66 



CLIMATIC CYCLES AND TREE GROWTH 



of spots. This polarity was observed to change from "north" direction to 

 "south" direction (or vice versa) at the sunspot minima, 1913, 1923, and 

 1933-34, thus raising many complex questions in the problem and locating 

 fundamental activity within the body of the sun. Thus we see that in 

 studying cycles in the sunspot numbers, the month is the logical time unit, 

 since in that time the entire circumference of the sun is reviewed. 



For certain terrestrial effects, however, we need the daily reports. Mag- 

 netic storms and the related auroral displays often show direct relation to 

 large spots near the center of the solar disk ; the terrestrial effect is observed 



I860 



1990 



,'» " 



—40 



1885 1890 1895 



Max. Min. Max. 



-Butterfly diagram (after Maunder). 



1900 



Min. 





to follow a day or so late. In consequence of this prompt response, the solar 

 influence in terrestrial magnetism has long been recognized. It is thought 

 to be due to streams of particles coming to the earth at high velocity from 

 the solar surface in or near the spot. Physical connection of these magnetic 

 effects with meteorological conditions and tree-ring growth has not yet been 

 established. It is thought possible that these streams perform some change 

 in our upper atmosphere that influences the transmission of heat. 



Meteorological changes depend primarily on the motion of our atmosphere 

 which results from heat that comes to us with the velocity of light from the 

 sun's surface. Here again there may be effects in our upper atmosphere that 



