the fall which would account 

 for their travel to Drununond 

 and Manitoulin Islands (fig. 

 5). It is conceivable that a 

 number of bottles landed on 

 these and other islands that 

 rim the northern boundaries of 

 Lake Huron, but because the 

 shores are so little frequented 

 only a few were found. 



WIND DATA 



After many years of work 

 on the North Sea, Carruthers 

 (1947) emphasized the impor- 

 tance of keeping suitable 

 records of wind for use with 

 studies of water movements. 



Wind data for this investigation were taken 

 from Coast Guard Stations on Lake Huron and 

 Saginaw Bay. (Other stations were held to 

 be too far distant for the records to be 

 useful in this study.) At no time were 

 bottles dropped more than 50 miles from a 

 source of wind information. The Tawas 

 Point, Bay City, and Harbor Beach Coast 

 Guard Stations submitted wind data consist- 

 ing of six observations daily, that is, an 

 observation every 4 hours. The wind direc- 



Table 5. — July 31 j 1956, wind data from Coast Guard Stations 

 on Saginaw Bay and Lake Huron 



HARBOR BEACH 



w- 



TAWAS POINT 



lOmph 



tion~and velocity were recorded for each 

 observation at each station and a prevail- 

 ing wind vector for the day at each station 

 determined. A wind track based on these 

 vectors was used in interpreting drift- 

 bottle movements. 



Even though the area studied lies 

 within the belt of prevailing westerlies, 

 frequent wind changes occur with the pas- 

 sage of the racuiy pressure systems through 

 the lakes cireas. These changes are 

 of paramount importance in affecting 

 surface-water flow. In addition, 

 there may be some tendency for off- 

 shore breezes at night and onshore 

 breezes during the daytime. Because 

 of these frequent wind changes, 

 local winds at stations no more than 

 50 miles apart may be blowing from 

 nearly opposite directions at the 

 same time (table 5). These differ- 

 : ences in wind direction among the 

 ■ stations are more common during 

 periods of light, variable breezes. 

 During strong blows and gales the 

 wind direction is less likely to 

 differ between stations. 



BAY CITY 



Figure 6. — July 31, 1956, wind vector at the 

 Tawas Point, Bay City, and Harbor Beach, 

 Michigan Coast Guard Stations. 



Although the prevailing winds 

 can differ between stations (fig. 

 6), the wind tracks in 1956 taken 

 over periods of a month, were simi- 

 lax (fig. 7). Similarity among 

 stations was greatest for August, 

 September, October, and November 

 and least in June and July when winds 

 were usually light. This seasonal 

 trend has prime significance in 



10 



