  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 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 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. Fig. 27.—Winds. Second Day. Fig. 28.—Winds. Third Day. Fig. 29.—Winds. Fourth Day. Fig. 30.—Winds. Fifth Day. 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 C. Write out a short connected account of the wind conditions and changes illustrated 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 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. |
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