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WIND AND WEATHER


[Illustration: Logo]

THE MACMILLAN COMPANY
NEW YORK · BOSTON · CHICAGO · DALLAS
ATLANTA · SAN FRANCISCO

MACMILLAN & CO., LIMITED
LONDON · BOMBAY · CALCUTTA
MELBOURNE

THE MACMILLAN CO. OF CANADA, LTD.
TORONTO


[Illustration: HOW THE WIND RUFFLES THE TOP OF A FOG BANK

_Frontispiece_]




WIND AND WEATHER

BY ALEXANDER McADIE

A. Lawrence Rotch Professor of Meteorology, Harvard
University and Director of the Blue Hill Observatory

New York
THE MACMILLAN COMPANY
1922

_All rights reserved_


Copyright, 1922,
By ALEXANDER McADIE.

Set up and electrotyped. Published November, 1922.




LIST OF ILLUSTRATIONS


        HOW THE WIND RUFFLES THE TOP
        OF A FOG BANK                  _Frontispiece_

                                                 PAGE
FIG. 1. THE TOWER OF THE WINDS                     13

 "   2. BOREAS--THE NORTH WIND                     19

 "   3. KAIKIAS--THE NORTHEAST WIND                23

 "   4. APHELIOTES--THE EAST WIND                  29

 "   5. EUROS--THE SOUTHEAST WIND                  33

 "   6. NOTOS--THE SOUTH WIND                      37

 "   7. LIPS--THE SOUTHWEST WIND                   41

 "   8. ALL STORMS LEAD TO NEW ENGLAND             45

 "   9. ZEPHYROS--THE WEST WIND                    49

 "  10. PATHS OF HIGH AND LOW, JANUARY, 1922       55

 "  11. SKIRON--THE NORTHWEST WIND                 59

 "  12. THE IDEALIZED STORM                        63

 "  13. TURNING OF WIND WITH ALTITUDE              67

 "  14. VELOCITY OF SUMMER AND WINTER WINDS        73

 "  15. BLUE HILL OBSERVATORY IN AN ICE STORM      79




WIND AND WEATHER




THE TOWER OF THE WINDS


In Athens on the north side and near the base of the hill on which the
upper city--the Acropolis--is built, there is a small temple still
standing, altho its walls were completed twenty-two centuries ago. It is
known as the Tower of the Winds; but as a matter of fact, the citizens
of Athens used it to tell the hour of the day and the seasonal position
of the sun. It was a public timepiece. It served as a huge sun dial.
Water from a spring on the hillside filled the basins of a water clock
in the basement of the Tower. And so, whether the day was clear or
cloudy the measure of the outflow of water indicated the time elapsed.
Also there were markings or dials on each of the eight walls of the
temple, and the position of the shadow of a marker indicated the
seasonal advance or retreat of the sun as it moved north from the time
of the winter solstice and then south after the summer solstice.

The sun is not an accurate time keeper and no one to-day runs his
business or keeps engagements on sun time. But the old Athenians were
quite content to do so; and their Tower served excellently for their
needs. And they did what we moderns fail to do, namely, give distinctive
names to the winds. They represented figuratively the characteristics of
the weather as the wind blew from each of the eight cardinal directions.

[Illustration: FIG. 1. THE TOWER OF THE WINDS

Erected in Athens, on the north side of the Acropolis, B. C. 150]

The allegorical figures of the winds used in this little book are
reproductions of the eight bas-reliefs in the library of the Blue Hill
Observatory, placed there by the late Professor A. Lawrence Rotch. They
are copied from the frieze of the Tower of the Winds at Athens.




THE NAMES OF THE WINDS


Boreas, the north wind, is perhaps the most important of all winds. At
Athens this a cold, boisterous wind from the mountains of Thrace. The
noise of the gusts is so loud that the Greek sculptor symbolized the
tumult by placing a conch shell in the mouth of Boreas. His modern
namesake, the Bora of the Adriatic, is the same noisy, blustering, cold
wind-rush from the north.

The northeast wind Kaikias is a trifle more pleasant looking than
Boreas, but still not much to brag about. Master of the squall and
thunderstorm, he carries in his shield an ample supply of hailstones,
ready to spill them on defenseless humanity. He might well serve as the
patron saint of air raiders dropping their bombs on helpless humans
below.

Apheliotes, the east wind, is a graceful youth, with arms full of
flowers, fruit and wheat. Naturally this was a favorite wind, blowing in
from the sea, with frequent light showers. Some of us who dwell on the
Atlantic Coast, in more northern latitudes than Athens, do not always
regard with favor the east wind, associating it with chilly, damp and
sombre weather. Yet it is the harbinger of good--tempering the cold of
winter and the heat of summer. It is an angel of mercy in mid-summer
when the temperature is above the nineties and there is no air stirring.
Then it is, that we all welcome the refreshing wind from the sea.

Euros, the southeast wind, and neighbor to Apheliotes, is a cross old
fellow, intent on the business of cloud making. He alone of all the
winds carries nothing in his hands. In the New Testament he becomes
Euroclydon, wind of the waves. He is no friend of the sailor; and the
seasick traveler prays to be rid of his company.

The figure on the south face of the tower, Notos, is the master of the
warm rain. He carries with him a water jar which has just been emptied.
Compare his light flowing robes and half-clad neck and arms with the
close fitting jacket of old Boreas. At his shrine, hydraulic engineers
well might worship.

Next, the Mariner's wind, Lips, the southwest favoring breeze bringing
the ships speedily into harbor; yes, into that Piraeus, famed in classic
history. Incidentally it is the southwest wind which differentiates the
climate of Great Britain from that of Labrador. This wind makes
Northwest Europe habitable; while on the other side of the Atlantic, in
similar latitudes, but under the influence of prevailing northwest
winds, we find Labrador--a section certainly misnamed, for it is not the
abode of farmers, as the name implies--but barren and bleak. What a
difference it would make thruout this region if the Gulf Stream
continued north, close to the shore, and the prevailing winds were _from
the east_. Our North Atlantic Coast would then be _the land of zephyrs_,
using the word in the sense of pleasant, gentle winds.

[Illustration: FIG. 2. BOREAS--THE NORTH WIND]

Zephyros, the west wind, is represented as a graceful youth, scantily
clad, with his arms filled with flowers. In Greece this wind traversed
the Ionian Sea and the Gulf of Corinth before reaching Athens. It is
quite unlike our west wind which blows across a continent, and is
continuously robbed of its water vapor on the long passage. The Ionian
wind is pleasantly moist and refreshing.

Last of all, but by no means least important, is Skiron, lord of gusty
northwest gales. Freezing in winter, parching in summer, he carries with
him a brazen fire basket and spills a generous stream of hot air on all
below. His husky Highness might not inappropriately adorn legislative
halls and editorial sanctums. He would displace the blindfolded lady
holding scales very much out of balance. Think of the deep significance
of his presence.

In our country the northwest is of all winds, except the west, most
persistent. For 1600 hours in a year, this wind is with us. Joining
forces with the west wind, these directions prevail one third of the
time. These northwest-west winds also have the greatest speed and
gustiness. The climate of the United States is essentially determined by
the prevalence of the north, northwest and west winds.




FORECASTING THE WEATHER


In old days, the _haruspices_ (for this is what the Romans called
weather men in the days of Caesar) proclaimed the will of the gods by
consulting the entrails of some freshly killed animal. Evidently these
haruspices did not always make correct forecasts; for there were some
Romans who openly questioned their worth. Cato, the Censor, is on record
as saying "that he wondered how one haruspex could look another in the
face without laughing!"

[Illustration: FIG. 3. KAIKIAS--THE NORTHEAST WIND]

The modern professional forecaster would scorn to consult the entrails.
There are however many amateur forecasters who foretell weather by their
aches and rheumatic pains. Probably there is a high correlation factor
between body sensations and dampness; and some individuals are quite
sensitive to changes in both relative and absolute humidity. This,
however, does not always mean that a storm is approaching. Humidity or
dampness is only one factor and may be quite local, whereas most storms
are wide-spread.




THE WEATHER MAP


The official forecaster consults a daily weather map and certain
auxiliary maps which show changes in pressure and temperature for twelve
hours or more. He examines closely the contours of pressure as shown on
the map. The synoptic map, as it is called, because it is a glance at
weather conditions over a large area at one and the same moment, is a
map on which are plotted pressure, temperature, wind direction, velocity
and rainfall. The lines of equal pressure or isobars generally curve and
inclose what is known as a cyclonic centre, or depression or LOW. The
arrows point in, but not exactly toward the centre of the depression.

On the map there will probably appear also an area of high pressure
where the surface air flows leisurely outward and away from the place of
highest pressure. Such an area is called an anticyclone, a word first
used by Sir Francis Galton in 1863 to designate not only high pressure,
but general flow of the air in a reversed or opposite direction to that
of the low area or cyclone. The word cyclone was first used by
Piddington in 1843 in describing the flow of the air in the typhoons of
the East Indian Seas. It is from the Greek and literally means the coils
of a serpent. The word cyclone must possess some special merit in the
minds of journalists for it is quite commonly misused for tornado in
descriptions of the smaller and more destructive storm.




THE LOW


Cyclone is simply the generic name for a large rotating air mass. It is
a barometric depression or LOW and is characterized by a flow of air
inward and around a moving centre. The air circulation is
counter-clockwise in the northern hemisphere and clockwise in the south.

Perhaps if the earth stopped rotating and there was no planetary
circulation, with the great west-moving trades and east-moving
"westerlies," the arrows on the weather map would all point directly
toward the centre of the LOW; but, as things are, there are some very
good reasons why air can not move directly into a LOW, that is at right
angles to the isobars.

Moreover, the weather map does not indicate the true flow of the air,
for observations of the wind made at the ground tell only a part of the
story of the balance which the flowing air must maintain under the
action of various forces, such as gravitation, rotational deflection,
centrifugal tendency, and the various expansion and compression forces.

The winds near the ground are modified both in velocity and direction by
friction. The free flow is often interfered with by topography.




THE TRUE AIR FLOW


One must rise above the ground some distance to get the true air flow,
or what is known as the gradient wind, the flow which balances the
gradient, i.e. a flow along the isobars. The gradient velocity is found
about 300 metres above the ground, and the gradient direction a little
higher. The lower clouds as a rule indicate true wind values very well;
and so, it is desirable in studying winds to use cloud directions and
velocities rather than surface values. In cloud work a nephoscope is
essential. The unaided eye, unless properly shielded, suffers from the
glare of a sunlit sky; and moreover, there are no fixed points or
references. A black mirror, with suitable sighting rods and measuring
devices, enables an observer to follow the cloud, estimate its height
and determine with accuracy the direction from which it is moving. There
is an average difference of 30 degrees between the cloud direction and
the surface wind; the upper direction being more to the right. At times
the directions may be opposite.

[Illustration: FIG. 4. APHELIOTES--THE EAST WIND]

It may seem surprising but few of us, except at sunrise and sunset,
really see what is going on in cloud land.

Some meteorologists hold that the circulation of air 3000 to 5000
metres above the ground controls the path and perhaps the intensity of
storms. It is therefore important to know something of the flow at high
levels if we would improve the forecasts.




LIMITATIONS OF MAP


The weather map fails to indicate what shifts of direction and changes
in velocity are likely to occur. The forecaster tries to anticipate
these, but he bases his conclusions chiefly upon an expected movement of
the low area; using the accumulated records of the paths of past storms.
But each storm is in reality a law unto itself; and while we know
something of the relations between pressure and flow of the air; as yet
we know very little about the relations of wind and weather. The problem
is complicated by the behavior of the load of water vapor.

[Illustration: FIG. 5. EUROS--THE SOUTHEAST WIND]

The Chief Forecaster of one of the great national weather services
recently wrote:


     "Despite the fact that maps have now been drawn day by day for over
     half a century, we may safely say that no two maps have been
     identical."


It is perhaps unfortunate that so much attention has been given to the
cyclone or depression or LOW, and comparatively little to the HIGH or
anticyclone. For we are now beginning to understand that while there may
seem at first to be nothing specially noteworthy about a mass of air
where the pressure varies from 1020 to 1040 kilobars, that is, 2 to 4
per cent _above_ a standard atmosphere, with isobars irregularly curved
and feeble surface winds, yet the anticyclone is more important than the
cyclone in determining weather sequence; for the progressive motion of
the cyclone depends largely upon the strength of the anticyclone.




OCEAN STORMS


Sir Napier Shaw, who has written much on the weather of the British
Isles, may be quoted here.


     "Anyone who is interested in the weather is always on the lookout
     for 'lows' and is very keen to know whether he is going to be on
     the south of the centre or the north of it. He is, of course,
     interested in the anticyclone too, because as long as an
     anticyclone is there, there cannot be a depression; but it is the
     depression which has the life and movement about it, giving it a
     claim to the attention of everybody who wants to know what the
     weather and its changes are going to be.

     "This has been recognized from the very earliest days of weather
     maps with isobars. The depressions which pass over our shores
     (Great Britain) mostly come from the west. Some of them come all
     the way from America; one or two have been traced from the west
     coast of Africa and so have crossed the Atlantic twice, first to
     the westward and then to the eastward. Some have come all the way
     from a sort of parent 'low' in the North Pacific Ocean. So general
     is the tendency for 'lows' to go eastward that it was thought at
     one time, particularly by the 'New York Herald,' that their
     departure from the American Coast and subsequent arrival on our own
     shores could be notified by cable, and we (the British) might thus
     be forewarned of their approach, some three or four days in
     advance. The attempt was made by the 'New York Herald' acting in
     co-operation with the Meteorological Offices of the United Kingdom
     and France. But a depression keeps to no beaten track; it has as
     many paths for its centre as there are lines in a bundle of hay.
     Though groups can be picked out there are many strays, and,
     moreover, the depression changes its shape and intensity while it
     travels, so that if you lose sight of it for a day you cannot be at
     all sure of its identity."


[Illustration: FIG. 6. NOTOS--THE SOUTH WIND]




TRANSCONTINENTAL STORMS


If there is so much uncertainty in forecasting the path of a disturbance
at sea, how much more uncertain must it be on land? Elaborate statistics
of the average daily movement of various types of storms have been
officially published. The average speed of storms (not wind speeds)
across the United States is 11 metres per second or 25 miles an hour.
Storms travel more rapidly in winter than in summer, about half again as
fast; that is, summer storms travel 20 miles, and winter storms 30
miles, an hour.

[Illustration: FIG. 7. LIPS--THE SOUTHWEST WIND]

The paths vary widely; from the Gulf storms moving northeast and West
Indian hurricanes recurving on the southern coast, to the storms from
Alberta and the west which move south and east. Ten types of storms,
classified according to the place of origin, are recognized by the
official forecasters of the United States. These are North Pacific,
Alberta, Northern Rocky Mountain, Colorado, Central, South Pacific,
Texas, East Gulf, South Atlantic and West Indian Hurricanes. A better
nomenclature would be (1) Alberta, (2) Washington, (3) Kootenay, (4)
Utah, (5) Kansas, (6) California, (7) Texas, (8) Louisiana, (9)
Florida, and (10) Hurricanes.




HURRICANES


Type 10 is the general class of tropical storms occurring chiefly in the
summer and fall which, drifting west, slowly work northward. Similar
storms are the typhoons and baguios of the East Indian and China Seas.

The path and point of recurvature will be determined by the position of
the Bermuda Hyperbar, that is, the seasonal anticyclone of the Atlantic.
This accounts for the swinging east and north of these tracks as the
season progresses; for the hyperbar is slowly displaced east, the
maximum displacement occurring in September.

[Illustration: BASE MAP BY GOODE

FIG. 8. ALL STORMS LEAD TO NEW ENGLAND]

Individual anticyclones also influence individual hurricanes. Thus a
hurricane passing west over Havana, will go farther west if a vigorous
"high" is spreading southeast over the Gulf States. And when this "high"
passes seaward, the hurricane will work around the southwest quadrant of
the "high," recurving and moving northeast.




STORM RENDEZVOUS


Altho storms originate or are first detected in nine different sections,
it is a fact worth mentioning that they all leave the United States in
the vicinity of New England or Nova Scotia. Some of the southern
depressions starting near the coast, pass to sea south of New York, but
in general an observer standing on Plymouth Rock can virtually encompass
within a radius of 500 kilometres, 300 miles, the paths of ninety per
cent of the storms that traverse the country.

Thus a storm that originates in Texas (7) will probably pass close to
Cape Cod. Likewise, types (3) and (5); while the other types may pass a
little to the north or south. See Chart, Paths of Storms.




STORM PATHS


Forecasting then would seem to be very easy; for one would only have to
know the place of origin of the storm and the rate of travel, to
foretell exactly the time of arrival. Unfortunately these are only the
average paths; and as with most mean values, represent a value not often
experienced in fact. These paths then are not paths which any given
storm will follow. One must recall the story of the operating surgeon
who gave the average age of his patients in the operating room as 35.
There were but two patients, one 69 years old and the other 1 year old.

[Illustration: FIG. 9. ZEPHYROS--THE WEST WIND]

As a matter of fact the path of any individual depression depends upon
several factors, some of which are:--the prevailing eastward drift of
the air; the extent and motion of some anticyclone advancing before the
"LOW"; the duration and speed of relatively dry cold tongues of air from
the north; and the supply of water vapor brought from southern waters by
south winds. A depression can make little headway if to the north or
east the normal path is blocked by what is known as a stagnant "HIGH."
So therefore, if the anticyclone is a slow mover, a Texas storm, which
would normally pass not far from southern New England, may be deflected
farther north than when the HIGH moved rapidly east. So too, with the
storms which originate in the western part of the country. A slow moving
HIGH will prevent the LOW following it, from moving east at a normal
rate along the usual path.

Anticyclones then, are the real weather controls. There are various
types, but all drift from the north or west. Occasionally they enter the
country from the Pacific, but the great majority come from Alberta and
move leisurely southeast, often reaching the South Atlantic States; but
more frequently recurving and passing to the north.




STAGNANT HIGHS


HIGHS are sometimes reinforced and this results in what is called a
stagnant HIGH. A good illustration of such a slow moving HIGH and its
consequences occurred during the last week of January, 1922.

A surge of cold air from Alberta or farther north reached the
international boundary January 21st and spread slowly eastward, reaching
the Great Lakes on the 24th and the St. Lawrence Valley two days later.
Then seemingly it halted or moved slowly westward, retrograding. In
three days, that is, on the 29th, the centre of the HIGH was apparently
500 miles _west_ of where it had been on the 27th. After the 29th it
followed a normal track, moving slowly southeast, reaching the Atlantic
near Long Island.

Meanwhile a depression on the south coast of Texas on the 25th, moved
across the Gulf of Mexico, passing over Southern Florida on the 27th and
advanced steadily northeast, reaching Cape Hatteras in 24 hours. Owing
to the presence of the anticyclone referred to above, the depression
recurved off Hatteras. The result was a memorable snow storm in Northern
Virginia and Maryland. At 8 p.m. January 27th, there had been a fall of
5 cms. (2 inches). Within the following twenty hours the average depth
in the city of Washington was 66 cms. (26 inches). The weight of the
snow caused the collapse of the roof of the Knickerbocker Theatre and
the death of 97 persons.

The total snowfall in various coast cities was:


     Raleigh         24 cms.*
     Richmond        48  "
     Washington      71  "
     Baltimore       67  "
     Wilmington      46  "
     Philadelphia    31  "
     Trenton         27  "
     New York        18  "
     New Haven        8  "
     Boston           1  "

     *Note: To convert to inches multiply by 0.4.


The table shows clearly how the snow was formed. On the east side of the
LOW a stream of air, relatively warm, carried a load of water vapor,
approximately 13 grams in each cubic metre.

[Illustration: BASE MAP BY GOODE

FIG. 10. PATHS OF HIGH AND LOW, GREAT SNOW STORM OF JANUARY 27-28, 1922]

This current was steered around the north side of the LOW and met the
north-northeast wind. Under the new conditions the air saturated could
hold only 2 or 3 grams; and so condensation and heavy precipitation
resulted. The region of maximum snowfall was near Washington, and it
will be seen that there is a proportional decrease north and south. The
snowfall at Washington was the heaviest ever known at that city.

Unlike most storms, there was no strong cold northwest wind blowing into
the depression. The temperature rose slowly. It was less a contrast of
winds than a steady slow outward push of the anticyclone, and the
consequent turning of the path of the cyclone eastward.




LAWS OF FORECASTING


Buys Ballot's Law.

"If you stand with your back to the wind the pressure decreases toward
your left, and increases toward your right."

For navigators, this law is more generally expressed in the words of the
Hydrographic Office on "Cyclonic Storms."

"Since the wind circulates counter-clockwise in the northern hemisphere,
the rule in that hemisphere is to face the wind, and the storm centre
will be at the right hand. If the wind traveled in exact circles, the
centre would be eight points (90 degrees) to the right when looking
directly in the wind's eye. But the wind follows a more or less spiral
path inward which brings the centre from eight to twelve points (90 to
135 degrees), to the right of the wind. The centre will bear more nearly
eight points from the direction of the lower clouds than from the
surface wind."

[Illustration: FIG. 11. SKIRON--THE NORTHWEST WIND]

The law given on the preceding page is named after C. H. D. Buys
Ballott, a Dutch meteorologist. It was announced in a paper published in
the _Comptes rendus_ in 1857. Two American writers on the Winds, J. H.
Coffin and William Ferrell, had however earlier found the law to hold.

       *       *       *       *       *

While most of us study storms from a window at home and are not called
upon to handle a ship in a storm, yet it may not be out of place to
include here the diagram of the winds in an ideal storm and give the
rules for maneuvering. See Figure 12. The Winds in an Idealized Storm.
The rules apply only to storms in the northern hemisphere.

"_Right or dangerous semicircle_,--Steamers: Bring the wind on the
starboard bow, make as much way as possible, and if obliged to heave-to,
do so head to sea. Sailing vessels: Keep close-hauled on the starboard
tack, make as much way as possible, and if obliged to heave-to, do so on
the starboard tack.

_Left or navigable semicircle_,--Steam and sailing vessels: Bring the
wind on the starboard quarter, note the course and hold it. If obliged
to heave-to, steamers may do so stern to sea; sailing vessels on the
port tack.

_On the storm track in front of center_,--Steam and sailing vessels:
Bring the wind two points on the starboard quarter, note the course and
hold it, and run for the left semicircle, and when in that semicircle
manoeuvre as above.

On the storm track, in rear of center,--Avoid the center by the best
practicable route, having due regard to the tendency of cyclones to
recurve to the southward and eastward."

[Illustration: FROM HYDROGRAPHIC OFFICE

FIG. 12. THE WINDS IN AN IDEALIZED STORM]




WIND AND ALTITUDE


The law of the turning of the wind with altitude.

A casual observation of the lower clouds where no means of measuring
small angles is available will not usually show any difference between
the motion of the clouds and the surface wind; but with the upper clouds
the case is different, and one readily detects a difference.

Several thousand observations with various agencies, such as kites and
pilot balloons and more especially measurements made with theodolites
and nephoscopes, show that there is a definite twist to the right with
elevation. The amount of the deflection is shown in Figure 13. Turning
of the Wind with Altitude. Here the average yearly values are given for
directions and velocities. Thus if the mean wind direction at Blue Hill
is from a point a little to the north of west, 306 grads or 275 degrees,
and the mean velocity 7 metres per second; the clouds at 1000 metres
elevation will move from 312 or 280 degrees and at a speed of
approximately 11 metres per second (24 miles an hour).

These however, are average values. In individual cases the difference
between surface winds and stratus clouds may be considerably greater. It
may be as much as 180 degrees; that is, the cloud may move directly
opposite to the wind. In general there will be a difference of 10 to 20
degrees.




WIND AND RAIN


The law of wind direction, approximate cooling and rain.

When the lower clouds are moving from the north or northwest, without
sharply defined edges, the LOW is east or northeast of the observer; and
rain or snow is not likely unless there is a rapidly falling
temperature.

[Illustration: TURNING OF WIND WITH ALTITUDE, BLUE HILL

FIG. 13. TURNING OF WIND WITH ALTITUDE]

When a stream of warm air with a high absolute humidity flows north on
the east side of a LOW, and a cold northwest wind follows quickly after
the LOW, rain or snow may be expected.

Any rapid chilling of warm, moist air produces cloudiness and rain or
snow; but a cold stream blowing into a warm area will not produce as
much rain as a warm stream blowing into a cold area.




DURATION OF WIND


The average duration of wind from various directions is as follows:

From the north about 16 hours each week; from the northeast, the same;
from the east, 11 hours; from the southeast, 10 hours; from the south,
24 hours; from the southwest, 27 hours; from the west, 33 hours; and
from the northwest 31 hours.

During an individual disturbance lasting about 36 hours, we may have 8
hours of southwest wind; 4 hours of west wind, backing during the next 4
hours to south; 2 hours of south wind; 2 hours of southeast wind; 2
hours of east wind; 8 hours northeast wind and 4 hours north wind, 2
hours northwest, when it may be considered that a new pressure
distribution prevails.

The above values hold only for a storm moving with normal velocity. LOWS
are often blocked by slow moving HIGHS in advance. In such cases the
duration of east winds is greater.




THE WINDS OF A YEAR


The following table shows the marked increase in the prevalence of
northwest and west winds during winter months, the decrease in north
winds during July, the increase in northeast winds in May, also in east
winds; the increase of south and southwest winds in July; and the
falling off of southeast winds in December. See Table, page 72.

In cities near the Atlantic Coast, a continuance of northeast wind,
especially in the fall and winter months, results in frequent altho not
necessarily heavy rains. On the other hand a period of continued
northwest and west wind is a dry period.

In summer, southeast and east winds bring fog and cooler weather; while
southwest winds are favorable for the development of thunderstorms.


WINDS OF A YEAR

TABLE I.--Number of Hours the Wind Blows from Different Directions.

    --------------------------------------------------------------------
                    Jan.    Mar.    May     July    Sept.   Nov.    Year
                        Feb.    Apr.    June    Aug.    Oct.    Dec.

    Boreas     (N)   98  74  71  70  60  40  59  59  67  80  82  96  850
    Kaikias   (NE)   41  46  65  94 101  55  79  79  77  91  48  30  819
    Apheliotes (E)   34  37  52  58  63  48  51  51  52  58  34  31  576
    Euros     (SE)   37  37  45  41  54  45  62  62  52  45  39  34  534
    Notus      (S)   82  66  95  99 143 155 128 128 118  93  81  65 1245
    Lips      (SW)  112  77  81  79 118 170 135 135 133 108 119 131 1402
    Zephyros   (W)  180 177 155 125 107 137 125 125 108 131 169 194 1732
    Skiron    (NW)  160 162 183 154  98  94 105 105 113 138 148 163 1607
    --------------------------------------------------------------------


[Illustration: FIG. 14. VELOCITY OF SUMMER AND WINTER WINDS IN METRES
PER SECOND]




THE SEA BREEZE


When the weather has been clear and moderately warm for two or more
days, and the winds are light and variable, there may occur on the third
day a moderate wind from the east, known as the sea-breeze. This occurs
during anticyclonic conditions. Preceding the sea-breeze, the winds are
very light, there are no clouds, and the temperature rises rapidly
during the forenoon. This heating is due to a slow dynamic compression
as the air slowly descends and the surface air does not flow away. There
is no cooling because there is no evaporation due to air movement. The
absolute humidity is low, often less than ten grams per cubic metre.
Cumulus clouds do not form because there is no uplift of the lower air
and consequently no chance for condensation of whatever water vapor may
be present. No thunder-heads form notwithstanding the heat. The heat,
while dry, is nevertheless extremely trying to men and animals. Relief
comes in the early hours of the afternoon by the arrival of the
sea-breeze.

The usual explanation of the origin of the sea-breeze is that the land
being excessively warm, the air over a relatively cool ocean moves in to
take the place of the warm and therefore lighter air, which it is
assumed has risen. Unfortunately for this explanation, the air over the
land has _not_ risen; but on the contrary is falling slowly. Again the
sea-breeze does not begin at the place where the temperature contrast is
greatest, namely, just inside the shore line; but comes in from the sea.
Nor does the flow extend far inland, which would be the case if there
were up-rising currents. The sea-breeze is very shallow, generally not
extending upward more than 200 metres, and often not above 100 metres.
It does not penetrate far inland, as a rule not more than 15
kilometres, 9 miles.

The sea-breeze is probably caused by a slow descent of dry, warm air, on
an incline sloping from northeast to southwest. As it reaches the
surface it is twisted more to the right; that is, becomes an east wind.
It carries inland with it some of the air over the ocean which is much
cooler and heavily saturated.




MUGGY DAYS


There are certain days, more noticeable in summer than at other times,
when the air is heavily laden with water vapor; and there is little or
no cooling of the body due to evaporation. We perspire freely but as the
sweat does not evaporate, there is a constantly increasing amount of
water on the skin.

[Illustration: FIG. 15. BLUE HILL OBSERVATORY DURING ICE STORM, NOVEMBER
29-30, 1922]

It is not altogether a question of temperature, for another day may have
as high or even higher temperature. It is essentially a matter of
ventilation. On muggy days we are somewhat in the condition of the
unfortunate prisoners in the Black Hole at Calcutta. They did not die by
poisoning, as has generally been accepted, that is, lack of sufficient
oxygen and an excess of carbon dioxide; but because they were unable to
keep the skin sufficiently cool. There was no ventilation; no movement
of the air and the body became over-heated and exhaustion followed. No
matter how much water there may be on the skin if the surrounding space
is saturated, one feels oppressed. A vigorous fanning of the air helps
evaporation and cools us. That is why a brisk northwest wind routs a
muggy condition.




CASTILIAN DAYS


John Hay wrote of such days spent in Spain. We who live in a land where
the winds are more boisterous, occasionally experience what we call a
perfect day. Such days have easterly winds of two metres per second or
less than five miles an hour. The temperature is midway between freezing
and normal body temperature or about 70° F. The relative humidity is
approximately 75% and the absolute humidity 12 grams per cubic metre.
The table on page 72 explains the paucity of perfect days. The gusty,
boisterous winds, Skiron and Zephyros, blow too frequently.

Perhaps certain of our national characteristics may be traceable to
this flow of the air and our climatic environment.