with the exception of the mountains,
a much milder climate than one would expect from a mere knowledge
of its latitude. It will be instructive to search out the reasons
for the remarkable contrasts in climate presented by different
portions of the slope.

The imaginary lines passing through points of equal temperature
upon the earth are called "isotherms." These lines rarely accord
in direction with the parallels of latitude, but curve far to the
north or south. The irregular course of the isotherms is due to
many causes. Among these are the distribution of the land and water,
the direction of the prevailing wind, the position of the mountain
ranges, and the elevation above sea-level.

[Illustration: WEATHER MAPS

Fair weather over central portion of Pacific slope. Storm coming
in upon coast of Washington

Stormy weather over the western half of the United States]

In winter the isotherms curve far to the north over the North Pacific
and North Atlantic oceans; but over the intervening land they curve
as much to the south. In summer the isotherms are almost reversed
in position, at least as far as the land is concerned, for they
bend to the north in the heart of the continent. There are important
reasons for the slight variation of the isothermal lines upon the
western borders of North America and Europe, and their great change
of position in the interior from winter to summer, but these reasons
are not at all difficult to understand.

The temperature of large bodies of water changes but little throughout
the year, for water warms and cools slowly. The surface of the
land, on the contrary, heats rapidly, and then as quickly loses
its heat with the changing season. The air over the ocean is cooler
in summer and warmer in winter because of the influence of the
water, but over the land, in districts far from a large body of
water, the changes in temperature between day and night, summer
and winter, are very great.

It was formerly thought that the warm Japan current, which flows
against the western shore of North America, was responsible for
the exceptionally mild climate there, and that the Gulf Stream
produced a similar climate upon the coast of western Europe. More
careful study, however, has shown that not the warm ocean currents,
but rather the winds blowing from the water, are the cause of the mild
climate in those lands across which they blow. In temperate latitudes
there is a slow movement of the air in a