152 Part II. Chapter 2. 
The greatest falls of temperature have occurred in December in both cases, 
and these great descents have been rare in the sixteen years compared. The 
distribution of heat in the winter, as well as in other seasons, is varied. 
Consequently the inconveniences caused by the extreme limits are not pro- 
longed for many hours; in general, the temperature in winter is mild from 
eleven o’clock a. m. to five o’clock p. m., and in the other seasons the morn- 
ings and nights are always cool. 
The principal elements which modify the normal course of the temperature 
are the winds, the clouds, and the rains. The winds from the two southern 
quadrants increase the heat and dry the air; the currents from the north cool 
and moisten it. Generally the winds from the winds from the first quadrant 
clear up the clouds suddenly and induce a decided lowering of the temperature. 
A sky entirely covered with clouds keeps the temperature high, while the 
passage of loose clouds at the hours of maximum temperatures impedes its 
rise. The hottest month is April; the coolest, December. 
The course of the mean monthly temperature of the ground at the depth 
of 0.85 (33.75 inches) meter varied from 56° F. (13.3° C.) to 63.5° F. (17.5° C.) 
rising from the first to the sixth month of the year and falling from June to 
December. The lowest reading corresponds to January and the highest to 
the month of June, and the regularity of its course indicates that it is free 
from external influences. The annual mean of 60° F. (15.6° C.) differs only 
two-tenths of a degree at most from the mean of the surrounding air. 
Rain occurs generally in all months of the year, although in no regular 
manner in the spring. The rainy season so called, can be said to begin in 
May, to be fully established in June, and to end in October, August being 
the most rainy and most stormy month. The mean quantity of water which 
has collected in this latter month amounted to 129.8 mm., and the greatest 
depth of rain corresponding to any one day in the same month amounted to 
63.5. The mean annual depth of rainfall in sixteen years is found to be 
593.5 mm., and the mean of the ten years from 1880—ı890 is 614.5. The 
greatest annual depth registered in sixteen years amounted to 892.6 mm., 
and the minimum to 444.2. Generally, the greatest quantity of “water falls 
on the mountains of the Valley of Mexico, whither the clouds are driven by 
the winds. 
The cloudiness in Mexico increases in the summer months, there being 
in the other seasons a great number of entirely cloudless days, with a clear 
sky of beautiful blue. Cirrus veils are precursors of storms and last but a 
few days. Haze on the horizon is prevalent in some of the spring months, 
but it disappears in the rainy season, during which the atmosphere becomes 
notably transparent, the twilight lasting remarkably and presenting brilliant 
displays at sunrise and sunset. The prevailing direction of the clouds is from 
the southwest, but in the rainy season they proceed from the first quadrant. 
The dominant wind in the City of Mexico is the northwest, which pre- 
vails the greater part of the year, especially in autumn and winter. It is the 
