ORNITHOLOGIST 



— AND — 



OOLOGIST. 



$1.50 per 

 Annum. 



FRANK B. WEBSTER, Publisher. 

 Established, March, 1876. 



Single Copy 

 15 Cents. 



VOL. X. 



PAWTUCKET, R. I., MAY, 1885. 



No. 5. 



Mississippi Valley Migration. 



TilK IIKI.ATION OK M ICUfATTON Td I;AI!()MKT1!IC 

 I'KESSl'RK AND TEMPEHATIKK. 



liY PROF. W. W. COOKE, MOORnEAD, MINN. 



Tlie materials for tlie present article were ob- 

 tained principally from tlie daily weather reports 

 of the Signal Service for 1884. During the 

 Spring migration of that year, I received tlie full 

 tri-daily reports from all the stations (one hundred 

 and twenty in number,) of the Signal Service in 

 the United States and Canada; of which, about 

 fifty stations are in or near the Mississippi Valley, 

 and their reports were used in making my maps. 

 These reports give the temperature, barometer, 

 dew point, direction and force of the wind, 

 amount of rain-fall, and character of the sky. 

 They are made three times a day, at 7 a. m., 3 p. 

 m., and 11 p. m. In our study, the 11 p. m., 

 records were used for those Ijirds which migrate 

 by night, and the 7 a. m., for such species as 

 Ducks and Gee.sc, which perform the bulk of 

 their movements in the forenoon. 



To render the study more ca.sy, weather maps 

 were made, one for each day, based on the 11 p. 

 m. . observations. The maps were made as nearly 

 as possible like those now jirinted daily by the 

 Signal Service at Washington, that is, the .state 

 of the weather and the direction of the wind arc 

 marked at each station on the map in .sj'inbols 

 which are plain and easily comprehended, so that 

 the eye can take in at a glance the general .state 

 of the weather in the whole Mississippi Valley. 

 At each station is then marked in figures the tem- 

 perature, barometer and force of the wind. 

 Dotted lines are then drawn, connecting all places 

 having the same temperature, and solid lines con- 

 necting places of the same barometric pressure; 

 the former, called isothermal lines, are drawn for 

 eveiy five degrees, while the latter, called iso- 

 baric lines are drawn for every tenth of an inch 

 of pressure, The only difference from the Signal 

 Service maps is that the Isotherms are drawn 



every five degrees, while the Government maps 

 put them ten degrees apart. 



Studying these maps, we find that the area of 

 lowest pressure is not always in the same place, 

 but is con.stanlly moving and aheays in an easkv- 

 ly directinii. It may be moving northeast, east, 

 or southeast, and rarely north or south, but never 

 northwest, west or southwest. The usual direc- 

 tion is a little south of east. Though it may 

 move north or .south for a time, it will surely 

 turn east in the end. It .so happens that the low 

 pressure area, which we particularly study at the 

 latter end of this article, moves toward the north- 

 east, but this is not the usual direction. One 

 may ask, what has all this to do with birds? Let us 

 .see. We said in the February "O. and O." page 

 17, that the warm waves began in the north and 

 travelled southward. It might have been a little 

 more e.xact had wc said they begin in the north- 

 west and move toward the southeast, but the 

 cause of this movement lies in the movement of 

 the area of low pressure. It is a law of the move- 

 ment of the winds, that they go toward an area of 

 laiD pressure, and from an area of high pressure. 

 If then an area of low pressure develops, .say in 

 .southwestern Dakota, it will be but a few hours 

 before a south or southeast wind will be blowing- 

 over Nebraska and Kansas, and a warm wave 

 will be started in those states. As this area passes 

 ea.stward to Minnesota, it.s effect will licgin to be 

 felt in Iowa, Missoini and .\rkaiisas, while liy the 

 time it reaches Lake ^lidiigaii, it will iirobably 

 have produced southeast winds, even to the Gulf 

 of Mexico. But an area of low pressure is imme- 

 diately followed by one of high pressure, pro- 

 ducing an opposite effect, and the isotherms 

 which bent north to welcome the coming of the 

 low area, turn rapidly southward l)efore the icy- 

 breath which blows from an area of high prcssm-e. 

 Thus the cold and warm waves both come from 

 the same quarter, and both move in the same di- 

 rection, that is, the direction in which the area of 

 low pressure is progre.s.sing. When we read that 

 low pressure is generally accompanied b}' clouds 



Copyright, 18S5, by Frank B. Websxbb. 



^,v\ill i"SU.ii;<a 



