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115
i 10 Sill <p
G eos a
k
Disse Formler gjælde strengt taget kun for det
Tilfælde, at man har retliniede, æquidistante Isobarer og jevn
Bevægelse. Af Kartet, Pl. XXXI, vil man se, at disse
Betingelser temmelig nær tilfredsstilles for store Strækninger
af Nordhavet. Centrifugalkraften, der nærmest skulde
blive virksom i Østhavet, bliver ringe, da Vindens
Hastighed paa de respective Steder ikke er stor. Da, som senere
skal vises, Vindens Virkning ikke overalt directe kan
overfores paa Havets Bevægelse, bliver den exacte Bestemmelse
af Vindens Retning og Hastighed af mindre Betydning.
Jeg har derfor anvendt de ovenstaaende Formler uden
Modification.
Gradienten er taget af Isobarerne paa Kartet PI.
XXXI. Ved Hjelp af Tversnit, lagte lodret paa Isobarerne,
eller i Gradientens Retning, i hvilke 10 mm i vertical
Retning forestillede en Lufttrykforskjel af 1 mm, og
Grundlinien havde Kartets Maalestok, bestemtes Beliggenheden af
Isobaren for hver Tiendedel Millimeter og indtegnedes i
Arbejdskartot. Dernæst construeredes paa Millimeterpapir
en Skala, der med Argument (i horizontal Retning):
Afstand paa Kartet gav som Function (i vertical Retning,
Gradient af 1 mm = Ordinat paa 100 mm.): den
tilsvarende Gradient. Da Gradientens Størrelse er omvendt
proportional med Isobarernes indbyrdes Afstand, havde
denne Skala Fonn af en ligesidet Hyperbel. Ved at
udmaale paa Kartet Afstanden mellem Isobarerne for en
Lufttrykforskjel af 1 mm, der svarer til det Punkt, for
hvilket man vil beregne Vindens Retning og Hastighed
(lu Gange Afstanden mellem Isobarerne for 0.1 mm),
afsætte denne Afstand som Abscisse paa Skalaen, og søge
Størrelsen af den dertil svarende Ordinat, finder man i
denne den søgte Størrelse af Gradienten.
Størrelsen q, Massen af en Kubikmeter Luft,
bestemmes paa følgende Maade. Ved det absolute Lufttryk
760 mm, 0° og Normaltyngden vejer et Kilogram tør Luft1
1.293052 Kilogram. Sættes Normaltyngden (45° Bredde,
Havets Overflade) efter Listing til 9.806165 bliver Massen
af en Kubikmeter Luft under de anførte Forhold
1.293052
o, = = 0.1318611
* 9.806165
Er det absolute Lufttryk b mm, Luftens Temperatur
t° C., og indeholder den Vanddamp af e mm Tryk. bliver
G sin « G eos «
’ — 2 to sin tf K’.
These fornmlæ apply in a strict sense only for
rectilinear, equidistant isobars and a uniform motion. From
the map, Pl XXXI, we see that full compliance with such
conditions is nearly found for extensive tracts of the North
Ocean. Centrifugal force, which might be assumed to
exert its chief influence in the Barents’ Sea, is but
trifling, the wind having no great velocity in the respective
localities. But since, as will subsequently appear, the effect
of the wind cannot be everywhere transferred direct to
the motion of the sea, the exact determination of the wind’s
direction and velocity is of less moment. I have therefore
applied the above-given fornmlæ without modification.
The gradient has been taken from the isobars in the
map, Pl. XXXI. By means of transverse sections, laid
perpendicular to the isobars, or in the direction of the
gradient, in which 10 mm. in a vertical direction
represented a difference in atmospheric pressure of 1 mm. and in
which the scale of the base was that of the map, the position
of the isobar was determined for every tenth of a millimetre
and marked off in the working-map. I then constructed on
ruled paper a scale, which, with argument (horizontal
direction): distance on map, gave as function (vertical
direction, gradient of 1 mm. = ordinate of 100 mm.): the
corresponding gradient. The magnitude of the gradient being
inversely proportional to the respective distances between
the isobars, this scale had the form of an equilateral
hyperbola. By measuring out on the map the distance between
the isobars for a difference in pressure of 1 mm., that
corresponds to the point for which the direction and velocity
of the wind has to be computed (10 times the distance
between the isobars for 0.1 mm.), then setting off this distance
as an abscissa on the scale and seeking the magnitude of
the corresponding ordinate, we shall find therein the required
value of the gradient.
The quantity q, or the mass of a cubic metre of air,
was determined in the following manner. At the absolute
pressure 760 mm., 0°, and the normal gravity, one kilogramme
of dry air1 weighs 1.293052 kilogramme. Now, if we put
the normal gravity (lat. 45°, sea-level), according to
Listing. at 9.8°6165, the mass of a cubic metre of air under
the said conditions will be —
1.293052
0.806165 "
0.1318611.
Assuming the absolute pressure at b mm., the
temperature of the air at t° C., and the latter to contain aqueous
vapour of e nun. pressure, then
1
1 + 0.00367 t
0.3779 e 273 b—0.3779 e
= * 760 • 878 +« = °-047366’ 273 + †
O. J. Broch. Poids du litre d’air atmosphérique. Travaux et mémoires du comitc international des poids et mesures.
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