Book IV -- Meteorology
The Science of the Weather
The Atmosphere; the Colors of Sunset and Sunrise . . . . . . .161
Eruption of Krakatoa (1883) . . . . . .162
Twilight . . . . . . . . . . . . . . . . . . . . . .163
Dust in the Atmosphere . . . . . . . . . 163
The Rainbow . . . . . . . . . . . . . . . . . 164
Halos . . . . . . . . . . . . . . . . . . . . . . . 165
Fog and Clouds . . . . . . . . . . . . . . . 165
Dew . . . . . . . . . .. . . . .. . . . . . . . . . 167
Height of Clouds . . . . . . . . . . . . . . .167
Rain . . . . . . . . . . . . . . . . . . . . . . . . .168
Size of Raindrops . . . . . . . . . . . . . .168
Hail, Snow, and Sleet . . . . . . . . . . .168
The Snow Line (Line of Perpetual Snow) . . . . . . . . . . . . . . .
.168
Snow Crystals . . . . . . . . . . . . . . . .169
Uses of Snow . . . . . . . . . . . . . . . . 169
Irrigation of Farming Lands . . . . . 169
Frost . . . . . . . . . . . . . . . . . . . . . . . . . .170
Rainfall . . . . . . . . . . . . . . . . . . . . . . .170
Rainfall and Crops . . . . . . . . . . . . . . .170
Winds . .. . . . . . . . . . . . . . . . . . . . . . . .171
Wind Vanes . . . . . . . . . . . . . . .171
Force of the Wind . . . . . . . . .. 171
Hurricanes . . . . . . . . . . . . . . . .171
Causes of the Winds . . . . . . . . 172
Land and Sea Breezes . . . . . .. 174
Weather .. . . . . . . . . . . . . . . . . . . . . .. .174
The Seasons (Spring, Summer, Autumn, Winter). . . . . . . . . .175
Storms . . . . . . . . . . . . . . . . . . . . . . . . .175
Weather Predictions . . . . . . . .175
United States Weather Bureau 176
Storm and Other Signals . . . . .176
Value of Weather Predictions .178
Summer Thunderstorms . . . . . 179
Lightning . . . . . . . . . . . . . . . . .180
Thunder . . . . . . . . . . . . . . . . . .180
Distance of a Thunderstorm From the Observer . . . . . . . 181
Lightning Rods . . . . . . . . . . . .182
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pg 160
FIG. 143. DIFFERENT
FORMS OF CLOUDS
a, cirrus; b, cumulus;
c, stratus; d, nimbus (rain cloud).
pg 161
The Atmosphere; the Colors of Sunset.--"I
wonder why it is," said Agnes, "that sunsets and sunrises are red. It
is the same sun at noon and at sunset, and the same sky; but sunsets
are red, and the sky is never red at noon."
Jack. There are two main
reasons, Agnes. In the first place, we are looking at the sun through
an air that is full of dust; and in the second place, the more dust you
look through the redder a thing looks that is beyond. At sunset (and
sunrise) you see the sun through a greater thickness of air than you do
at noon.
Mary. I do not understand how
that is.
Jack. Tom, see if you can
explain it by a little drawing.
Tom. Isn't it like this? When
the sun is nearly overhead at noon we see it through a less thickness
of air than when it is setting (or rising). (See Fig. 144.)
Jack. That is right. The
greater the thickness of air the more dust there is in it; and,
moreover, the more dust the redder the sun looks.
Agnes. How do you know that,
Jack?
Jack. Well, you could try the
experiment by pointing a long wooden box filled with dusty air at the
sun, and then by taking
pg 162
a box twice as long and doing the same thing. But the simplest proof is
this: In 1883 there was a huge volcanic eruption of a mountain in Java,
called Krakatoa. The whole air for hundreds of miles round was darkened
with the dust from the volcano. The winds scattered this dust round the
whole earth, so that for two years afterwards all the sunsets in
FIG. 144. A person on
the earth's surface at A sees the
sun overhead at noon through a thickness of air (AB), and the sun at
sunset through a thickness of air (AC). AC is considerably greater than
AB.
Europe and America were very red indeed, much redder than usual. There
was an extra amount of dust in the air at that time, and so the sunsets
and sunrises were redder than usual. It is the same thing in sand
storms on deserts. The sun looks red through them.
Fred. Suppose you should go up
on a high mountain, what then?
pg 163
Jack. The higher up you go the
less dust you look through. If you are on Mount Washington in the White
Mountains (5000 feet high), or on Mount Hamilton in California (4000
feet), the sky looks very pure and blue, and if you go to the top of
the high Alps or on Pikes Peak (14,000 feet), the sky is a dark violet
color--it begins to look a little black even.
Fred. And in balloons?
Jack. It is blacker yet. The
less dust you are looking through the whiter, or the bluer rather, the
sun looks to you. If you were quite outside the earth's atmosphere--on
the moon, for instance--the sun would not look yellow at all; it would
be bluish.
Mary. Where does the dust come
from, Jack?
Jack. Oh, from dusty plains,
from smoke, the pollen of plants, and from volcanoes. Just think of the
millions of tons of coal that are burned every winter.
Mary. Well, then, why doesn't
the air become thick with smoke by and by and stay so?
Jack. See if you can answer
that, Tom.
Tom. Is it because every rain
storm carries the dust particles down with the raindrops? I have
noticed that the air is clearer after rain.
Jack. Yes, that is a good
reason; and a great part of the dust falls on the ocean, too, and is
lost in that way.
Twilight.--"If you will look
out any evening half an hour after sunset, you will see a faint arch in
the sky in the west that is a little brighter than the rest. That is
the twilight arch, and it is caused by the sun's rays reflected and
scattered from dust high up in our air. You had better look for it on
the next clear evening. It is easy to see.
Dust in the Atmosphere.--"One
of the things that physicians want to know is how pure the air is at
any place--how free
pg 164
from dust. They put little plates of glass covered with sticky varnish
out of doors and then count the pieces of dust on the glass with a
microscope. High mountains and the snowy arctic regions have the purest
air of course but even there there is a great deal more than you would
think."
The Rainbow.--"Is the rainbow
caused by dust, Jack?" said Agnes; "part of it is red."
Jack. No, Agnes; that is
different. You see all the colors are in the rainbow, not red alone.
The white light from the sun is split up into colors by each raindrop
much as it would be by a glass prism, and then the light is scattered
by the different drops as light is scattered
<>FIG. 145. White Light
Entering a Raindrop is Split Up
Into Colored Lights--A white sunbeam enters
a hollow raindrop, and its different colors are
separated by the water of the drop as they would be by a prism of
glass. The white color is separated into red, yellow, blue, and so
forth, and is refracted by the drop down to the ground where you are
standing. (See Fig. 146.) You see the drops by these refracted
colors--red, yellow, blue--and all of these colors show in the rainbow.
pg 165
from mother-of-pearl shells. It is not very easy to explain in simple
words, but that is the main cause.
Halos.--"You have seen rainbows
round the moon, haven't you? and halos--bright circles--round the moon?
They are
FIG. 146. The Rainbow--Sometimes two bows are seen. Both are formed in much
the same way. The
ordinary bow is formed by sunlight that enters the top of the raindrops
and is refracted to the eye. The secondary bow is formed by sunlight
that enters the bottom of the raindrops. (Examine the picture
carefully.) SSS'S' are rays from the sun; HH' is the horizon. The
center of the bow is always exactly opposite to the sun from where you
stand.
caused by little prisms of ice floating high up in the atmosphere,
which scatter the moonlight in a regular way."
Fog and Clouds.--"The air is
full of dust that we can see," said Jack, "and it is full of the vapor
of water that we cannot
pg 166
FIG. 147. A Complete
Solar Halo (parhelion, sundog)--Sometimes the
complete halo is seen as in the picture,
but more often
only parts of it. These halos are caused by light refracted from small
prisms of ice in our atmosphere.
see, too. When I put a pan of water out of doors, Agnes, what becomes
of the water?"
Agnes. It disappears somehow,
if there is not much of it. I don't know where it goes.
Tom. It evaporates; it rises
up into the air like a gas. I suppose it is a gas.
Jack. Yes, it is a gas, like
invisible steam. Real steam is invisible, and water vapor is invisible.
When the water vapor in the air turns into visible water what do you
see?
pg 167
Agnes. Fogs and clouds and
mist.
Mary. Yes, and rain and dew.
Jack. Mist and fog are made of
millions and millions of little drops of water.
Agnes. Why don't they fall
down in rain, then, Jack? Water is heavy.
Jack. The drops are hollow,
and they are very small and they float in the air just as soap bubbles
do.
Dew.--"When you breathe on a
cold windowpane the invisible water vapor in your breath," said Jack,
"condenses on the pane and makes a mist which is just like the dew that
falls at night. Take a tumbler of ice water and set it in a warm room
and you will see dew form on the outside of the tumbler. The cold
tumbler condenses the invisible water vapor just as the cold water of a
pond condenses the vapor of the air above it into a fog or mist. The
reason is because a cubic foot of warm air can hold more water vapor
than a cubic foot of cold air. When you cool air, no matter how you do
it, you squeeze some of its water vapor out of it."
Tom. When the sun rises the
fogs over ponds vanish. Is that because all the air gets warmer and can
hold more vapor?
Jack. Exactly so, and when all
the air is warm you have no clouds either. Clouds are a sure sign that
the air where they are is colder than the other air in the neighborhood.
Agnes. How high are the
clouds, Jack?
Jack. Oh, they are at very
different heights. Why, don't you know, Agnes, that you are sometimes
in the very midst of a rain cloud? The cirrus clouds (see Fig. 143) are
sometimes ten miles high, but usually less. They are probably made of
little ice crystals, for the air at that height is very cold indeed.
pg 168
The cumulus clouds are a mile
high, or so. The stratus clouds
are the
lowest.
Tom. If clouds are made of
hollow water drops like soap bubbles floating in the air, how is it
that we ever have rain? Why don't the bubbles always float?
Jack. You have seen two soap
bubbles come together and burst? They become nothing but two heavy
drops of water, or even one drop, and the water falls. That is rain.
Rain.--"A little sphere of
water that is not hollow is a good deal heavier than the air, and a
hollow sphere of water is often lighter than air. There are millions of
drops in a cloud, and when they are blown about by winds they come into
collision and fall in rain.
Size of Raindrops.--"The next
time it rains try to measure the diameter of the raindrops. It is not
very easy, but you can find some way to do it. I leave it to you boys
to invent a way. The raindrops of a heavy pattering summer shower are
large--about a tenth of an inch in diameter. Fine rain is made of drops
one twentieth to one fiftieth of an inch in size."
Hail and Snow and Sleet.--"I
suppose hail is nothing but frozen rain," said Mary.
Agnes. And snow and sleet,
too, for that matter.
Tom. Sleet is nothing but snow
that is driven about by the wind. In calm weather you get the little
snow crystals; but when the wind blows, a dozen of them are blown into
one and they come down in little lumps of ice; sleet, that is.
Jack. Or else the snow falls
through a layer of rather warmer air and is partly melted.
The Snow Line.--"The higher up
you go," said Jack, the colder is the air, and by and by you come to a
height above which there is never rain, only snow. That is the line of
pg 169
perpetual snow. In our Rocky Mountains the snow line is about 13,000
feet or so. Above that height the snow never melts at all, and you have
snow mountains. In Alaska the snow line is nearly at the level of the
sea. That is one reason why Alaska scenery is so impressive. A low snow
line makes fine mountains.
Uses of Snow.--"If no snow fell
in the winter time, seeds would have a hard time to grow, as the ground
would be frozen
FIG. 148. SNOW CRYSTALS
Notice that all snow
crystals are six sided.
stiff; but the snow fall covers it up like a blanket. The ground is not
frozen so very deep, and the seeds have a chance.
Irrigation.--"Snow has another
great use. When it melts in the spring the water can be used for
irrigating arid lands. We in the United States let our snow go mostly
to waste. We ought to save it in great reservoirs in the western and
southwestern states and let it out during the summer when it is sadly
needed. Nevada and Arizona and other states could be made into gardens
if people would take a little trouble."
Tom. That is something for the
government to do. The government must build the reservoirs, and the
people will do the rest.
pg 170
Frost.--"I suppose frost is
nothing but frozen dew," said Mary.
Jack. It is not quite that,
Mary, though it looks so. The dew does not fall first as water and then
freeze; but it really is water vapor frozen in the air, and it falls in
fine spikelets of ice and covers everything.
Rainfall.--"How much rain falls
in a year, Jack?" said Fred.
Jack. Fred, that is like
asking how long the nose of a man is. Why, in some parts of the world
almost no rain falls--on the deserts of Sahara and of Arizona, for
instance. The average rainfall of the whole world is about thirty-three
inches in each year; the water would be about a yard deep at the end of
a year if all of it were saved--if it did not get into the soil. But
there is an enormous difference in rainfall at different places. On the
arid plains of Arizona there are often less than two inches a year. In
some parts of the Himalaya Mountains forty feet of rain fall in a year.
FIG. 149. One Form of
Wind Vane--The arrow points to the direction
from which the wind
is coming. If you should move the arrow so as to point in any other
direction and then let go of it, you can see that the pressure of the
wind on the tail of the vane would soon bring it back. A wind vane put
into a rapid stream of water (a brook, a river) would always point
upstream. The four arms are set, once for all, so as to point north,
east, south, west.
Rainfall and Crops.--"Wheat
will not grow by itself where the rainfall is less than about eighteen
inches a year, unless there are plenty of fogs. In the arid (dry)
regions the farmers have to irrigate their crops."
pg 171
Winds.--"A wind," said Jack,
"is a current of air moving near the surface of the earth. How can you
tell which may the wind blows?"
Mary. A wind vane will do
that. (See Fig. 149.)
Force of the Wind.--"Here is a
table," said Jack, that scientific men and sailors use to express the
velocity of the wind, or else its pressure on a square foot.
"You can describe a wind by using this little table. A wind that blows
about eighteen miles an hour--one that would carry a feather or a
little toy balloon about eighteen miles in sixty minutes--is called
four (4). (See the first
column of the table.) Such a wind presses on
every square foot of a house nearly two pounds. Hurricanes travel at
the rate of seventy-six miles an hour--faster than express trains--and
press on every square foot of houses nearly thirty pounds."
Agnes. And the houses are
often blown down, too.
pg 172
Jack. They aren't built to
resist such winds. We very seldom have them in our part of the world,
I'm thankful to say.
Causes of the Winds.--"Whenever
the surface of the earth is warm," said Jack, "the air over that part
rises and other air
FIG.150. A Map of the
General Winds of the Earth--The arrows show their general direction.
The dark spots mark places
where there is much rain. These winds blow over large regions of the
earth. There are particular winds over smaller regions; but these are,
of course, not shown on the map.
from a colder place flows in to take its place. If you boys build a
bonfire, the air rises and the smoke rises with it. Other air comes in
to take its place, and if your fire was big enough--if a city were
burning--it would create a really
pg 173
FIG. 151. Diagram to
Show How Winds Arise--If any region (D) is
warmer than near-by regions (C,C),
the air over D is warmed and rises. As it rises it cools, and the air
near B,B moves downwards and inwards to take its place. The air over a
bonfire moves in this way, and we have a little local wind. The air
over a large part of the Mississippi valley may move in the same way
for like reasons, and then we have winds covering several states.
I. In January II. In July
FIG 152. Winds of the
Atlantic Ocean--The arrows show which way the winds blow. Charts like
these are made
for every ocean and for each month, and sailing ships go by tracks
where the winds are favorable.
pg 174
strong wind. The sun warms the hot regions of the earth, near the
equator, more than the arctic regions; the hot air rises and the cold
arctic air flows southwards to take its place."
Tom. You have to add that the
earth is turning round all the time and so the winds do not flow
straight to the equator but in spirals.
Jack. The sun is warming the
earth all day--land and water, mountains and valleys; and all night the
heat from the warmed places is rising up.
Land and Sea Breezes.--"The
land gets warm more quickly than the sea, so that all day the breeze
blows from sea to land.
FIG. 153. The Sun (S)
Shining on the Earth Illuminating and Heating the Hemisphere Turned
Towards Him--It is daytime in that hemisphere.
As the earth revolves
on its axis
(NS) every twenty-four hours both hemispheres are lighted and heated in
turn.
At night the land gets cool sooner than the sea, so that all night the
breeze blows from land to sea. The next time you go to the seashore see
if this is not true. Of course there will be other winds, too; but
every hot day you will notice the sea breeze that springs up in the
morning and blows till nightfall."
Weather.--"Weather depends upon
a great many things," said Jack. "See if you children can tell me some
of them."
pg 175
Agnes. Well, we have warm days
and cooler nights because the earth turns round. We are in the sun's
rays in the daytime and out of them at night. (See Fig. 153.)
Mary. And we have cold winters
and warm summers because--I don't believe I quite know why. Is it
because the earth is nearer to the sun in summer?
Jack. No, the earth is a
little nearer to the sun in December and January than it is in June and
July--a little, though not much; there is a different reason, Mary.
(See Fig. 154.)
FIG. 154. The Earth in
its Path Round the Sun--The earth is at A in
December, at B in March, at C in
June, at D in September. NS is the earth's axis, and N is the earth's
north pole (in all four positions). At A (December) the arctic regions
are dark. As the earth turns round on its axis a person at N is not
brought into the light. In the northern hemisphere in March the days
are shorter than the nights. As the earth turns a person anywhere in
the northern hemisphere is in the lighted half of the earth for a
shorter time than in the dark. But in June (C) a person in the northern
hemisphere is longer in the light than in the dark--longer in the
region heated by the sun.
Storms.--"Our weather depends
on the earth's turning on its axis then," said Jack, "and on its motion
round the sun. Those causes are working all the time. Then there are
storms that
pg 176
travel over the whole country from west to east (1) and others that
come
up from the Gulf of Mexico along the Gulf Stream. These storms reach
us, and our weather on Thursday, we may say, depends upon the weather
some one else had on Monday. The Weather Bureau in Washington gets
reports of all the storms in the whole country by telegraph several
times a day and makes up a prediction about the weather we are going to
have. You see the Weather Bureau predictions in the newspaper every
day. (2)
Storm and Other Signals.--"Whenever
you see a red flag with a black center expect a storm. The triangular
pennants tell which way the wind will blow. (See the titles to the
cuts.) A
FIG. 155. UNITED
STATES WEATHER BUREAU STORM SIGNALS
square white flag predicts fair weather; a square blue flag predicts
rain or snow; a flag half white and half blue predicts local rain or
snow storms. A square white flag with a black center indicates that a
cold wave is to arrive. If the black pennant (No. 4) is hoisted above
any flag, it means that the weather is going to be warmer. If it is
hoisted below any flag, it means that the weather is going to be
colder. (3)
(1) See Book II (Physics), page 87.
(2) Ibid.,
page 88.
(3) These flags are displayed in all towns where there
is an observing
station of the United States Weather Bureau, and children who live in
such towns should learn them by heart.
pg 177
FIG. 156. Hurricane
Signal
FIG. 157. Information
Signals--On the Great Lakes a
red pennant denotes easterly, a white pennant
westerly, winds. A red pennant at
seacoast stations indicates a storm.
FIG. 158. Weather
Signals--By a cold wave is meant a fall of temperature of at least 20
degrees in twenty-four hours.
N.B.--In all the foregoing pictures a red flag is marked by vertical
lines, a blue flag by horizontal lines.
pg 178
"In some regions the Weather Bureau signals are given by steam
whistles. A long blast is sounded to attract attention, then follow the
signals for weather, and next those for temperature. The signals for
weather are long blasts; those for temperature are shorter.
"One long blast means 'expect fair weather.'
Two long blasts mean 'expect rain or snow.'
Three long blasts mean 'expect local rains or snows.'
"One short blast means 'expect lower temperature.'
Two short blasts mean 'expect higher temperature.'
Three short blasts mean 'expect a cold wave.'
"You have no idea how useful these weather predictions are nor how many
people read them and follow their indications. Think about it a moment.
Suppose there is a cold wave far up in Winnipeg moving eastward. Often
it makes cold north winds in Texas--a 'norther'--and northers are
destructive to crops and to cattle. The whole of the United States from
the Mississippi River eastward to Maine and southward to Florida is
going to feel it, and every one is warned to get ready. The railway
people are all ready with snowplows; stock raisers herd their cattle
into shelters and provide food for them; people who are shipping fruit,
etc., on trains take warning and wait; orange growers in Florida light
fires to protect their trees; ice companies prepare to get in their
crop of ice; householders see that there is plenty of coal for their
furnaces; firemen take extra precautions about their hydrants. There
are millions of people who are affected in thousands of ways. The
government Weather Bureau warns them all, and every man must look out
for himself and for his business. That is the way a government like
ours should be, I think. It ought to do
pg 179
FIG. 159. Summer
Thunderstorms--A summer thunderstorm usually
occurs in the late
afternoon and usually
moves from west to east. Several hours before the storm you will see a
layer of cirro-stratus cloud (CC) high up, with festoons (ff) below it.
These cirro-stratus clouds may be from ten to fifty miles in advance of
the true storm. The air is hot and oppressive--thunderstorm weather.
When the cloud (cc) covers the sun the air is slightly but noticeably
cooler. About an hour later the "thunderheads" (t) begin to appear on
the western horizon. They are a dull leaden color and threatening.
Sometimes distant thunder is heard. The thunderheads rise higher in the
air (they are coming nearer), and you can see their bases (b) and the
gray curtain of rain (r) below. The thunderheads are about ten times as
high as the curtain of rain. Smaller detached clouds (d) are often seen
in front of the main storm, which advances eastward. Then comes the
"thunder squall" (see the following picture) brushing up the dust in
front of it and bringing a rush of cool air. The arrows show which way
the winds are blowing in the different parts of the storm. The rain
comes in large pelting drops and then in a steady downpour, sometimes
with hail, nearly always with thunder and lightning. In half and hour
or so the rain is over, the storm has passed, blue sky appears, a
rainbow shows in the east, the clouds tower high in the east, and the
air is fresh and cool.
pg 180
the things that no single man can do--like this weather prediction--and
leave every man to take care of his own affairs afterwards."
Lightning.--"The clouds in
storms are electrified," said Jack, "and lightning is electric sparks
on a large scale exchanged between one cloud and another. Thunder is
the crackle of the spark
echoed among the clouds and mountains. Sheet lightning is usually the
reflection of distant forked lightning from the surface of high clouds."
FIG. 160 Thunder
Squalls--A part of the preceding picture (within the space marked d b q
in Fig. 159) is drawn on a larger scale here. The first picture shows
the thunderstorm as it moves across the country at the rate of twenty
to fifty miles an hour. This picture shows the thunder squall as it
reaches any particular place. The arrows indicate how the different
winds are blowing. If the two pictures are carefully studied, and
especially if the reader will compare them with the summer
thunderstorms seen at his own home, they will explain most of the
appearances he sees.
pg 181
Agnes. Thunder is the echo
that we hear, and sheet lightning is a kind of echo that we see.
Tom. How fast does lightning
travel, Jack?
Jack. Exactly as fast as light
does--at the rate of 186,000 miles in a second--so that the duration of
a lightning flash is only a very small fraction of a second. After the
flash comes the thunder. Do you know how to tell how far away a
thunderstorm is?
Distance of a Thunderstorm from the
Observer.--Tom. You
notice the flash of lightning and then count the number of seconds till
you hear the thunder; I know that much, but I forget the rest.
Jack. It's like this. The
lightning flash and the thunder occur in the storm at exactly the same
moment.
FIG. 161. Lightning
Flashes
You are far off from it. You see
the flash the moment it occurs because
light travels so fast; but as sound travels only 1100 feet in a second,
it takes time for the sound of the thunder to reach you. You have to
multiply the number of seconds between the time of the flash and the
time of the thunder by 1100, and you'll have the distance of the storm
in feet.
It takes sounds about five seconds to travel a mile.
pg 182
Lightning Rods.--"Some people
say," said Fred, "that lightning rods aren't of any use. How is it,
Jack?"
Jack. Well, no lightning rods
are so good that you can be certain your house will not be struck. The
government takes the greatest pains to protect its powder magazines,
but once in a while they are struck. Still, a lightning rod really does
protect. It should be a good-sized copper rod that goes deep down into
the ground--far enough to reach moist earth--and it should extend ten
or twelve feet above the roof, and end in a sharp point. Three or four
good rods will protect an ordinary house almost always. It is better to
have them; you are safer.
