PHYSICAL HYDROGRAPHY AND TEMPERATURE 



Charles K. Green, Hydrographer 

 U. S. Coast and Geodetic Survey 



CONTENTS 



Page 



Introduction 11 



Methods 11 



Meteorological conditions 12 



Temperatures 15 



Condition at time of different 1929 cruises. . 15 



Temperatures in 1928 52 



Summary of temperature observations 52 



Water movements 56 



Currents 56 



Subsurface movements of the Deep Hole 



waters 59 



Oscillations 64 



Transparency 69 



INTRODUCTION 



The limited survey of the eastern portion of 

 Lake Erie in 1928 disclosed the presence in this area 

 of unusual hydrographic conditions. The 1929 pro- 

 gram called for a more comprehensive investigation 

 of these conditions and the extension of the survey 

 to include the main body of the lake. As biologi- 

 cal, chemical, and geological researches were 

 equally important branches of the survey, it was 

 necessary to make definite and regular cruises; and 

 the Shearwater was therefore not available to man- 

 euver solely in accordance with the needs of the 

 hydrographer. Nevertheless, the general physical 

 properties of the lake were well covered. The data 

 obtained during the survey are included in this re- 

 port in the form of tables and figures. 



METHODS 



The hydrographic apparatus, with the excep- 

 tion of the Secchi disc and the surface -current pole, 

 was operated on a 1/4 -inch galvanized iron wire 

 and hand winch. The depths were obtained from 

 a high-grade meter sheave which was hung from a 

 davit about 7 feet from the quarter deck. This ar- 

 rangement permitted the thermometers to be 

 hoisted to the height of the observer's eye where 

 they could be read quickly and accurately. 



Negretti-Zambra and Richter -Wiese deep- 

 sea reversing thermometers were used for water 

 temperatures. Readings were made to 0. 01° C. on 

 the latter type and to the nearest 0. 05° C. on the 

 Negretti-Zambra instruments. All the thermometers 

 had been standardized at the Bureau of Standards and 

 stem corrections were applied to the readings when 

 the correction was large enough to affect tenths of a 

 degree. The results are tabulated to the nearest 

 0.01°C. 



Throughout this report all temperatures are given 

 in Centigrade, depths in meters, distances in statute 

 miles, and velocities in statute miles per hour. 



Transparency was determined by observations 

 with a 20 -centimeter white Secchi disc attached to 

 a line graduated in meters. The depth, in meters 

 and tenths, at which the disc disappeared from view 

 is recorded as the transparency. The chemist made 

 top and bottom turbidity determinations at each sta- 

 tion also. 



The standard Coast and Geodetic Survey current 

 pole was used for surface -current measurements. This 

 equipment consists of a 15 -foot pole, 3 inches in 

 diameter, and weighted with lead at one end so that 

 it floats in an upright position with about a foot of 

 its length showing above the surface. A graduated 

 cotton line is attached to the pole by means of a 

 simple bridle; the stray line is about 100 feet long. 

 The line is so graduated in knots and tenths (or 

 statute miles per hour and tenths) that the number 

 of graduations passing out in 60 seconds, when the 

 pole is free to drift with the current, gives the 

 velocity directly. 



A Price (Gurley) current meter was used for 

 measurement of subsurface velocities. This instru- 

 ment is accurate only when it is suspended from a 

 dock or some other stable structure. If it is sus- 

 pended from a boat, the vertical movement, even 

 in a sea which may be called "smooth", increases 

 the revolutions, and the greater the movement the 

 greater will be the error. As subsurface velocities 

 of the lake are low, the error thus introduced may be 

 as much as 50 percent on choppy days. In a current 

 of 1 or 2 m. p. h. , the percentage of error due to 

 vertical movement of the boat is small, but when 

 we are dealijig with velocities of only 0. 1 or 0. 2 

 m.p. h. , the percentage error is obviously greatly 



11 



