LIGHT AND PRESSURE 63 
snows and the beginning of the rainy season. The drainage area of Lake 
Michigan is very small and has little relief, and the amount of sediment 
carried in is small at all times. The depth of light penetration is there- 
fore not so much influenced by these factors as in Lake Geneva. Wave- 
action is also important in stirring the bottom materials near shore. 
We would expect the light penetration in Lake Michigan to be least 
during the rainy and windy seasons, and greatest in calm, dry weather— 
late summer and autumn.’ All of the surrounding physiographic con- 
ditions are factors controlling light. Table IV shows the seasonal 
distribution of rainfall and light penetration in Lake Geneva, and the 
seasonal distribution of winds and rainfall at Chicago. 
TABLE IV 
SHOWING DEPTH OF LIGHT PENETRATION IN LAKE GENEVA AND CONDITIONS AFFECT- 
ING THE SAME IN BotH LAKE GENEVA, AFTER FoREL (76, Vol. IT, 
p. 439), AND LAKE MICHIGAN 
In the eighth column the results are given in seconds, in terms of the effect on the 
photographic plate, of equivalent exposures to the sun. 
Lake MIcHIGAN | LAKE GENEVA, SWITZERLAND (AFTER FOREL) 
Rainfall Neloeitay ea Wind | Rainfall and Light | Light and Depth 
Montu | | i" Nl wen 
| Intensity 
Meters Light of Light 
ch Centi- |Miles per per Prec. in | Limit at (March) | Depthin 
) teehee meters Hour Second Cm. Depth in | at Depth | Meters 
Meters in Next 
| Column 
January 2.0 Roig sp dlypace | 8.0 4.87 ar 500 sec. 0.0 
February BASE) ile 6 5).:2 20.0] 9.0 3.65 ene 500 SeC.| 19.6 
March: = -=.. Ps 6.4 20,4 |) Oa Aas7D IIo 500 sec.| 25.2 
Aprile heen - Any 6.9 ROMAN Oey 5.68 Bec 400 SEC.) 45.5 
IMT AN or ev ons 355 8.9 mse gt Shoe 7.91 75 | 360 sec.) 55.5 
AUN eres. aur, 9.4 DAN 4y i Ons 7 SOMA neta 120 sec.| 65.6 
Alirl yeaeee eee 3.6 9.2 bol) Ge Ih “Geel 45 60 sec.| 75.6 
August 2.8 Be 13.4, | (O.o0 Vip ratoy ee 25 sec.) 85).7 
September Soe Winey 16.7 Tegel OMA 2 50 Io sec.| 95.8 
October. .... 2.6 6.6 L705 FO | 10.10 Biches 2 sec.| 105.4 
November...| 2.6 6.6 TOLOult oS neem 85 © S€C.| IT5.6 
December. -| 2.1 Soe 19.9 8.9 | gute | oy 
4. PRESSURE (76) 
Pressure in water increases with depth. The results given by Forel 
are shown in Table V. 
tThe Lake Michigan Water Commission has reported greatest turbidity in 
January, February, March, and April. 
