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_.

_B._ Compare each map of the series with the map preceding it. Note what changes in direction and velocity have taken place at individual stations. Group these changes as far as possible by the districts over which similar changes have occurred. Compare the wind systems on each map with those on the map for the preceding day. Has there been any alteration in the position or relation of these systems? Write for each day an account of the conditions on that map, and of the changes that have taken place in the preceding 24-hour interval.

_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 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_.