Chapter - 22

Weather

Scientific Forecasting

Many factors enter into the problem of making successful weather forecasts from the examination of weather maps. The experienced forecaster, for example, has due regard for the area under consideration, the topography of the land, and the distribution and effect of land and water areas.

Professional forecasters, from long experience, have found however that certain general conclusions may be drawn which are of help in analyzing probable changes in the weather map. Some of the more important ones are summarized in the Weather Bureau's publication, Weather Forecasting, as follows:

1. When there is an area of high pressure over the southeast and a cold wave in the northwest threatens, there will be storm development in the southwest and precipitation will be general.

2. If a storm forms in the southwest and is forced to the left of a normal track, another storm will immediately begin to develop in the southwest and it becomes a sure rain producer. Storms that develop in the southwest and move normally are quickly followed by clearing weather.

3. Troughs of low pressure moving from the west are of two types —the narrow and the wide. The former moves eastward slowly and storm centers develop in the extreme northern and the extreme southern ends. When the trough is wide the development of an extensive storm area is not uncommon, especially if the wide intervening area between the Highs shows relatively high temperatures.

4. When the northern end of a trough moves eastward faster than the southern end the weather conditions in the south and southwest remain unsettled and the chances are that a storm will form southwest of the High that follows. When the southern end moves fasterthan the northern end settled weather follows.

5. Storms that start in the northwest and move southeastward do not gather great intensity until they begin to recurve to the northward. At the time of recurving they move slowly, as a rule, and care must be exercised in predicting clearing weather.

6. Marked changes in temperature in the southeast and northwest quadrants imply an increase in the storm's intensity. Small temperature changes do not indicate a further development of the storm.

7. Abnormally high temperatures northwest of a storm indicate that it will either retrograde or remain stationary.

8. East of the Rocky Mountains a storm which moves to the left of its normal track increases in intensity.

9. Storms with isobars closely crowded on the west and north west generally move slowly and to the east or southeast, and the precipitation and high winds are maintained unusually long in the northern and western quadrants.

10. Storms with the isobars closely crowded in the south and southeast quadrants move rapidly northeastward, and the weather quickly clears after the passage of the storm center.

Systems of communication throughout the country are today so highly developed that, through the medium of newspapers, the telegraph, telephone, radio and even the radiotelephone, yachtsmen are generally able to avail themselves of the scientific weather information and forecasts issued by the United States Weather Bureau, rather than being forced to rely on their own doubtful capacity to predict by means of local observations. (Of course, during a war such systems of weather communications are suspended.)

Local Observations

The local indications which might afford a clue to weather changes rarely give more than twelve hours' warning. In many cases the storm which is strictly local may not give more than even an hour's notice. Coupled with this is the fact that local indications vary greatly all over the country. The yachtsman who aspires to any considerable skill as a weather forecaster might well make a detailed study of the latest findings in the science of meteorology and then supplement this with careful, scientific and systematic analysis of his own observations as they relate to his own particular locality.

The actions of birds and animals, changes in vegetation, planetary movements and similar bases for prognostication have all been largely discredited, and have little foundation in fact. The signs that are worth testing for one's own locality include those which furnish some clue to the state of the atmosphere—for example, the appearance of the sky, the nature and movement of clouds, direction and velocity of the wind, the trend of barometric pressure, etc. Many fine, inexpensive instruments are available today, suitable not only for installation aboard boats, but also excellent for use ashore, which record with great accuracy the basic factors which give the necessary information for a study of weather forecasting.

While the expert meteorologist is steadily increasing the range of his forecast, being able today to make a general forecast for a week in advance, with reasonable accuracy in detailed forecasts up to about 36 or 48 hours, the amateur prediction based on observations of local conditions can hardly be expected to be reliable for more than a few hours in advance.

A fundamental point to remember is the fact that weather changes can always be traced to physical conditions resulting from unequal heating of the earth's atmosphere in various localities. Coupled with this is the fact that such conditions are modified by the nature of the surrounding country. Generally speaking, a continuation of existing weather conditions may be expected unless there is some definite indication of a coming change.

The Barometer

The barometer gives the key to the one most important factor in weather changes—the trend of atmospheric pressure. Taken together with the direction of the wind and its shifts a reasonably accurate forecast can generally be made. On every weather map the following information is given under the heading, Wind-Barometer Indications: When the wind sets in from points between south and southeast and the barometer falls steadily a storm is approaching from the west or northwest, and its center will pass near or north of the observer within 12 to 24 hours with the wind shifting to northwest by way of southwest and west. When the wind sets in from points between east and northeast and the barometer falls steadily, a storm is approaching from the south or southwest, and its center will pass near or to the south or east of the observer within 12 to 24 hours with wind shifting to northwest by way of north. The rapidity of the storm's approach and its intensity will be indicated by the rate and the amount of the fall in the barometer.

This general rule can be regarded as applicable in all parts of the country. The following table, from the Weather Bureau's pamphlet, Weather Forecasting, is also generally applicable throughout the United States, and shows how barometer fluctuations are to be interpreted, depending on the rapidity of the change, whether it is rising or falling, the height at which the mercury stands at the time of observation and the wind direction.

It will be noted that the readings in the table are "reduced to sea level" and of course practically all observations made by yachtsmen will be at sea level. The reason for the necessity of such corrections is the fact that atmospheric pressure is reduced with increases in altitude. Standard works on the care and use of the barometer give tables showing the pressure, in inches, corresponding to changes of 100 feet in elevation. Such a table will show, for example, altitudes from 0 to 7000 feet with pressure changes at intervals of 500 feet for temperatures from 20 degrees below zero to 90 above.

Wind                                                                Character of weather
direction           Barometer reduced                   indicated
                        to sea level                              

SW. to NW.    30.10 to 30.20 and steady        Fair, with slight temperature
                                                                        changes, for 1 to 2 days.

SW. to NW.    30.10 to 30.20 and rising          Fair, followed within 2
                        Rapidly                                     days by rain.

SW. to NW.    30.20 and above & stationary   Continued fair, with no
                                                                        decided temperature change.

SW. to NW.    30.20 and above and falling..    Slowly rising temperature
                        Slowly                                      and fair for 2 days.

S. to SE           30.10 to 30.20 and falling         Rain within 24 hours.
                        slowly

S. to SE           30.10 to 30.20 and falling         Wind increasing in force,
                        rapidly....                                  with rain within 12 to 24
                                                                        hours.

SE, to NE.       30.10 to 30.20 and falling         Rain in 12 to 18 hours.
                        slowly…..

SE. to NE.,      30.10 to 30.20 and falling         Increasing wind, and rain
                        rapidly                                      within 12 hours.

E. to NE           30.10 and above and falling      In summer, with light
                        slowly..                                     winds, rain may not fall for
                                                                        several days. In winter, rain
                                                                        within 24 hours.
E. to NE           30.10 and above and falling      In summer, rain probable
                        rapidly..                                    within 12 to 24 hours. In
                                                                        winter, rain or snow, with
                                                                        increasing winds, will often
                                                                        set in when the barometer
                                                                        begins to fall and the wind
                                                                        sets in from the NE.

SE. to NE        30.00 or below and falling         Rain will continue 1 to 2
                        slowly....                                   days.

SE. to NE        30.00 or below and falling         Rain, with high wind,
                        rapidly....                                  followed, within 36 hours, by
                                                                        clearing, and in winter by
                                                                        colder.

S. to SW          30.00 or below and rising         Clearing within a few
                        slowly                                       hours, and fair for several
                                                                        days.

S. to E              29.80 or below and falling         Severe storm imminent,
                        rapidly....                                  followed, within 24 hours, by
                                                                        clearing, and in winter by
                                                                        colder.

E. to N             29.80 or below and falling         Severe northeast gale and
                        rapidly...                                   heavy precipitation; in
                                                                        winter, heavy snow, followed
                                                                        by a cold wave.

Going to W      29.80 or below and rising,        Clearing and colder.
                        rapidly...
 
The broad rule covering general conditions is stated as follows: Winds from easterly quadrants with falling barometer indicate foul weather to come. Winds shifting to westerly quadrants promise clearing and fair weather. A single observation, that is, one reading of the barometer or one glance at the existing position of a High or a Low on the weather map is never sufficient knowledge on which to base a prediction. While it is generally true that a high or rising barometer commonly forecasts fine weather, and a low or falling barometer is the customary forerunner of storm, there are many occasions when the High is attended by wind and rain, the Low by clear skies.

Storm And Weather Signals

Storm signals are displayed by the Weather Bureau at many stations along the Coast and Great Lakes. (See Chapter XX.)

Coast Guard vessels now display storm warning signals. Headquarters of the Coast Guard are supplied with weather information by the Weather Bureau and Coast Guard vessels receive instructions to fly the proper signals when storms are approaching. The shore stations where storm warning signals are displayed are listed in the various Coast Pilots. The Pilots also contain general information as to the prevailing weather in areas covered by each book.

Force Of Wind

When it is necessary to refer to the force of wind, this is commonly done in terms of numbers from 0 to 12, using the Beaufort Scale. Zero is a Calm (0-1 statute m.p.h.); 1 is Light Air (1-3 m.p.h.) ; 2 is Light Breeze (4-7 m.p.h.); 3 Gentle Breeze (8-12 m.p.h.) ; 4 Moderate Breeze (13-18 m.p.h.); 5 Fresh Breeze (19-24 m.p.h.); 6 Strong Breeze (25-31 m.p.h.); 7 Moderate Gale (32-38 m.p.h.) ; 8 Fresh Gale (39-46 m.p.h.); 9 Strong Gale (47-54 m.p.h.) ; 10 Whole Gale (55-63 m.p.h.); 11 Storm (64-75 m.p.h.); and 12 Hurricane (above 75 m.p.h.). Forces 0, 1 and 2 are referred to by the term Light in Weather Bureau forecasts; 3 Gentle; 4 Moderate; 5 Fresh; 6 and 7 Strong; 8 and 9 Gale; 10 and 11 Whole Gale; and 12 Hurricane.

Yachtsmen will find much interesting information on the various pilot charts of the North Atlantic and North Pacific Oceans issued by the Hydrographic Office of the Navy Department. These amount practically to monthly weather maps of the oceans and in addition give a vast amount of valuable data on subjects closely allied to weather and weather forecasting.

NOTE—The new modified Beaufort wind scale distinguishes between hurricanes of varying intensity, from force 12 (64-71 knots) up to 17 (110-118 knots).

Weather Jingles The Barometer

"When the glass falls low, prepare for a blow;
When it rises high, let all your kites fly."

"When the wind backs and the weather glass falls
Be on your guard against gales and squalls."

Cirrus Clouds

"Mackerel scales and mares' tails Make
tall ships carry short sails."

and

"Trace in the sky the painter's brush, The
winds around you soon will rush."

"Whene'er the clouds a cloth shall weave,
 Look for storms before they leave."

Cumulus Clouds

"Morning towers bring evening showers."

Cumulonimbus Clouds

"When the clouds appear like rocks and towers, The
earth's refreshed by frequent showers."

The Sun

"Above the rest, the Sun who never lies Foretells the
change of weather in the skies."
—Virgil.

"The sun sets weeping in the lowly west, Witnessing
storms to come, woe and unrest."

Color of the Sky

"A red morn that ever yet betokened
Wreck to the seaman, tempest to the field,
Sorrow to shepherds, woe unto the birds,
Gusts and foul faws to herdsmen and to herds."

"Red in the morning, sailors take warning,
But red at night is the sailor's delight."

Wind

"When the wind is in the north
Sailors seldom venture forth;
When the wind is in the east
Sailors venture forth the least.
When the wind is in the west
Then the weather's at its best;
But when the wind is from the south
It blows the rain from the storm king's mouth."

"A veering wind means weather fair
A backing wind, foul weather's near."

Fog

"An early morning fog generally indicates a clear day."

"Mists dispersing on the plain
Scatter away the clouds and rain;
But when they rise to the mountain tops
They'll soon return in copious drops."

Rain

'Long threaten, long last;
Short notice, soon past."

"Rain before seven, clear before eleven."

The Rainbow

"Rainbow to windward, foul fall the day;
Rainbow to leeward, damp runs away."

Dew

"When the dew is on the grass,
Rain will never come to pass."

"When the grass is wet at night,
Look for rain before the light.
When the grass is dry at morning light
Look for rain before the night."

Halos

"A ring around the moon means a storm."

"The larger the ring, the nearer the rain."

The Moon

"The moon and the weather may change together,
But a change of the moon doesn't change the weather.
If we'd no moon at all (and this surely seems strange),
We still should have weather that's subject to change."

Stars

"When the stars begin to huddles,
The earth will soon become a puddle."

Tides

"Storms burst as the tides turn"

and

"Showers are most frequent at the turn of the tide."

Sight and Sound

"The farther the sight, the nearer the rain."

St. Elmo's Fire

"Last night I saw Saint Elmo's stars
With their fiery lanterns all at play,
On the tops of the masts and the tips of the spars,
And I knew we should have foul weather today."

Human Reactions

"Old sinners have all the points
O' the compass in their bones and joints."

"A coming storm your shooting corns presage,
And aches will throb, your rheumatiz will rage.”

"Grumphie smells the weather,
An' Grumphie sees the wun’,
He kens when clouds will gather,
And smoor the blinkin' sun."

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