CLIMATE.

 

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power of the more nearly vertical rays is greater
than that of the rays which strike obliquely.

236. Variations in Temperature.—The differ-
énces in the heating power of the vertical and ob-
lique rays of the sun. cause the temperature of
the earth’s surface to decrease gradually from the
equator toward the poles. The differences of tem-
perature thus effected are further increased by the
difference in the length of daylight and darkness.
While the sun is shining on any part of the earth
the air is gaining heat; when it is not shining the
air is losing heat.. When the length of daylight
exceeds that of the darkness, the gain exceeds
the loss; when the darkness exceeds the day-
light, the loss exceeds the gain.

The excessively low temperatures that would
result from the oblique rays in high latitudes are
prevented by the great length of daylight during
the short summers, thus allowing the sun to con-
tinue heating the surface during longer periods.
The warmest part of the day in high latitudes
sometimes equals that in the equatorial regions.
During the long winters, however, the continued
loss of heat makes the cold intense.

Hence in the tropics we find a continual sum-
mer; in the temperate-zones, a summer and winter
of nearly equal length; and in the polar zones,
short, hot summers, followed by long, intensely cold
winters.

The true temperature of the air is ascertained by hang-
ing a thermometer a few feet above the ground, so as to be
shielded from the direct rays of the sun, and yet be in free
contact on all sides with the air.

237. Manner in which the Atmosphere re-
ceives its Heat from the Sun.—The atmosphere
receives ‘its heat from the sun—

(1.) Directly. As the sun’s rays pass through
‘the air, about 28 per cent. of the vertical rays
are directly absorbed, thus heating the air. The
remainder pass on and either heat the earth, or
are reflected from its surface.

(2.) From the heated earth. The sun’s rays
heat the earth and the heated earth heats the air.
It does this in three ways:

(a.) By the air coming in contact with the
heated earth.

(b.) By the heated earth radiating its heat, or
sending it out through the air in all directions.

After the sun’s heat has been absorbed by the
earth and radiated from it, a change occurs. which
renders the rays much more readily absorbed by
the air.

(¢.) By the heat being reflected from the earth

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and again sent through the air. But little heat
is imparted to the air in this way.

It is mainly the aqueous vapor the atmosphere
contains that absorbs the sun’s heat. Dry air
allows the greater part of the heat to pass through
it; therefore variations in the quantity of vapor in
the air must necessarily produce corresponding
variations in the distribution of heat.

238. Isothermal Lines are lines connecting
places on the earth which have the same mean
temperature.

The Mean Daily Temperature of a place is ob-
tained by taking the average of its temperature
during twenty-four consecutive hours.

The Mean Annual Temperature of a place is
the average of its mean daily temperature
throughout the year.

If the physical climate were the same as the
astronomical, the isothermal lines would coincide
with the parallels of latitude.

An inspection of the map of the isothermal lines shows
that their deviations from the parallels, though well

/~marked in all parts of the earth, are greatest in the north-

ern hemisphere. Wherever, from any cause, the mean tem-
perature of a place is higher, the isothermal lines are found
nearer the-poles ; when lower, nearer the equator. The former
effects are noticed~particularly in portions of the ocean
traversed by warm currents; the latter, in crossing por-
tions of the ocean traversed by cold currents. In the map
of the isothermal lines the influence of elevation is re-
moved by adding 1° for every 1000 feet of elevation.

239. Physical Zones.—The Physical Torrid
Zone lies on both sides of the equator, between -
the annual isotherms of 70° Fahr.

The Physical Temperate Zones lie north and
south of the Physical Torrid Zone, between the
annual isotherms of 70° and 30° Fahr.

The Physical Frigid Zones lie north and south
of the Physical Temperate Zones, from the an-
nual isotherms of 30° Fahr. to the poles.

The greatest mean annual temperature in the

‘eastern hemisphere is found in portions of North

Central Africa, and in Arabia near the Red Sea,
in the southern part of Hindostan, and in the
northern part of New Guinea and the neighbor-
ing islands; in the western hemisphere, in the
northern parts of South America and in Central
America.

‘240. Modifiers of Climate:— The principal
causes which prevent the isothermal lines from
coinciding with the parallels of latitude are:

(1.) The Distribution of the Land and Water
Areas.—Land heats or cools rapidly, absorbing or
emitting but little heat. This is because the land