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Vejrmodeller
Mange af DMI's produkter og serviceydelser er idag afhængige af et numerisk vejrforudsigelsessystem af høj kvalitet. Et
sådant system er omfattende og kræver betydelige EDB-resurser. Systemet består, nævnt i naturlig rækkefølge, af følgende
komponenter: præprocessering (forbehandling af data), analyse, initialisering, prognose og postprocessering
(efterbehandling af data).
Præprocesseringen
Præprocesseringen dekoder bl.a. de mange forskellige typer af kodede vejrobservationer, der fra hele jordkloden ankommer
til DMI via det globale telekommunikationssystem (GTS). Observationerne ankommer i klumper efter
observationsterminerne hver tredje time døgnet rundt. En automatisk kvalitetskontrol frasorterer åbenlyst fejlagtige
observationer, mens dekodningen sørger for at bringe observationerne på en sådan form, at de kan benyttes af analysesystemet og den efterfølgende numeriske vejrforudsigelsesmodel, herefter omtalt som vejrmodellen.
Analyse
DMI foretager idag analyser af atmosfærens tilstand hver tredje time. Antallet af observationer er størst ved
hovedterminerne 00, 06, 12 og 18 UTC. Som et første gæt på atmosfærens tilstand til analysetidspunktet benyttes en
numerisk vejrprognose gældende til analysetidspunktet. Under analysen sammenlignes observationerne med de tilsvarende
første gæt værdier. Observationer kan undertiden være behæftet med store fejl. For at undgå at sådanne fejlagtige
observationer påvirker analysen forkastes observationer, der afviger for meget fra første gæt værdierne.
Den færdige analyse er således en korrigeret prognose, der i størst mulig omfang stemmer overens med de (normalt
mange) observationer, som systemet har accepteret. Processen omtales som data-assimilering, og gennem denne procedure
skabes en kontinuert udnyttelse af tilgængelige data. Analysen er det bedste bud på atmosfærens tilstand til
analysetidspunktet og er samtidig i princippet begyndelsestilstanden for vejrmodellen, der er "krumtappen" i hele
vejrforudsigelsessystemet. Vejrforudsigelse af høj kvalitet forudsætter således ikke blot en god vejrmodel men tillige
nøjagtige observationer med en tilstrækkelig tæthed i tid og rum.
Initialisering
I det nuværende analysesystem er der ingen garanti for, at de resulterende masse- og vindfelter er i fuldstændig balance.
Det betyder, at der i begyndelsen af prognoseberegningerne kan optræde urealistiske bølgevægelser (d.v.s. bølger af
samme natur som ringe på en vandoverflade), som bl.a. viser sig ved betydelige kortperiodiske svingninger i lufttrykket ved
jordoverfladen. For at komme disse svingninger til livs underkastes analysen en såkaldt initialisering, der er en matematisk
metode til at eliminere de uønskede bølger. Det er derfor den initialiserede analyse, som er begyndelsestilstanden i
vejrmodellen.
Vejrmodel
Beregningerne i vejrmodellen er i princippet numeriske løsninger af bevægelsesligningerne, den termodynamiske
energiligning og kontinuitetsligningen for atmosfæren. Disse ligninger er i rækkefølge matematiske formuleringer af
bevarelseslovene for impuls, energi og masse.
En prognose består i princippet af et antal på hinanden følgende numeriske løsninger af atmosfæreligningerne. I
beregningerne indgår et tidsskridt, hvis længde er tidsforskellen mellem gyldighedstidspunktet for en numerisk løsning og
den næstfølgende.
For finmaskede modeller er tidsskridtet typisk på 2 minutter. Det betyder, at en 48 timers prognose i virkeligheden består
af 1440 på hinanden følgende numeriske løsninger at atmosfæreligningerne. Beregningerne foretages i hvert af modellens
gitterpunkter, hvis antal typisk er omkring 2,5 millioner. I løbet af en 48 timers prognose skal atmosfærens tilstand derfor
beregnes ca. 3,6 milliarder gange. Oven i kommer både et stort antal fysikberegninger og et stor antal beregninger af
jordlagenes og jordoverflades tilstand. De førstnævnte beregninger modificerer atmosfærens tilstand i gitterpunkterne som
følge af fysiske processer (f.eks. turbulens og skydannelse) som for en stor del foregår på en rumlig skala, som er mindre
end modelgitterets.
Det samlede antal beregninger for en 48 timers prognose når derfor "astronomiske" højder i størrelsesorden 10 trilliarder
(10 x 1012)
Postprocesseringen
Postprocesseringen er en efterbehandling af de "rå" data i modellens beregningspunkter. Der beregnes nye data i hvert
eneste tidsskridt, men p.g.a. begrænset lagerkapacitet gemmes der dog indtil videre kun data for hver tredje time.
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Under postprocesseringen bliver dataene bl.a. interpoleret til standard trykflader. Der beregnes også et mean sea level
(m.s.l.) lufttryk, som i princippet er det lufttryk, man vil forvente ved havniveau, hvis man erstattede landmassen over
havniveau med luft. Den grafiske præsentation i DMI's Vejrtjeneste af vejrmodellens prognoseberegninger er baseret på de
postprocesserede data.
DMI-HIRLAM
DMI's nuværende vejrmodel, DMI-HIRLAM anvendes i tre versioner som alle har 40 lag i atmosfæren. Herudover anvendes
en model af landoverfladerne som bruges til beregningen af jordoverfladens temperatur, fugtighed, vandindhold og
snedybde. Modelatmosfæren påvirker ikke oceantemperaturen, og modellen har derfor ikke tilsvarende lag i toppen af
oceanet. Det betyder bl.a. at havoverfladens temperaturmønster holdes konstant under hele prognosekørslen.
Kobling til en ydre model
DMI's vejrmodel er koblet til den globale atmosfæremodel, som er i operationel drift ved det europæiske meteorologicenter
ECMWF, som Danmark er medlem af. ECMWF-modellen anses for at være den mest avancerede globale atmosfæremodel i
verden. I princippet består koblingen i, at atmosfærens tilstand i en zone langs DMI-vejrmodellens rand (fire lodrette flader)
bliver sat lig med den beregnede atmosfæretilstand i ECMWF-modellen.
DMI's vejrmodelsystem er opbygget som et "æske system", der består af modelversionerne DMI-HIRLAM-T15 og DMIHIRLAM-S05 samt DMI-HIRLAM-Q05. Bortset fra beregningsområde og horisontal tæthed af beregningspunkter er
modelversionerne ens. Afstanden mellem beregningspunkterne er ca. 15 km i DMI-HIRLAM-T15 medens den er ca. 5 km i
DMI-HIRLAM-S05 og DMI-HIRLAM-Q05, som er hhv.en lokal model med høj opløsning for Danmark og omkringliggende
farvande, og en lokal model med høj opløsning for Grønland. DMI-HIRLAM-T15 anvendes til regionale udsigter for
Nordatlanten og Europa. Det større område for DMI-HIRLAM-T15 betyder endvidere at modellen kan anvendes til 60 timers
prognoser.
I tabellen betyder mlon og mlat antallet af gitterpunkter i hhv. vest-øst og nord-syd retningen. Afstanden mellem
nabogitterpunkter er i grader (1° svarer til ca. 110km), og tidsskridts længde er i sekunder. Randværdier er prognosedata
fra en ydre vejrmodel, som bestemmer atmosfæretilstanden langs den indre models rande. Randværdier for DMI-HIRLAMT15 er prognoser fra den globale model fra ECMWF, mens randværdier for DMI-HIRLAM-S05 og DMI-HIRLAM-Q05 er
prognoser fra DMI-HIRLAM-T15.
T15
S05 Q05
Gridpunkter (mlon)
610
496
550
Gridpunkter (mlat)
568
372
378
Antal vertikale lag
40
40
40
Horisontal opløsning
0,15°
0,05° 0,05°
Tidsskridt
360s
120s 120s
Randværdier: model ECMWF
T15
T15
(DMI.dk)
The early history of Numerical Weather Prediction
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A century ago, in 1904, the Norwegian hydrodynamist V. Bjerknes suggested that the weather could be
quantitatively predicted by applying the complete set of hydrodynamic and thermodynamic equations to carefully
analysed initial atmospheric states. Lacking both the theoretical and practical means to make any quantitative
predictions he initiated instead the qualitative approach that has became known as the "Bergen School".
After the Second World War two technological developments appeared to make mathematical weather forecast
along the lines suggested by Bjerknes possible: the establishment of a hemispheric network of upper-air stations
and the development of the first electronic computers. In 1948 a young meteorological theoretician, Jule Charney,
succeeded to derive simplified mathematical models of the atmospheric motions, based on the quasi-geostrophic
approximations. These equations would be able to forecast the large scale flow in spite of minor inaccuracies in the
initial analyses.
When the first NWP experiments were conducted in 1950, due to the limited computer capacity, only the most
simple of Charney's models could be used, the barotropic equation of atmospheric motion. The results were
surprisingly successful: the general 500 hPa flow pattern over North America was forecast 24 hours in advance with
greater skill than previous subjective methods.
From this successful start two different strategies developed: countries with limited computer resources, preferred to
explore the potential of the barotropic model, whereas countries like the US and Britain took a more ambitious
approach by developing baroclinic models where forecasts of vertical motion were possible. It soon turned out that
the nature of the problem was much more complicated than envisaged. That is why during the 50's the first
operationally useful NWP forecasts were barotropic: in Sweden in 1954, in the US in 1958 and Japan in 1959. Only
in 1962 could the US launch the first operational quasi-geostrophic baroclinic model, followed by Britain in 1965. By
that time, work was already under way, to introduce more realistic numerical models, based on the primitive
equations (PE).
In a PE-model changes in wind and geopotential fields are not restricted by any quasi-geostrophic constraint, but
are allowed to interact freely. The physical parametrizations such as convection, which are difficult to handle in the
quasi- geostrophic model, could now be realistically incorporated, so that the tropical regions, essential for forecasts
over Europe beyond two or three days, can be included. The first global PE model began operating in 1966 at NMC
Washington, with a 300 km grid and six-layer vertical resolution. During the 70's several other PE models were
implemented, global, hemispheric or as Limited Area Models, which ran with a higher resolution over a smaller area
and took boundary values from a larger hemispheric or global model.
Interest in ocean wave forecasting started during the Second World War when it was realised that information on
the sea state could be of vital importance. The first operational predictions were based on the use of empirical wind
sea and swell laws. An important advance was the introduction of the concept of a wave spectrum in the mid
1950's, followed by a dynamical equation describing the evolution of the wave spectrum, the energy balance
equation.
During the 1980's it became evident that wave forecasts did not only have an intrinsic value, but that they also
provide a means for increased realism of the atmospheric system through incorporating the friction the waves
except on the wind, which on its turn affects the ocean circulation and the storm surge. Ultimately, it is expected to
have a model consisting of the atmosphere and the oceans where the ocean waves are the agent that transfer
energy and momentum across the interface in accordance with the energy balance equation. Presently, we have
taken the first step by coupling the IFS atmospheric model with the wave model in a two-way interaction mode.
This coupled model provides the 10 day weather and wave forecast since the 29th of June 1998. As a next step
ECMWF is developing a coupled atmosphere, ocean-wave, ocean-circulation model. This coupled model will be
used in seasonal forecasting and monthly forecasting in the near future.
With the increasing number of satellites providing observations also from the upper atmosphere, the atmospheric
models have been extended to ever higher altitudes. One of the major breakthroughs in the last 15 years in NWP
came from an enormous improvement in data assimilation techniques together with the availability of an increasing
number of remotely sensed observations from satellites, providing a global and high frequency data coverage. The
development of variational techniques has progressively allowed for a direct assimilation of infrared and microwave
sounder radiances which impact on analysed temperature and humidity fields. This technique also ensures that the
information coming from satellites is dynamically consistent.
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Recent studies have shown that in terms of NWP performance, satellite observations are now equally important as
radiosondes, not only in the Southern Hemisphere (void of conventional observations), but also in the Northern
Hemisphere
(http://www.ecmwf.int/products/forecasts/guide/The_early_history_of_Numerical_Weather_Prediction.html)
Is the old adage “Red sky at night, sailor’s delight. Red sky in morning,
sailor’s warning” true, or is it just an old wives’ tale?
Within limits, there is truth in this saying.
Have you ever heard anyone use the proverb above?
Shakespeare did. He said something similar in his play, Venus and Adonis. “Like a red morn that ever yet betokened, Wreck to the seaman,
tempest to the field, Sorrow to the shepherds, woe unto the birds, Gusts and foul flaws to herdmen and to herds.”
In the Bible, (Matthew XVI: 2-3,) Jesus said, “When in evening, ye say, it will be fair weather: For the sky is red. And in the morning, it will
be foul weather today; for the sky is red and lowering.”
Weather lore has been around since people needed to predict the weather and plan their activities. Sailors and farmers relied on it to
navigate ships and plant crops.
But can weather lore truly predict the weather or seasons?
Weather lore concerning the appearance of the sky, the conditions of the atmosphere, the type or movement of the clouds, and the direction
of the winds may have a scientific basis and likely can predict the weather.
In order to understand why “Red sky at night, sailor’s delight. Red sky in morning, sailor’s warning” can predict the weather, we must
understand more about weather and the colors in the sky.
Usually, weather moves from west to east, blown by the westerly trade winds. This means storm systems generally move in from the West.
The colors we see in the sky are due to the rays of sunlight being split into colors of the spectrum as they pass through the atmosphere and
ricochet off the water vapor and particles in the atmosphere. The amounts of water vapor and dust particles in the atmosphere are good
indicators of weather conditions. They also determine which colors we will see in the sky.
During sunrise and sunset the sun is low in the sky, and it transmits light through the thickest part of the atmosphere. A red sky suggests an
atmosphere loaded with dust and moisture particles. We see the red, because red wavelengths (the longest in the color spectrum) are
breaking through the atmosphere. The shorter wavelengths, such as blue, are scattered and broken up.
Red sky at night, sailors delight.
When we see a red sky at night, this means that the setting sun is sending its light through a high concentration of dust particles. This
usually indicates high pressure and stable air coming in from the west. Basically good weather will follow.
Red sky in morning, sailor’s warning.
A red sunrise reflects the dust particles of a system that has just passed from the west. This indicates that a storm system may be moving to
the east. If the morning sky is a deep fiery red, it means a high water content in the atmosphere. So, rain is on its way.
To learn more about weather lore and proverbs see the following Related Web Sites and For Further Reading sections.
(http://www.loc.gov/rr/scitech/mysteries/weather-sailor.html)
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Introduction
Weather — the condition of the atmosphere at a particular place and time. Lore — a traditional belief. From these two definitions, we get
weather lore, which is a collection of proverbs and sayings that have been passed on from generation to generation over hundreds of years,
generally in rhyme.
The purpose of weather lore was to instruct early farmers, sailors, herdsmen, and others on how to predict the weather. Its poetic nature
made it easier to pass on to later generations. People who make their living outdoors depend on the weather. That has always been the case.
Today, meteorologists make use of satellites, weather balloons, super computers, Doppler radar, and a complex communications network to
produce reasonably accurate daily weather forecasts.
In earlier times, however, folks had to rely on other weather indicators to advise them on what kind of plans to make. Some of these
indicators have a true correlation with factors that do affect the weather. Others have no relationship at all to the weather. The purpose of
this publication is two-fold. First, it is designed to be informative. Many of the weather signs and sayings given in the pages that follow can
really be used as a guide to how the weather is likely to develop 12 to 24 hours in the future. By making correct use of weather lore, you
may find yourself with the ability to outguess the real weatherman with your own forecasts. At any rate, it may provide you with a greater
appreciation of how the weather is interrelated with other elements of the natural environment.
The second reason for writing this is to provide pure entertainment. The weather is often a topic of conversation between two people who
have nothing else to talk about. This may give some of you something to say in that situation! It also is fun to see how our ancestors coped
with the unpredictable nature of the weather.
Weather Indicators
In order to determine whether or not a proverb or saying has any correlation to the weather, it is helpful to understand exactly which sayings
are based on true weather indicators that have a cause-and-effect relationship with atmospheric conditions. Here is a listing of relevant
weather indicators and how they may affect the weather.
HUMIDITY (MOISTURE IN THE AIR)
There is always a certain amount of water vapor in the air, even in deserts. The amount of moisture in the air is also dependent upon the
temperature. The higher the temperature, the more moisture the air can hold.
Relative humidity is a term meteorologists use to describe the amount of moisture the air is holding compared to the maximum amount it
could hold at a given temperature. For example, a relative humidity reading of 50 percent means that the air is holding half as much
moisture that it could hold at that temperature.
As the temperature falls during the evening, the air is able to hold less moisture than it could during the warmest part of the day. The
relative humidity, therefore, rises. You could compare it to a container of water. If the container is flexible so that its height can change, and
if it is half-full of water, we could say that its “humidity” is 50 percent. If we kept the amount of water the same, but reduced the size of the
container so that its size was equal to the amount of water it held, its “humidity” would have increased to 100 percent, because it is now
holding all that it is capable of holding (at its new size).
If you reduced the size of the container any further, water would spill out. The same thing is true of the atmosphere. If the temperature of
the air falls to a point where it can hold no more moisture, the relative humidity is 100 percent, and moisture begins to condense out of the
air by forming small water droplets on such things as grass, windows, and cars. This moisture is called dew.
The instrument used to measure atmospheric moisture is called a psychrometer or a hygrometer. The higher the relative humidity, especially
during the daytime, the greater the probability of rain. Fog may also form if the humidity is near 100 percent.
AIR PRESSURE
The air is a fluid. As with any other fluid, it has internal pressure due to the force of Earth’s gravity. Normally, the weight of the air is 14.7
pounds per square inch measured at sea level. It gets lower with increasing altitude.
Air pressure is typically measured by reading how high a column of mercury can be suspended in a glass tube by the pressure of the air
against the mercury in a cup in which the tube has been inverted. At sea level, this is normally 29.92 inches high, or about 760 millimeters.
We call this normal atmospheric pressure.
Pressure differences are caused by the uneven heating of the surface of the earth by the sun. An area that is receiving a lot of solar radiation
will become wanner, and the air volume will expand. As air becomes warmer, its molecules move faster and therefore, bump each other
farther apart. This increases the volume of the air. A volume of warm air will contain fewer air molecules than an equal volume of cold air.
When the weight of the air over one region of earth becomes lower than the surrounding area, the parcel of air in that region begins to rise,
being pushed upward by the higher pressure of the surrounding air. Air under low pressure, therefore, rises.
As air rises, it cools. As has already been stated, cooler air can hold less moisture. So if the rising air reaches an altitude where it is too cool
to hold the amount of moisture it had on the ground, that moisture condenses out as clouds. Thus, low pressure areas produce cloudy and
rainy weather.
High pressure areas are produced by heavy, sinking air. They are characterized by clear weather. An area of high pressure is sometimes call
a high pressure cell, or simply, a “high.” Low pressure cells are usually just called “lows.”
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The instrument used to measure air pressure is called a barometer. The change in the pressure, and how fast it is changing, is more
indicative of the weather than the pressure itself. Rapidly falling pressure almost always means an approaching storm system. Rapidly rising
pressure almost always means clearing and cooler weather is ahead.
WIND
Wind is the movement of air in a horizontal direction over the earth’s surface. The direction from which the wind is blowing can be a good
indicator of weather to come.
Wind always blows in a circular pattern around high and low pressure cells. It blows clockwise around a high and counterclockwise around a
low. This circulation is a direct result of the earth’s rotation and is termed the Coriolis effect.
Since atmospheric pressure helps determine the weather, knowing from which way the wind is blowing can help you to locate where highs
and lows are relative to your position. For example, if you stand with your back to the wind, a high pressure cell will probably be to your
right, If your right is west, then you can predict fair or improving weather, because weather systems usually move from west to east.
The way the wind direction changes can also help predict the weather. If the wind is out of the south, then it changes to the southwest, then
west, then northwest, it is changing in a clockwise direction and is said to be veering. If it changes in a counterclockwise direction, such as
first blowing out of the west then southwest, then south, then southeast, the wind is said to be backing. Sometimes a backing wind is a sign
of an approaching storm front.
The speed of the wind also is an indicator of the weather. A strong wind usually means a big differential in the air pressure over a small
space. This means that if a low pressure system is approaching you, it will likely be intense.
The instrument used to measure wind direction is called a wind vane. The instrument that tells how fast the wind is blowing is called an
anemometer.
Many of the sayings of weather lore make use of the correlation between these weather indicators and the affect they may have on
observable phenomena. For example, as the humidity becomes higher, human hair becomes longer. It follows, then, that if your hair seems
to curl up at the end and seems more unmanageable, it could be a sign of rain.
Another good sign of high humidity is salt. Salt tends to become sticky and clog the holes in the salt shaker if the humidity is high. The
weather lore that follows has been derived from several sources. Most of the sayings are very old. When possible, the derivation of the saying
is given. Whether or not the saying is a valid predictor of the weather is also explored. However, given that even with the vast resources of
the National Weather Service, their predictions are quite often off the mark, how good can an old weather proverb be at predicting the
weather? Use the ones listed here to find out for yourself!
Weather Lore
Lore Related to Clouds or Moisture
Moisture in the air (humidity) as well as the visual results of this moisture (fog and clouds) are good indicators of what the weather may be in
the near future. Sayings that relate to humidity or clouds may, therefore, be accurate predictors.
“A summer fog for fair,
A winter fog for rain.
A fact most everywhere,
In valley or on plain.”
Most fog is formed in one of two ways. One way is for the temperature to fall to a point, called the dew point, in which the humidity rises to
100 percent. This happens on clear, calm summer nights. A cloudy sky acts like a blanket and holds in the heat of the day, so dew and fog
won’t form because the air temperature doesn’t fall to the dew point. So a summer fog, or dew, means clear, calm weather conditions.
Another way fog can form is for warm, moisture-laden air to blow in over a cold surface. This is how most winter fogs form. Warm, moist air
is a harbinger of rainy weather.
A similar saying which relates to due rather than fog is...
“When the dew is on the grass,
Rain will never come to pass.
When grass is dry at morning light,
Look for rain before the night.”
Again, if there is no dew on the grass, it either means the sky is cloudy or the breeze is strong, both of which may mean rain.
“If a cat washes her face o’er her ear,
‘tis a sign the weather will be fine and clear.”
Cat fur can build up static electric charges when it gets very dry. During times of low humidity and fair weather, especially in the winter time
when it is very dry, a cat may lick its fur. In order to moisten it. Moist fur will shed electric charge and prevent static discharges, which annoy
the cat.
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“If a circle forms ‘round the moon,
‘Twill rain soon.”
The circle that forms around the sun or moon is called a halo. Halos are formed by the light from the sun or moon refracting (bending) as
they pass through the ice crystals that form high-level cirrus and cirrostratus clouds. These clouds do not produce rain or snow, but they
often precede an advancing low pressure system which may bring bad weather.
“Trace in the sky the painter’s brush,
The winds around you soon will rush."
The “painter’s brush” are cirrus clouds. These are high-level ice clouds that often precede the approach of a storm system.
“Rainbow in the morning, Shepherds take warning.
Rainbow at night, Shepherd’s delight.”
A rainbow in the morning is formed when light from the rising sun in the east strikes and refracts through the water droplets in a rain cloud in
the western sky. Rainbows always occur in the part of the sky opposite the sun. Since most storms (though not all) come out of the west, a
rainbow in the western sky is a sign of rain. A rainbow in the eastern sky, as would occur in the evening, is a sign the rain has passed.
“When clouds look like black smoke,
A wise man will put on his cloak.”
Thick clouds laden with large droplets of water look darker than the fair-weather cumulus clouds,
“When leaves turn their back ‘tis a sign it’s going to rain.”
Some trees, such as oak and maple, have leaves that will curl when the humidity is very high and the wind is blowing strongly. Both these
conditions indicate an approaching storm.
“Evening red and morning gray
Helps the traveler on his way.
Evening gray and morning red
Brings down rain upon his head.”
Weather systems usually move from west to east. A reddish evening sky can be caused by sunlight shining through dry dust particles in the
western sky. This dry sky may move overhead by morning. If the morning is gray in the east, it means the clouds have already passed you.
Conversely, if the evening is gray, it means the clouds have not yet reached you. Rain may be on its way.
There are many other similar sayings relating to sky color in the morning or evening Here are but a few:
“Evening red and morning gray,
Two sure signs of one fine day.”
“Evening red and weather fine.
Morning red, of rain’s a sign.”
“An evening gray and a morning red
Will send the shepherd wet to bed.”
The “evening red, morning gray” sayings are among the more widely-recognized of all weather lore.
“The higher the clouds, the finer the weather.”
Clouds are formed by moisture that condenses out of rising air currents. The higher the air must rise before condensation begins, the drier it
was to begin with.
“When clouds appear like rocks and towers,
The earth will be washed by frequent showers.”
Towering clouds, lofted high by strong updrafts, are cumulonimbus clouds. These are the thunderstorm clouds that produce heavy showers,
wind, and lightning. They are not, however, associated with steady rain.
“I know ladies by the score
Whose hair foretells the storm;
Long before it begins to pour
Their curls take a drooping form.”
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Human hair, especially blond hair, has a tendency expand in length as the humidity rises. This may cause naturally-curly hair to droop. Or it
may cause straight hair to curl up a little. The higher the humidity, the more likely it is to rain.
“When chairs squeak
It’s about rain they speak.”
Wooden chairs will absorb moisture from the air when the humidity rises. This causes them to squeak
“If salt is sticky and gains in weight,
It will rain before too late.”
Salt tends to draw moisture from the air. If the humidity is high, as it is during or preceding a rain, salt will soak up this atmospheric
moisture and clog the saltshaker.
“Pale moon rains; Red moon blows.
White moon neither rains or snows.”
The more dust particles there are in the air, the greater the chance that moisture will have something on which to form raindrops. Drops of
rain cannot form unless they can form around a “condensation nucleus,” which is a dust particle, ice crystal, or some similar tiny object
suspended in the air. When moonlight passes through air laden with dust particles, it appears pale or reddish. When the air is very clear, it
appears white.
“If smoke hovers low near the ground it is likely to rain.”
Smoke particles tend to absorb moisture from the air. The more moisture present in the air, the more a particle of smoke will absorb, and the
heavier it gets. Heavy, moisture-laden smoke particles do not disperse as easily as the lighter, dry ones do.
“When sounds travel far and wide,
A stormy day will betide.”
Sound travels at different speeds through different substances. It travels faster through a solid substance than it does through air, for
instance. Sound travels better in air that is heavily laden with moisture than it does in dry air.
“Cold is the night
When the stars shine bright.”
The more moisture there is in the sky, the more the light from the sun, moon, and stars is dimmed or reddened. A very clear sky permits
more starlight to penetrate, thus the stars appear brighter. Moisture tends to hold in the day’s heat, like a blanket. The less moisture there is
in the air at night, the more the temperature tends to fall. Thus, the brighter the stars appear, the cooler is the night.
“Mares’ tails and mackerel scales
Make lofty ships carry low sails.”
Mares’ tails are cirrus clouds, called this because they sometimes resemble the flowing tail of a horse in the wind. Mackerel scales are
altocumulus clouds. They appear broken and scaly. Neither of these cloud types will bring rain or snow themselves. They do, however,
precede an approaching storm front by a day or two.
Lore About Air Pressure
As with those sayings related to humidity or clouds, much of the weather lore that is based on changes in air pressure or the wind can
accurately predict the weather. Air pressure and wind are two weather indicators that can be used to determine future weather conditions
over the short term.
“When the wind is out of the east,
‘Tis neither good for man nor beast.”
Easterly winds usually indicate an approaching weather front or low pressure area. Low pressure generally brings bad weather. Remember
that wind circulates around a high pressure cell in a clockwise direction. So if the wind is out of the east, a “high” that came from the west
has already passed you by, or is currently passing by on the north. A low pressure system is sure to follow since highs and lows usually tend
to alternate in progression.
“When the wind is in the north. The skillful fisher goes not forth;
When the wind is in the cast, ‘Tis good for neither man nor beast;
When the wind is in the south, It blows the flies in the fish’s mouth;
But when the wind is in the west, There it is the very best.”
“Fish bite least
With wind in the east.”
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In the U.S. the prevailing winds are from the west, northwest, or southwest. As a general rule, winds coming from a westerly direction
signifies good weather. It is only a matter of conjecture, however, if fish react to the direction of the wind.
“When the wind backs; and the weather glass falls
Prepare yourself for gales and squalls.”
A backing wind is one which changes direction in a counterclockwise manner—usually starting in the west, then changing to the southwest,
south, and then southeast. A backing wind indicates the approach of a low pressure cell from the southwest.
The “weather glass” was an early term for a crude barometer. When the weather glass falls, the atmospheric pressure is lowering, signaling
the approach of a storm system.
A similar rhyme goes like this:
“When the glass falls low,
Prepare for a blow;
When it rises high.
Let all your kites fly.”
Again, the “glass” referred to here represents a barometer, which measures air pressure. High pressure means fair weather; low pressure
indicates rain or storms.
“When the ditch and pond affect the nose,
Look out for rain and stormy blows.”
High air pressure, associated with fair weather, tends to hold earthly scents to their source. When low pressure arrives, the odors are
released and can be sniffed.
“A coming storm your shooting corns presage,
And aches will throb, your hollow tooth will rage.”
“If your corns all ache and itch,
The weather fair will make a switch.”
Studies have shown that some people experience increased pain when the barometric pressure falls. This is not the case with everyone, but
changes in air pressure do seem to cause aches and pains to increase for some folks.
“If birds fly low
Expect rain and a blow.”
When the air pressure is high, it is easier for birds to fly at a higher altitude. If the air pressure is low, indicating bad weather, birds can’t fly
as high because the air is less dense.
“If the rooster crows on going to bed,
You may rise with a watery head.”
It is thought that birds, and other animals, react negatively to a decrease in atmospheric pressure; it makes them restless. A restless rooster
tends to crow more.
“Trout jump high
When a rain is nigh.”
Lowering of air pressure sometimes causes trapped gases, created by decaying plant matter on the bottom of a lake or pond, to release.
This, in turn, causes microscopic organisms that hide in the plant debris to be dispersed. This, in turn, stimulates small fish to start feeding,
which causes larger fish to start feeding on them. A whole “feeding frenzy” may develop, which cause the fish to become active and jump.
“If clouds move against the wind, rain will follow.”
Clouds that are moving in a direction that differs from the way the wind is blowing indicates a condition known as wind shear. This sometimes
indicates the arrival of a cold front. Weather fronts usually bring rain.
“Cats and dogs eat grass before a rain."
Cats and dogs eat grass when they are feeling gastrointestinal distress and need to vomit. Changes in air pressure may affect animals in this
manner.
“A wind in the south
has rain in her mouth.”
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A southerly wind usually carries moisture from the Gulf of Mexico. It causes the air to become more humid, and thus, more likely to form rain
clouds.
General Weather Lore
Here is a collection of weather sayings that do not fit neatly into the preceding categories. They may or may not have any scientific validity.
Most of them are probably not very reliable but a few may have merit.
Included here are the weather sayings that attempt to predict the long-range weather, such as whether the winter will be cold or the summer
rainy. Almost all long-range forecasts, even by the National Weather Service, are not very accurate.
“Onion skins very thin
Mild winter coming in;
Onion skins thick and tough
Coming winter cold and rough."
“A swarm of bees in May
Is worth a load of hay.”
”If March comes in like a lamb, it goes out like a lion; if it comes in like a lion, it goes out like a lamb.”
“Plant your beans when the moon is light,
You will find that this is right;
Plant potatoes when the moon is dark,
And to this line you’ll always hark;
But if you vary from this rule,
You will find you are a fool;
Follow this rule to the end.
And you’ll have lots of dough to spend.”
“When oak is out before the ash,
‘Twill be a summer of wet and splash.
But if the ash before the oak,
‘Twill be a summer of fire and smoke."
“The first snow comes six weeks after the last thunderstorm in September.”
“If February brings drifts of snow
There will be good summer crops to hoe.”
“When sheep gather in a huddle,
tomorrow we will have a puddle.”
“Expect the weather to be fair
When crows fly is pairs”
“If woolly worms are dark, the coming winter wilt be severe.”
“When ladybugs swarm,
Expect a day that’s warm.”
The above saying may be true because lady bugs store heat in their shells. If it gets too warm, they start flying to dissipate the heat. Of
course, this means it is already warm, not that it is going to be warm!
“When chickens scratch together,
There’s sure to be foul weather.”
“If the groundhog sees his shadow on February 2nd, there will be six more weeks of winter.”
“When pigs carry sticks,
The clouds will play tricks;
When they lie in the mud,
No fears of a flood.”
“When cattle lie down during a light rain, it will pass soon.”
“When walls in cold weather begin to show dampness, the weather will change.”
This is probably a good indicator that the humidity is increasing and may mean cloudy or rainy weather ahead.
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“If the sparrow makes a lot of noise, rain will follow.”
“The moon and the weather
May change together;
But change of the moon
Does not change the weather”
“If the moon lies on her back, She sucks the wet into her lap.”
“Tipped moon wet: cupped moon dry.”
The moon has virtually no affect at all on the weather. Certainly, the way a crescent moon’s horns point has nothing to do with the weather.
This is purely a function of the moon’s orbit.
Here's a little poem that seems to tie it all together:
When the sky is red in the morning,
And sounds travel far at night;
When fish jump high from the water
And flies stick tight, and bite;
When you can't get salt from your shaker,
And your corn gives you extra pain,
There's no need to consult an almanac,
You just know it's going to rain.
Ode to the Weatherman...
And in the dying embers
These are my main regrets:
When I’m right no one remembers;
When I’m wrong no one forgets.
-----
Copyright © 2001 by Jerry Wilson. Get permission to reprint this article.
(http://wilstar.com/skywatch.htm)
Predicting weather through the ages
Imagine a rotating sphere that is 12,800 kilometers (8000 miles) in diameter, has a bumpy surface, is
surrounded by a 40-kilometer-deep mixture of different gases whose concentrations vary both spatially and
over time, and is heated, along with its surrounding gases, by a nuclear reactor 150 million kilometers (93
million miles) away. Imagine also that this sphere is revolving around the nuclear reactor and that some
locations are heated more during one part of the revolution and other locations are heated during another part of
the revolution. And imagine that this mixture of gases continually receives inputs from the surface below,
generally calmly but sometimes through violent and highly localized injections. Then, imagine that after
watching the gaseous mixture, you are expected to predict its state at one location on the sphere one, two, or
more days into the future. This is essentially the task encountered day by day by a weather forecaster (Ryan,
Bulletin of the American Meteorological Society,1982).
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Earth rotates on its axis once every 23 hours, 56 minutes, and completes one revolution around the sun every 365.25 days. (Courtesy of Rob
Simmon.)
Early History
The art of weather forecasting began with early civilizations using reoccurring astronomical and meteorological events to help them
monitor seasonal changes in the weather. Around 650 B.C., the Babylonians tried to predict short-term weather changes based on the
appearance of clouds and optical phenomena such as haloes. By 300 B.C., Chinese astronomers had developed a calendar that divided
the year into 24 festivals, each festival associated with a different type of weather.
Around 340 B.C., the Greek philosopher Aristotle wrote Meteorologica, a philosophical treatise that included theories about the
formation of rain, clouds, hail, wind, thunder, lightning, and hurricanes. In addition, topics such as astronomy, geography, and
chemistry were also addressed. Aristotle made some remarkably acute observations concerning the weather, along with some
significant errors, and his four-volume text was considered by many to be the authority on weather theory for almost 2000 years.
Although many of Aristotle’s claims were erroneous, it was not until about the 17th century that many of his ideas were overthrown.
Throughout the centuries, attempts have been made to produce forecasts based on weather lore and personal observations. However,
by the end of the Renaissance, it had become increasingly evident that the speculations of the natural philosophers were inadequate
and that greater knowledge was necessary to further our understanding of the atmosphere. In order to do this, instruments were needed
to measure the properties of the atmosphere, such as moisture, temperature, and pressure. The first known design in western
civilization for a hygrometer, an instrument to measure the humidity of air, was described by Nicholas Cusa (c.1401-1464, German)
in the mid-fifteenth century. Galileo Galilei (1564-1642, Italian) invented an early thermometer in 1592 or shortly thereafter; and
Evangelista Torricelli (1608-1647, Italian) invented the barometer for measuring atmospheric pressure in 1643.
While these meteorological instruments were being refined during the seventeenth through nineteenth centuries, other related
observational, theoretical, and technological developments also contributed to our knowledge of the atmosphere; and individuals at
scattered locations began to make and record atmospheric measurements. The invention of the telegraph and the emergence of
telegraph networks in the mid-nineteenth century allowed the routine transmission of weather observations to and from observers and
compilers. Using these data, crude weather maps were drawn and surface wind patterns and storm systems could be identified and
studied. Weather-observing stations began appearing all across the globe, eventually spawning the birth of synoptic weather
forecasting, based on the compilation and analysis of many observations taken simultaneously over a wide area, in the 1860s.
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A schematic sounding of air temperature and dewpoint derived from radiosonde data. This sample schematic
sounding includes a temperature "inversion" (temperatures increasing with height) at about 800 hPa and
reflects atmospheric conditions that frequently precede the development of severe thunderstorms and
possibly tornadoes. [1 hectoPascal (hPa) = 1 millibar (mb).] (Courtesy of Rob Simmon.)
With the formation of regional and global meteorological observation networks in the nineteenth and twentieth centuries, more data
were becoming available for observation-based weather forecasting. A great stride in monitoring weather at high altitudes was made
in the 1920s with the invention of the radiosonde. Small lightweight boxes equipped with weather instruments and a radio transmitter,
radiosondes are carried high into the atmosphere by a hydrogen or helium-filled balloon that ascends to an altitude of about 30
kilometers before bursting. During the ascent, these instruments transmit temperature, moisture, and pressure data (called soundings)
back to a ground station. There, the data are processed and made available for constructing weather maps or insertion into computer
models for weather prediction. Today, radiosondes are launched every 12 hours from hundreds of ground stations all over the world.
The North American network of upper air ground stations, each indicated by a three-letter station identifier. Radiosondes are launched
and tracked from each location every twelve hours. (Courtesy of NOAA.)
Towards Numerical Prediction
Over the past few centuries, physical laws governing aspects of the atmosphere have been expressed and refined through
mathematical equations. The idea of numerical weather forecasting—predicting the weather by solving mathematical equations—was
formulated in 1904 by Vilhelm Bjerknes (1862-1951, Norwegian) and developed by British mathematician Lewis Fry Richardson
(1881-1953, British). Despite the advances made by Richardson, it took him, working alone, several months to produce a wildly
inaccurate six-hour forecast for an area near Munich, Germany. In fact, some of the changes predicted in Richardson’s forecast could
never occur under any known terrestrial conditions. Adding to the failure of this effort, a six-hour forecast is not particularly useful if
it takes weeks to produce.
Courageously, Richardson reported his results in his book Weather Prediction by Numerical Process, published in 1922. In one of the
chapters of this work, Richardson describes a scheme for predicting the weather before it actually happens, a scheme involving a
roomful of people, each computing separate sections of the equations, and a system for transmitting the results as needed from one
part of the room to another. Unfortunately, Richardson’s estimated number of human calculators needed to keep pace with weather
developments was 64,000, all located in one very large room.
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Richardson’s work highlighted the obvious fact that a large number of calculations had to be made very rapidly in order to produce a
timely forecast. In the late 1940s, using one of the earliest modern computers, significant progress toward more practical numerical
weather forecasts was made by a team of meteorologists and mathematicians at the Institute for Advanced Study (IAS) in Princeton,
New Jersey. Mathematician John von Neumann (1903-1957, Hungarian-American) directed the construction of the computer and put
together a team of scientists led by Jule Charney (1917-1981, American) to apply the computer to weather forecasting. Charney
determined that the impracticality of Richardson’s methods could be overcome by using the new computers and a revised set of
equations, filtering out sound and gravity waves in order to simplify the calculations and focus on the phenomena of most importance
to predicting the evolution of continent-scale weather systems. In April 1950, Charney’s group made a series of successful 24-hour
forecasts over North America, and by the mid-1950s, numerical forecasts were being made on a regular basis.
Modern technology, particularly computers and weather satellites, and the availability of data provided by coordinated meteorological
observing networks, has resulted in enormous improvements in the accuracy of weather forecasting. Satellites, in particular, have
given forecasters routine access to observations and data from remote areas of the globe. On April 1, 1960, the polar-orbiting satellite
TIROS 1 (the first in the series of Television and Infrared Observation Satellites) was launched. Although the spacecraft operated for
only 78 days, meteorologists worldwide were ecstatic over the pictures of the Earth and its cloud cover that TIROS relayed back to
the ground.
The first picture of Earth from a weather satellite, taken by the TIROS-1 satellite on April 1, 1960. Although
primitive in comparison with the images we now receive from satellites, this first picture was a major advance.
Over the past 40 years, satellite sensor technology has advanced enormously. In addition to providing visual images, satellites can
also provide data that allow calculation of atmospheric temperature and moisture profiles and other environmental variables. This is
done using a variety of instruments, among them atmospheric sounders, which measure quantities at various levels in atmospheric
columns. The data retrieved from sounder measurements taken from a satellite can be made similar to radiosonde observations, with
the major advantage that the satellite data are more complete spatially, filling in gaps between weather ground stations, which often
are hundreds or even thousands of kilometers apart.
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Full-disk GOES-8 water vapor image from September 5, 1995. (Courtesy of Marit Jentoft-Nilsen.)
In 2002, the Atmospheric Infrared Sounder (AIRS), the Advanced Microwave Sounding Unit (AMSU), and the Humidity Sounder for
Brazil (HSB) will be launched together on NASA’s Earth Observing System (EOS) Aqua satellite, a satellite that will also carry a
Moderate Resolution Imaging Spectroradiometer (MODIS), two Clouds and the Earth’s Radiant Energy System (CERES) sensors,
and an Advanced Microwave Scanning Radiometer for EOS (AMSR-E). The AIRS/AMSU/HSB sounding system will provide the
capability for determining atmospheric temperature and moisture more accurately than ever before from space-based measurements.
These measurements will be provided to the National Oceanic and Atmospheric Administration (NOAA), the European Centre for
Medium-Range Weather Forecasts (ECMWF), and the weather community at large for assimilation into operational numerical
weather prediction models. It is expected that assimilation of global AIRS/AMSU/HSB data, complementing other operational
observations, should lead to a substantial improvement in the accuracy of mid- and long-range weather forecasts.
The Aqua spacecraft, with pointers to the AMSU-A1, AMSU-A2, AIRS, and HSB instruments.
The AIRS/AMSU/HSB instrument suite builds on the technical heritage of NOAA’s High Resolution Infrared Sounder (HIRS) and
Microwave Sounding Unit (MSU). The HIRS/MSU combination was the National Weather Service’s (NWS’s) operational weather
sounding system for nearly twenty years, flying on numerous NOAA polar orbiting satellites. This system was enhanced in the late
1990s by the replacement of the four-channel MSU by a 20-channel AMSU, which includes Aqua’s AMSU and HSB channels.
Looking toward further improvements in weather forecasts, the NWS has set measurement requirements for temperature at an
accuracy of 1°C in layers 1 km thick and humidity at an accuracy of 20% in layers 2 km thick in the troposphere (the lower part of the
atmosphere, where weather systems are of most relevance to human life and property). AIRS/AMSU/HSB will meet these
requirements, allowing meteorologists to improve and extend their predictions and reduce the number of significant prediction
mistakes, like failing to predict a major storm prior to a few hours before its arrival.
Benefits to Society
The potential economic benefits of more accurate weather forecasts are immense. For example, a more accurate 24-hour forecast of
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heavy rain and thunderstorms along a cold front could allow airline dispatchers enough time to reroute their airplanes appropriately
and thereby help alleviate costly delays. Being able to pinpoint a wintertime low temperature in Florida could be the deciding factor in
whether orange grove farmers make the correct decision regarding deployment or non-deployment of freeze prevention methods to
save their crops. Better information on wind patterns could aid the National Hurricane Center in producing a more accurate forecast of
a hurricane’s track and might enable a reasoned decision to be made regarding evacuating thousands of families out of Miami or
Jacksonville. In military operations, there is a considerable historical record of instances when weather conditions have altered the
course of battles, including examples when accurate forecasting has been a deciding factor in one side’s victory.
Sample crops that could benefit from more accurate weather forecasts aiding farmers in selecting harvesting times and in protecting their
crops from freezing temperatures.
Closing
Only fifty years ago, weather forecasting was an art, derived from the inspired interpretation of data from a loose array of land-based
observing stations, balloons, and aircraft. Since then it has evolved substantially, based on an array of satellite and other observations
and sophisticated computer models simulating the atmosphere and sometimes additional elements of the Earth’s climate system. All
this has been made possible by advances in satellite technology, a sweeping acceleration in worldwide communications, and
overwhelming increases in computing power. Aqua’s AIRS/AMSU/HSB combination should further these advances, enabling more
accurate predictions over longer periods.
Sources and Suggested Readings
Burroughs, William J., Bob Crowder, Ted Robertson, Eleanor Vallier-Talbot, and Richard Whitaker, 1996: Weather, a Nature
Company Guide, Time-Life Books, Sydney, Australia, 288 pp.
Frisinger, H. Howard, 1977: The History of Meteorology: to 1800, Science History Publications, New York, 148 pp.
Fung, Inez Y., 1996: Charney, Jule Gregory. In Encyclopedia of Climate and Weather, edited by Stephen H. Schneider, Oxford
University Press, New York, vol. 1, pp. 109-111.
Goldstine, Herman H., 1980: The Computer from Pascal to von Neumann, Princeton University Press, Princeton, New Jersey, 378 pp.
Lutgens, Frederick K., and Edward J. Tarbuck, 1998: Weather analysis and forecasting. In The Atmosphere: An Introduction to
Meteorology, seventh edition, Prentice Hall, Upper Saddle River, New Jersey, pp. 278-302.
Monmonier, Mark, 1999: Air Apparent: How Meteorologists Learned to Map, Predict, and Dramatize Weather, University of
Chicago Press, Chicago, 309 pp.
Moran, Joseph M., and Michael D. Morgan, 1994: Weather analysis and forecasting. In Meteorology: The Atmosphere and Science of
Weather, fifth edition, Prentice Hall, Upper Saddle River, New Jersey, pp. 374-401.
Parkinson, Claire L., 1985: Breakthroughs: A Chronology of Great Achievements in Science and Mathematics, 1200-1930, G. K. Hall,
Boston, 576 pp.
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Ryan, Robert T., 1982: The weather is changing... or meteorologists and broadcasters, the twain meet, Bulletin of the American
Meteorological Society, vol. 63, No. 3 (March 1982), p. 308.
Shuman, Frederick G., 1989: History of numerical weather prediction at the National Meteorological Center, Weather and
Forecasting, vol. 4, pp. 286-296.
(http://earthobservatory.nasa.gov/Library/WxForecasting/wx7.html)
Weather Proverbs
True or False?
People have been forecasting the weather for centuries. They once looked to plants and animals for hints about what the weather would do. For example, before it
rained, some people often observed that ants moved to higher ground, cows lay down, pine cones opened up, frogs croaked more frequently, and sheeps' wool
uncurled. Over the years, people began to notice other natural clues to upcoming weather, and several weather "sayings" grew up over the years.
When looking at weather proverbs, keep this in mind: They are usually based on someone’s observations and not on scientific studies. Because climates and
weather patterns differ throughout the world, a weather proverb based on observations in one location may not be valid in another location. Some proverbs arose
simply from coincidence, not weather patterns, and therefore may seldom hold true. But under certain circumstances, some proverbs do hold up to science. Here
are some that, under the right circumstances, have proven valid.
"Red Sky at night, sailor's delight. Red sky in the morning, sailor take warning."
This one has been around a long time. In fact, compare it with this Biblical passage from Matthew 16:1-3 : "When evening comes, you say,
'It will be fair weather, for the sky is red,' and in the morning, 'Today it will be stormy for the sky is red and overcast.' You know how to
interpret the appearance of the sky, but you cannot interpret the signs of the times."
When the western sky is especially clear, there is often a red sunset. That's because as the sun sets, its light shines through much more of
the lower atmosphere, which contains dust, salt, smoke and pollution. These particles scatter away some of the shorter wavelengths of light
(the violets and blues), leaving only the longer wavelengths (the oranges and reds.) If an area of high air pressure is present, the air sinks.
This sinking air holds air contaminants near the earth, making the sunset even redder than usual. This would be the “red sky at night.” In the
middle latitudes of the northern hemisphere, weather systems most often approach from the west. Since high pressure generally brings fair
weather, this type of red sky at sunset would indicate that clear weather is approaching, which would "delight" a sailor. If the sky is red in the
eastern morning sky for the same reasons as above, then the high pressure region has most likely already passed from west to the east, and
an area of low pressure may follow. Low pressure usually brings clouds, rain or storms, a warning for sailors.
"Mare's tails and mackerel scales make tall ships take in their sails."
A mackerel sky refers to cirrocumulus clouds, which often precede an approaching warm front, which will eventually bring veering winds
(changing from northeast and east over to southwest and west) and precipitation.
"Clear moon, frost soon."
If the atmosphere is clear, the surface of the earth will cool rapidly as heat is radiated away at night. There is no "blanket" of clouds to keep
the heat that the ground absorbed during the day from radiating back up into space. If the temperature is low enough on these clear nights
and there's no wind, frost may form.
"A year of snow, a year of plenty."
A continuous covering of snow on farmland and orchards delays the blossoming of fruit trees until the season of killing frosts is over. It also
prevents the alternate thawing and freezing which destroys wheat and other winter grains.
"Halo around the sun or moon, rain or snow soon."
The halo around the sun or moon is a layer of cirrus clouds made of ice crystals. These ice crystals act as tiny prisms, forming a white or
sometimes colorful halo around the sun or moon. This cirro-stratus cloud often indicates an approaching warm front and an associated area
of low pressure. Rain or snow will not always follow, but there is a higher probability of it after a halo is seen, and the brighter the circle, the
greater the probability.
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"Rainbow in the morning gives you fair warning."
In the morning, when the sun is in the east, the shower and its rainbow are in the west. As the weather in the mid-latitudes of the northern
hemisphere moves mostly from west to east, the morning rainbow indicates that rain is moving from the west toward the observer.
"When the stars begin to huddle, the earth will soon become a puddle."
When clouds increase, whole areas of stars may be hidden by clouds with groups of stars, still in the clear sky, seem to huddle together. The
clouds are increasing, so the chance of rain is increasing too.
A few more clues from nature:
Most animals are vulnerable to environmental changes that humans often can't detect. Swallows flying low may indicate the air pressure is
dropping. Falling pressure may affect the digestive system of cows, making them less willing to go to pasture, causing them to lie down.
Static electricity may increase the grooming activities of cats. The calls of some birds, including crows and geese, have been known to be
more frequent with falling pressure. Deer and elk sometimes react to wind and air pressure by coming down from mountains and seeking
shelter. Some species from rabbits to rattlesnakes to certain kinds of fish may feed more before a storm so they can seek shelter.
Some flowers close up as the humidity rises so rain doesn't wash away their pollen. The leaves of some trees curl just before a storm.
The higher the humidity, the better sound travels. Some English people gauged the chances of rain by the clarity with which they heard
church bells sound.
A drop in barometric pressure often affects people with joint diseases, bad teeth, recently healed broken bones, or corns and bunions,
bringing pain or pressure to those areas of the body.
Cicadas can't vibrate their wings when the humidity is very high, so may be silent when rain is approaching. Flying insects are more active
when the air pressure drops and stay closer to the ground, so they seem to be swarming before a rain storm.
The chirping of a cricket has been shown to provide a close indication of air temperature. By counting the number of cricket chirps in a 14second period and adding 40, the total will equal the air temperature to within one degree 75% of the time.
A final note:
Most of these natural forecasting methods are for the short range. Most long-range proverbs have no meteorological basis, including the
legend of the ground hog.
More about Weather Proverbs from USA Today
Special thanks to 1001 Questions Answered About the Weather by Frank H. Forrester and Weather and the Bible by Donald B. DeYoung
(http://www.wxdude.com/proverb.html)
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