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98. A Home Lesson for a Hot Day.

Find a place where the sun shines upon sand or
upon hard bare ground.

Hold your hand one inch from the sand or the
bare ground, then one foot away, and three feet
away. In which place is the air warmest?

Find a flat rock. Can you feel heat coming from it ?

Place your hand near the sand and then near some
water that stands in the sun. Which is warmer, the
air above the water or above the sand?

99. How the Air is Heated.

On a clear summer day, how hot the air is
over a sandy field, and how cool a breeze
from over the sea.

A flat rock in the sunshine may be hot, while
green grass close by it feels cold. On very
hot days, why do we like to sit in the shade?

The sun warms the surface of the earth,
and the earth then warms the air just as a
stove does.

Dust and clouds also receive warmth from
the sun, and help to
warm the air about
them. Can you now
tell why the air be-
comes cooler when
heavy clouds form be-
tween us and the sun?

When the sun is
shining, some parts of
the earth become
warmer than other
parts. For this reason,
the air in some places
is warmer than it is in others.

 

Figure I

100. How the Earth is Heated.

Let us suppose that the straight lines in
figure I. are rays coming from the sun to the

earth at noon and also at about nine o'clock
in the morning.

Count the slanting rays. Count the vertical
or upright ones. Which rays
farther over the surface
of the earth? | |

The more nearly over- ||
head the sun is, the
thicker its rays reach
the earth. In the morn-
ing, when the sun is
low in the sky, the rays
are very slanting, and
the earth warms slowly.

Which part of a day
is generally the warm-
est? Why does the
earth become cool or cold in the evening?

In summer the sun shines higher in the
sky than it does in winter. In the coldest
part of the year, the sun’s rays are more
slanting than they are in summer.

On places near the equator, the sun shines
almost straight down every day in the year,
and makes a wide hot belt round that part of
the earth.

As we travel north or south from the hot
belt, the sun’s rays become more and more
slanting, and therefore we reach cooler and
cooler parts of the earth. Round the poles
there are wide regions of ice and snow.

In figure II., as many rays fall upon B as
upon A. Over B the rays are nearly vertical,
but over A they strike the surface with greater
slant.

Which is the wider space, A or B? Upon
which do the rays strike nearer together ?

Figure I. shows that while the sun is shining
nearly straight down on the hot belt, its rays
fall with more slant on other parts of the earth.

   

Figure II.