Juday—Lakes in Central America . 
225 
place in all four lakes previous to the time of these observations. 
The minimum temperatures which were reached by the waters 
of these lakes in the winter of 1909-10 are shown by the bottom 
temperatures obtained in these observations. The minimum 
temperature depends upon the severity of the winter and doubt¬ 
less it varies somewhat from year to year. In late January and 
early February, 1906, Meek found that the temperature of the 
bottom water of lake Amatitlan was 20.5° C. while on Febru¬ 
ary 5, 1910, it was 19.6°. In lake Atitlan the bottom tempera¬ 
ture was 20.0° in February, 1906, and 19.2° in February, ’1910. 
In this latitude day and night are about equal in length so that 
the diurnal period of radiation is substantially equal to that of 
insolation. Such a condition is not favorable for the storing 
of a large amount of heat in the upper stratum, but during the 
latter part of January this water begins to gain heat and this 
process continues until the temperature of the upper water rises 
a few degrees above that of the lower. As the temperature of 
the surface water rises it becomes lighter than the water below 
and offers a resistance to mixture with it. Soon this thermal 
resistance reaches a point where the wind is no longer able to 
mix the upper water with the somewhat cooler water below and 
the water of these tropical lakes becomes directly stratified just 
as it does in temperate lakes in early summer. Such a stratifi¬ 
cation was found in these southern lakes in February, 1910. 
The temperature differences were not great enough to show the 
exact limits of the three different strata, that is, the layers which 
correspond to the epilimnion, the thermocline or mesolimnion, 
and the hypolimnion in temperate lakes after their stratification; 
but taken in connection with the results obtained for dissolved 
gases, they show that similar strata existed at the time of these 
observations. Owing to differences in area, depth, and climatic 
conditions, the thickness of these layers differed in the different 
lakes. 
In lake Amatitlan, the middle stratum or mesolimnion lay be¬ 
tween 10 m. and 20 m. and the temperature at the latter depth 
was only 0.5° less than at the former. In lake Ilopango this 
zone had the same extremes or limits with a difference in tem¬ 
perature of 0.6°. In lake Coatepeque, however, the middle 
stratum lay between 20 m. and 30 m. and there was a decrease 
of 0.5° in this layer. In lake Atitlan the difference in tempera¬ 
ture between surface and bottom was only about 0.4° in the early 
15—S. A. 
