Climate of North America, 131 
less marked degree delayed after the sun’s lowest altitude. In marine climates 
on the other hand, the delay in the time of maxima and minima is much 
greater. The lowest temperature does not occur until two, or even three, 
j.e., in February and March. The highest temperature is similarly delayed 
after the greatest altitude of the sun, although to a less degree, the warmest 
month being August. 
The covering of snow in the northern part of North America has an 
important influence on climate, because it alters the effect which land has upon 
the temperature of the air by substituting its own influence for that of the 
ground which has then no control over the temperature of the air. Snow 
during the season of frost increases radiation and at the same time prevents 
the flow of heat from the ground by conduction, and in this respect, it is very 
beneficial. For this reason, says HANN, radiation from a snow surface under 
the clear sky of a continental winter considerably reduces the winter tem- 
peratures and increases the annual range of temperature. The increase of 
temperature in the spring is much retarded by the presence of snow, because 
the heat of the sunshine and of the warmer currents of air is almost wholly 
expended in melting the snow and ice. 
The diurnal range of temperature increases with an increase of distance 
from the ocean, as does the annual range. The greater and more rapid 
warming of the land surface, and the stronger insolation, increase the daily 
temperature maxima; while on the other hand the dry air and clear sky at 
night favor rapid radiation and cooling of the earth’s surface, and produce low 
nocturnal minima. Thus, there is a large diurnal and a large annual range. 
The diurnal range is greatest on deserts, and especially on dry plateaus. In 
Death Valley, California, the mean diurnal range of temperature in August, 
1891, was 64.2° F. (36° C.) and the greatest daily range in the same month 
was 73.9° F. (41°C.). The variability of the monthly means of temperature 
is greater in a continental than in a littoral, or insular climate. Thus in the 
interior of North America in February, the mean departure is 36.6° F. (20° C.), 
in August, 33.9° F. (19° C.) and for the year, 35.0° F. (19,5° Ku) 
With respect to temperature, it may be stated as a general rule that the 
temperatures in a littoral, or an insular climate possess a greater uniformity 
than a continental one. This is due, in the first place, to the influence of 
neighboring large bodies of water and secondly to the moist atmosphere, the 
effect of which is to minimize the influence of cooling agencies by causing a 
condensation of water vapor and thus by means of the latent heat liberated 
diminishing the fall of temperature, which would otherwise take place. 
The influence of continents upon humidity, cloudiness and precipita- 
tion is marked. As the water vapor of the atmosphere is supplied by eva- 
poration of the ocean waters, the amount naturally decreases toward the in- 
terior of the large continents, not considering the local sources of supply such 
as rivers, lakes and vegetation. The lowest mean annual relative humidities 
in the United States for instance are in the dry southwest. Yuma, Arizona, 
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