102 PHYSICAL GEOGRAPHY.

which comes in contact with the cool sides of the
glass has its temperature lowered below the dew
point, and deposits as vapor the moisture it no
longer can retain.

The dew which is deposited during certain sea-
sons of the year on plants and other objects on
the earth, has a similar origin. Objects on the
earth cool more rapidly than the surrounding air,
which deposits its moisture on them whenever
they lower its temperature below the dew point.
When the objects are colder than 82° Fahr., the
dew is deposited as hoar-frost.

Dew falls or is deposited more heavily on some
objects than on others; this is because some ob-
jects radiate or give off their heat more rapidly
than others, and thus becoming cooler, they con-
dense more of the moisture of the air.

More dew is deposited during a clear night than
during a cloudy one, because objects cool more rap-
idly when the sky is clear than when it is cloudy.

Thick clothing keeps the body warm, not because the
clothes give any heat to the body, but because they are non-
conductors,and prevent the escape of heat from the body.

In like manner the clouds, acting as blankets to the earth,
prevent its losing heat rapidly.

More dew falls or is deposited during a still night
than during a windy one.

The air must remain long enough in contact
with cold objects to enable them to lower its tem-
perature and collect its moisture. Powerful winds
prevent this, while gentle breezes favor the depo-
sition, by bringing fresh masses of air into contact
with the cold objects.

In the tropics, during seasons when the sky is clear, the
dew is so copious that it resembles a gentle rain.

In the deposition of dew, the moisture is derived from a
comparatively thin stratum of air in the immediate neigh-
borhood of the cool object. All other kinds of precipita-
tions are produced by the cooling of a large mass of air.

“~ 268. Fogs and Clouds—Whenever the tem-
perature of a large mass of air is reduced below
its dew point, its moisture begins to collect in
minute drops, which diminish the transparency
of the air, and form fogs or mists, when near the
surface, and clouds, when in the upper regions of
the atmosphere. fogs and clouds are the same in
their origin and composition, and differ only in
their elevation.

The minute drops of water that form clouds
and fogs, though formed of a substance about
eight hundred times heavier than air, are pre-
vented from settling rapidly by the resistance of
the air. This is rendered possible by the minute

 

size of the drops, which are much smaller than
the relatively heavier dust-particles, which are
wafted about by the winds. Whenever the drops
exceed a certain size, they fall as rain or snow.**

It was once believed that the moisture in fogs and clouds
existed in the form of hollow bubbles or vesicles, filled
with air, and that the clouds or fogs ascended, whenever
the contained air expanded the bubbles and rendered

them specifically lighter. This idea is now generally
abandoned.

Clouds or fogs result whenever a mass of air is
cooled below the temperature of its dew point,
as, for example, when two bodies of air of dif-
ferent temperatures are mingled, especially if,
as is generally the case, the warmer of the two
is the moister. On the contrary, clouds or fogs
disappear on the approach of a dry, warm wind.
Clouds are higher in the tropics than in the polar
regions, and generally are higher during the day
than during the night.

Off the banks of Newfoundland, the warm, moist air of
the Gulf Stream is cooled by the cold, moist air of~the

Labrador ocean current. Hence result the dense fogs so
frequent over this part of the ocean.

‘_ 269. Classification of Clouds—Clouds assume

such a variety of shapes, that it is difficult to
classify them. Meteorologists, however, have rec-
ognized the existence of four primary forms: the
cirrus, the cumulus, the stratus, and the nimbus.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

     

: =
Fig. 90. Primary Forms of Clouds,
~v Cirrus. ~ + Cumulus.

The Cirrus Cloud consists of fleecy, feathery
masses of condensed vapor, deposited in the
higher regions of the atmosphere. The name
cirrus is derived from the resemblance the cloud
bears to a lock of hair. These clouds are called