METEOROLOGY AND CLIMATOLOGY. 35 
changes of weather. To understand these general conditions it is 
necessary to consider the elements of climatology. 
3. Outlines of Climatology. —The air is in constant movement on 
account of the unequal way in which the heat of the sun falls on different 
parts of the Earth's surface, and at different seasons of the year. All the 
conditions of the atmosphere show a certain diurnal periodicity which is 
most marked in the regions of steady climate between and near the 
tropics. Thus, as a rule, the minimum temperature of the air occurs 
just before sunrise, the maximum temperature from two to three hours 
after noon. The amount of difference between the maximum and 
minimum temperature of the day (daily range) is least near the sea 
or in wet regions (a maritime climate) and greatest in the interior of the 
continents, especially where the rainfall is slight (a continental climate). 
Over the sea itself the daily range of air temperature averages only 
3 Fahrenheit degrees; but in the heart of a continent, especially in 
a desert, it may exceed 60 Fahrenheit degrees. 
Diurnal changes of pressure are proportionally much smaller in amount 
than changes of temperature, and are to be observed as a regular 
phenomenon only in the tropics, or elsewhere during very settled 
weather. There are usually two maxima daily, about 10 a.m and 
10 p.m., and two minima occurring about 4 a.m. and 4 p.m. It is only 
in rare cases that the total barometric range exceeds O’lO inch, very 
frequently it is not greater than 0*04 inch. Still it is convenient to 
remember in the tropics that a fall of the barometer not greater than 
0*10 inch between 10 a.m. and 4 p.m. is to be expected, and does not 
indicate either the approach of a storm (if the observer is at rest) or the 
ascent of 100 feet (if he is on the march). 
Associated with the diurnal changes of temperature in settled weather 
are changes of wind due to local configuration of the ground. The wind,, 
for example, usually blows up a mountain side, or up a steep valley,, 
during the day, and down a mountain, or down a steep valley, during the 
night. So, too, the regular land and sea breezes found on the borders of 
the sea or of great lakes blow from water to land in the day time and 
from land to water at night. Here the determining cause is the fact 
that land is warmed and cooled by radiation, and in turn heats or chills 
the air much more than water does. 
Similar diurnal periodicities occur in the amount of cloud, in the 
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