BIOLOGICAL STUDY OF CHESAPEAKE BAY WATERS 
299 
were frequently below 0.0° C.; and it is the writer’s belief that the temperature of 
the surface water in the upper part of the bay reached temperatures below 0.0° C. — 
for example —0.2° C. (salinity 13.67) on March 7, 1920, at area W. 
SURFACE TEMPERATURE FROM MOUTH TO HEAD 
Leaving out of consideration, for the moment, the surface temperature condi- 
tions along the line E, F, G, which extends across the mouth of the bay, an examina- 
tion of the surface temperatures observed during the cruises of the coldest months 
of the year suggest that, in general, there is a decrease from a region near the mouth 
(line D, C, B, A) to the head. The data consistently show a graded decrease; and 
such a condition would be expected, but it must be remembered that our observations 
were not made simultaneously at the thirty-some areas distributed over the bay 
and that, in fact, it took several days to complete the collection of the data. It is 
hardly necessary to state that there was some change in the temperature conditions 
from day to day so that a map showing isotherms for a cruise can give only a general 
idea of the conditions over the whole bay. Such a map for the January, 1921, cruise 
is shown in Figure 13. The decreasing range of surface temperatures for January, 
1916, 1921, and 1922 may be seen well in Table 1 where series are given for areas 
from the mouth to head. 
During the cruises of the spring, summer, and fall the surface temperature values 
with one exception did not show the decreasing range from the mouth to the head. 
Although not taken simultaneously, they indicate a more variable condition and 
smaller range during those seasons. The exception mentioned above was found 
during the August, 1920, cruise, when, as may be seen from Table 3 and Figure 11, 
the data showed a decreasing range of surface temperatures from the mouth to the 
head. These figures were rather surprising until it was seen by reference to the 
weather map of the United States Weather Bureau that shortly before the observa- 
tions were made at areas D, C, B, and A (August 21) the air temperature at Norfolk 
reached 90° F. (32.2° C.) and at Baltimore only 70° F. (21.1° C.). 
Much variability in temperature distribution is to be expected, especially at the 
surface, in a shallow body of water where a difference of 20° F. in the temperature of 
the air over two different areas may occur at the same time so that maps showing 
isotherms can give only approximate pictures of conditions. The map for the cruise 
of August, 1920 (fig. 11), shows a range of surface temperatures from 23° near the 
head to 27° near the mouth, an unusual condition for which an explanation has just 
been offered. The 27° isotherm is of special interest in this connection. A more 
usual condition for the warmer months is shown on the map for June, 1916. (Fig. 15.) 
The greatest differences in surface temperatures per unit of distance from mouth 
to head were found, as in the case of salinity, near the mouth of the bay. They 
occurred during the cruises of the warmer months, when the heated waters of the 
rivers and bay meet the colder waters of the ocean. As examples, in August, 1920, 
there was a difference of almost 5°C. between E and A; in June, 1916, there was almost 
4° C. difference between the two areas. During the cruises of the colder months, 
however, such a rapid change in passing from the line G, F, E to the line D, C, B, A 
was not observed. (Figs. 11, 12, 13, 14, and 15.) 
The range of surface temperatures passing from D, C, B, A out through the 
mouth of the bay by way of areas G, F, E showed almost invariably a decrease in 
temperature during the cruises of the warmest months, and an increase in tempera- 
ture during those of the coldest months. 
