changes in Time 



131 



controlling action of intermediate temperatures for all kinds of plants 

 and animals. 



Changes in Temperature 



Changes in Time. Let us consider first the temperature changes to 

 which an animal or plant may be exposed if it remains in the same 

 place. Time changes in temperature are controlled by various astro- 

 nomical and climatic cycles. Within a few hours the sun may move 

 so as to change the exposure of a sessile form from direct sunlight to 

 shade. On land a very great difference in the heat received may oc- 

 cur within a short period of time. In the water environment the 

 change from sun to shade produces only a minor effect— less than 

 0.1°C at a depth of 5 m. 



Similarly, the diurnal fluctuation in temperature is very much 

 damped in the aquatic environment. In a body of water of any con- 

 siderable size differences between day and night are commonly less 

 than 1°C. The maximum diurnal change in the ocean is about 4°C, 

 and with increasing depth the amplitude is reduced. Probably no 

 diurnal temperature change is detectible below a depth of 15 m. 



12 16 20 24 



Modified from Weaver and Clements, 1938 



Fig. 5.1. Portion of a soil thermogram made in June on the prairie near Lincoln, 

 Nebraska, showing the temperature fluctuations at depths of (A) 7.6 cm and 



(B) 30 cm. 



The temperature of the air near the surface of the land is sometimes 

 17°C higher in the daytime than at night, and in desert localities this 

 difference may be as much as 40°C. A still greater diurnal range in 

 temperature is reported for the surface of the soil in desert areas. In 

 every situation the amplitude of the change from day to night in the 

 soil is reduced with depth. The soil thermogram shown in Fig. 5.1 

 illustrates this point and also reveals the lag in the time of occurrence 

 of the maximum and minimum daily temperatures. At a depth of 



