mean. Find an isobar of 30; to what does this figure refer? Find all the towns on your map where the barometer stands at 30 inches. Is there more than one isobar on your map where the barometer stands at 30?

2. Where is the greatest air pressure on your map? How high does the barometer stand there? How are the isobars arranged with reference to this region? What word is printed in the center of this series of isobars?

3. What do the arrows indicate? What do the circles attached to the arrows indicate?

4. In general, what is the direction of the winds with reference to this high center?

5. Is the air rising or sinking at the center of this area? If the wind is blowing in all directions from a center marked high, what sort of weather must the places just east of the high be having? Do the arrows with their circles indicate this?

6. Find a center marked low. How high does the barometer stand there? Does the air pressure increase or diminish away from the center marked low, as indicated by the isobars? Do the winds blow toward this center or away from it?

7. What must the weather in the region just east of the low be? Why? Do the arrows and circles indicate this?

8. Is there a shaded area on your map? If so, what does this show?

9. Compare the map of the next date with the one you have just studied. Are the highs and lows in just the same position that they were the day before? Where are the centers high and low now? In what directions have they moved?

10. Look at the third map and compare the three maps. Where do the high and low centers seem to have originated? How long does it take a high or low to cross the United States? How far north and south does a high or low, with all its isobars, extend?

11. What do the dotted lines on your map mean? Do they follow exactly the isobars?

12. What is the greatest isotherm on your map? Through or near what towns does it pass?

13. Do the regions of high air pressure have the highest temperature or the lowest? Do high temperatures accompany low pressures? Why?

14. What is the condition of the sky just east of a low center? What is its condition just west of low?

15. If the isobars are near together in a low, it means that the wind is moving rather fast and that there will be a well marked storm. Look at the column giving wind velocity. Was the wind blowing toward the center of the low on the map? If so, does that mean it is coming fast or slow? How does this fact correspond with the indications shown by the distance between the isobars?

16. Describe the weather accompanying the approach and passage of a low in the region where your town is situated? What sort of clouds would you have, what winds, what change of the barometer and thermometer?

How to Find the General Direction and Average Rate of Motion of Highs and Lows

Observations—1. On the first map of the series of three given, put an X in red pencil or crayon at the center of the high and a blue one at the center of the low; or if you do not have the colored pencils, use some other distinguishing marks for the two. If there are two highs and two lows use a different mark for each one.

2. Mark the position of each center on this map for the following day with the same mark that you first used for that area. Do this for each of the highs and lows until it leaves the map or until your maps have been used. All the marks of one kind can be joined by a line, using a red line for the red marks and a blue line for the blue marks.

3. What do you find to be the general direction of the movement of the highs and lows?

4. Examine the scale marked statute miles at the bottom of the map. How many miles are represented by one inch on the scale?

5. With your ruler find out how many miles one area of high or low has moved in twenty-four hours; in three days. Divide the distance which the area has moved in three days by three and this will give the average velocity for one day.

6. In the same way find the average velocity of each of the areas on your map for three days and write down all your answers. From all your results find the average weekly velocity; that is, how many miles per hour and the general direction which has characterized the movement of the high and low areas.

Supplementary reading—The Wonderbook of the Atmosphere, Houston, Chapters XIV-XXIII.

How to Keep a Daily Weather Map

The pupils should keep a daily weather map record for at least six months. The observations should be made twice each day and always at the same hours. While it would be better if these records could be made at 8 o’clock in the morning and again at 8 o’clock in the evening, this is hardly practicable and they should, therefore, be made at 9 o’clock and at 4. The accompanying chart may be drawn enlarged. Sheets of manila paper are often used, so that one chart may cover the observations for a month.

Few schools are able to have a working barometer, but observations of temperature and sky should be made in every school. Almost any boy can make a weather vane, which should be placed on a high building or tree where the wind will not be deflected from its true direction when striking it. A thermometer should be placed on the north side of a post and on a level with the eyes; it should not be hung to a building, as the temperature of the building might affect it.

The direction of the wind and the cloudiness of the day may be indicated on the chart, as it is on the weather maps, by a circle attached to an arrow which points in the direction in which the wind is blowing.

References—Elementary Meteorology, Waldo, American Book Co., $1.50; Elementary Meteorology, Davis, Ginn and Co., $2.50; Bulletins from the United States Weather Bureau, Washington, D. C.

Chart for School Weather-Records.