TEMPERATURA - UMIDITA' - PRESSIONE
TEMPERATURA - UMIDITA' - PRESSIONE
TEMPERATURA - UMIDITA' - PRESSIONE
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<strong>TEMPERATURA</strong> - UMIDITA’ - <strong>PRESSIONE</strong><br />
Le capanne meteorologiche svolgono la funzione di schermi per la radiazione solare.<br />
Devono consentire la libera circolazione dell’aria attorno agli strumenti (termometri,<br />
igrometri) alloggiati all’interno.<br />
La capanna meteorologica della stazione di Trieste del DST (disegno e costruzione<br />
locali), sul tetto dell’Istituto Nautico in piazza Hortis. La porta è a nord; sul tetto della<br />
capanna il collettore del pluviografo.
Interno della capanna precedente con gli strumenti convenzionali, meccanici:<br />
barotermoigrografo SIAP, termoigrografo Salmoiraghi, pluviografo SIAP, evaporigrafo;<br />
psicrometro ad aspirazione di August (i due termometri verticali con ventola),<br />
termometro a massima (orizzontale, sopra) e a minima (orizzontale, sotto).<br />
Barotermoigrografo SIAP:<br />
viti di taratura per l’amplificazione (a) Termoigrografo SIAP<br />
e per la costante (c).
Termoigrografo Salmoiraghi: lamina bimetallica (temperatura) e fascio di capelli<br />
(umidità). Il modello a sinistra, al posto del diagramma su foglio, avvolge un rotolo<br />
mensile montato su di un supporto a parte.<br />
Modello di capannina commerciale (SIAP) con barotermoigrografo e pluviografo a<br />
vaschette oscillanti.
Schermi solari per i sensori di temperatura e umidità Micros (stazione di Trieste DST) e<br />
piranometro. La centralina per l’acquisizione programmata dei dati (a destra).<br />
Schermi solari per sensori di temperatura/umidità.
La capanna del Talassografico (modello Mazelle, 1903) a Trieste ed il suo interno:<br />
termoigrografo, evaporigrafo, termografo (grande modello), evaporimetro; psicrometro,<br />
termometri di massima e minima
Psicrometro ad aspirazione di August (fisso, da capanna, a sinistra); psicrometro ad<br />
aspirazione portatile di Assmann; psicrometri a rotazione manuale (sotto).
Termoigrometro digitale. Registra i dati su memorie interne ad intervalli di tempo<br />
programmabili
Istituto Talassografico di Trieste: stanza dei barometri. Barometri a mercurio di controllo<br />
(a sinistra) e barografo Sprung-Fuess (a destra).<br />
Barografo Fuess<br />
sistema Sprung<br />
E’ una bilancia:<br />
f fulcro<br />
P peso mobile<br />
F forza esercitata dalla pressione<br />
atmosferica sulla superficie So<br />
F a = P x<br />
La distanza x è variata<br />
automaticamente in modo da<br />
garantire l’equilibrio tra i punti 1 e<br />
2. I movimenti sono comandati<br />
dall’orologio. La tabella sostiene il<br />
diagramma e funge da peso che,<br />
cadendo, mantiene il moto del<br />
pendolo. Il contrappeso P sostiene<br />
il pennino scrivente. La densità e<br />
l’altezza h del mercurio sono<br />
irrilevanti: la sola funzione della<br />
canna è di mantenere il vuoto<br />
torricelliano di riferimento.
Barometro Salmoiraghi n. 437, tipo Fortin.<br />
(1) leggere la temperatura dello strumento (2) regolare il fondo per portare il mercurio<br />
nella vaschetta a coincidere con la punta (zero della scala) (3) leggere l’altezza del<br />
mercurio al nonio (4) applicare le correzioni.
Barografo aneroide. L’elemento sensibile è costituito da una batteria di capsule fissate<br />
sul fondo; l’estremità superiore si schiaccia o si innalza al variare della pressione<br />
atmosferica; il movimento amplificato da una leva aziona il pennino. Trasmissione e<br />
sistema di regolazione (amplificazione e costante) sono simili a quelli adottati per altri<br />
sensori (temperatura, umidità). A sinistra un barometro, stesso principio.<br />
Sensori di pressione Vaisala: modello analogico (sinistra) e digitale con uscita seriale,<br />
risoluzione 1 Pa (destra).
2. Psychrometric computations<br />
The percentual relative humidity is commonly measured, in practical meteorology, by means of<br />
the aspirating psychrometer. This<br />
instrument yields the dry (θ) and wet (θw ) bulb Celsius temperatures. Sprung's psychrometric<br />
formula:<br />
e(θ) = ew (θw ) - A p (θ - θw ) (1)<br />
relates the water vapour pressure e, the water vapour pressure at saturation ew , the psychrometric<br />
temperature lapse and the atmospheric pressure p. The Ferrel's coefficient:<br />
A = 0.000660 (1 + 0.00115 θw /°C) °C-1 (2)<br />
is used for ventilations in the range 4-10 m/s. The water vapour pressure at saturation as a<br />
function of the air temperature is given by the Goff-Gratch formula (Stravisi 1988); using the<br />
natural logarithm, this is:<br />
L1 = 10.79574 T2 - 2.183633 log(T3 ) + 0.78614 ,<br />
L2 = 1.50475 x 10-4 (1 - 10 (-8.2969(T3 - 1)) ) ,<br />
L3 = 4.2873 x 10-4 (10 (4.76955 T2) - 1) ;<br />
ew (T)= 10 (L1 + L2 + L3) ;<br />
T = θ + To ,<br />
To = 273.15 K ,<br />
T1 = 273.16 K ,<br />
T2 = 1 - T1 /T ,<br />
T3 = T/T1 .<br />
(3)<br />
The moist atmosphere is characterized by p, θ, θw , ε, εw above and by the following quantities:<br />
mole fraction : e/p<br />
mixing ratio : r = ε e/(p - e)<br />
virtual temperature : Tv = T(1 + r/ε)/(1 + r)<br />
adjusted virtual temperature : Tv' = 0.9995 Tv moist air density : ρ = p/(R Tv') specific humidity : q = r/(1 + r)<br />
absolute humidity : dv = ρ q<br />
relative humidity : u = (e/ew )(p - ew )/(p - e)<br />
percentual relative humidity : U = 100 u (4)<br />
R = 287.053 J.kg-1 .K-1 is the dry air constant and ε = 0.62198 is the molar mass ratio of water<br />
vapour and dry air. Further details are given by Stravisi (1988).<br />
Given p, θ, θw , quantities above are computed by the BASIC program "PSYBAR" described in<br />
section 4.<br />
3. The mercury barometer<br />
The mercury barometer is the reference instrument for measuring the atmospheric pressure in<br />
a meteorological station. The atmospheric pressure is balanced by the pressure of the mercury<br />
column plus the pressure (p c ) resulting from the different curvature of the meniscus at the top of<br />
the mercury column and in the container:<br />
p = ρ m g h + p c . (5)<br />
The mercury pressure is a function of the local gravity g, of the mercury density ρ m at the<br />
barometer temperature θ b and of the column height h above the mercury level in the container.<br />
The thermal dilatation of mercury inside the barometer is linear, and:<br />
ρ m = ρ o (1 - µ θ b ) , (6)<br />
µ = 1.818 x 10 -4 °C -1 .<br />
Introducing the standard gravity g o , (5,6) give:<br />
p = (g/g o ) (1 - µ θ b ) ρ o g o h + p c . (7)
Using SI units (IAPSO, 1979), the numerical values of the physical quantities in (7) are defined<br />
as follows:<br />
P = p/Pa ,<br />
Ro Go = ρ o /(kg.m -3 ) = 13.5951 x 10 3 ,<br />
= go /(m.s-2 ) = 9.80665 ,<br />
RoGo = 1.33322387 x 105 ,<br />
H = h/m , (8)<br />
The equation between numerical values corresponding to (7) is:<br />
P = (g/go ) (1 - µ θb ) Ro Go H + Pc . (9)<br />
The mercury column height h is commonly measured on a scale drawn on the barometer, using a<br />
conventional unit of length. We can call the length unit at present in use for barometers the<br />
"barometer hectopascal", symbol σ. The barometer hectopascal increases with temperature<br />
according to the linear dilatation of the material which supports the scale drawings:<br />
σ = σo (1 + β θb ) . (10)<br />
For a brass ruler it is:<br />
β = 0.184 x 10-4 °C-1 .<br />
The barometer hectopascal at 0 °C is a constant unit of length; it is defined by:<br />
1 σo = 102 (RoGo ) -1 m . (11)<br />
The mercury column height is therefore<br />
h = H m = B σ , (12)<br />
where B is its numerical value on the barometer scale, and<br />
H = B σ/m = 102 B (1 + β θb ) (RoGo ) -1 . (13)<br />
Substituting (13) in (9):<br />
10-2 P = (g/go ) (1 - a θb ) B + 10-2 Pc , (14)<br />
where a = µ - β = 1.6339 x 10-4 °C-1 (15)<br />
is the linear dilatation of mercury with respect to brass. The numerical value of the atmospheric<br />
pressure in hectopascals is therefore (14, 8, 9) given by:<br />
p/hPa = (g/go ) (1 - a θb ) h/σ + pc /hPa . (16)<br />
Formula (16) is used to convert the barometer reading B=h/σ in "barometer hectopascals" into the<br />
corresponding pressure value in hectopascals; local gravity, the capillary correction of the<br />
instrument and its temperature must be known. The reading reduction to 0 °C can be computed<br />
also as<br />
(1-a θb ) B = B - C(θb ) ;<br />
C(qb ) are reported in Table 1.<br />
If the barometer scale is in "millimeters", pressure values in (16)<br />
are substituted by the corresponding values expressed in<br />
"conventional millimiters of mercury". This unit, whose use is<br />
"strongly discouraged" (IAPSO, 1979), is defined by:<br />
1 mmHg = RoGo x 10-5 hPa = 1.333 223 87 hPa .<br />
Measurement of the mercury height. A reference point is<br />
used in the Fortin barometer to define h=0 (Fig.1); the mercury<br />
level is set to zero by adjusting the volume of the bulb. In the fixedbulb<br />
barometers this operation is avoided, since the scale is<br />
compensated for zero level changes according to the surface ratio<br />
between capillary and bulb. A third method used in some<br />
barometers is taking the difference between the heights of the<br />
upper and lower mercury levels. If the bulb diameter is the same of<br />
the barometer tube at the reading level, the capillary corrections<br />
cancel each other. The correction for the capillary depression is<br />
given by:<br />
pc /hPa = 0.087 + 0.063 X/σ ,<br />
Fig. 1. Fortin barometer.<br />
where X/σ is the meniscus height in barometer hectopascals.<br />
Pressure reduction to sea level. The exact computation of the
sea level pressure p(0) requires the knowledge of the vertical profile of the air density up to the<br />
station height z. An approximate solution is usually given by assuming a constant air temperature<br />
for the atmospheric layer below z. Integration of the equation of state then yields:<br />
p(0) = p(z) exp(g z/(R T v ')) ; (17)<br />
and, for typical atmospheric conditions,<br />
p(0) = p(z) exp(1.184 x 10 -4 z/m) . (18)<br />
For a small station height (below about 100 m) a constant density can be assumed, and<br />
p(0) = p(z) + ρ(z) g z . (19)<br />
°C 950 960 970 980 990 1000 1010 1020 1030 1040 1050<br />
1 0.16 0.16 0.16 0.16 0.16 0.16 0.17 0.17 0.17 0.17 0.17<br />
2 0.31 0.31 0.32 0.32 0.32 0.33 0.33 0.33 0.34 0.34 0.34<br />
3 0.47 0.47 0.48 0.48 0.49 0.49 0.50 0.50 0.50 0.51 0.51<br />
4 0.62 0.63 0.63 0.64 0.65 0.65 0.66 0.67 0.67 0.68 0.69<br />
5 0.78 0.78 0.79 0.80 0.81 0.82 0.83 0.83 0.84 0.85 0.86<br />
6 0.93 0.94 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 1.03<br />
7 1.09 1.10 1.11 1.12 1.13 1.14 1.16 1.17 1.18 1.19 1.20<br />
8 1.24 1.25 1.27 1.28 1.29 1.31 1.32 1.33 1.35 1.36 1.37<br />
9 1.40 1.41 1.43 1.44 1.46 1.47 1.49 1.50 1.51 1.53 1.54<br />
10 1.55 1.57 1.58 1.60 1.62 1.63 1.65 1.67 1.68 1.70 1.72<br />
11 1.71 1.73 1.74 1.76 1.78 1.80 1.82 1.83 1.85 1.87 1.89<br />
12 1.86 1.88 1.90 1.92 1.94 1.96 1.98 2.00 2.02 2.04 2.06<br />
13 2.02 2.04 2.06 2.08 2.10 2.12 2.15 2.17 2.19 2.21 2.23<br />
14 2.17 2.20 2.22 2.24 2.26 2.29 2.31 2.33 2.36 2.38 2.40<br />
15 2.33 2.35 2.38 2.40 2.43 2.45 2.48 2.50 2.52 2.55 2.57<br />
16 2.48 2.51 2.54 2.56 2.59 2.61 2.64 2.67 2.69 2.72 2.74<br />
17 2.64 2.67 2.69 2.72 2.75 2.78 2.81 2.83 2.86 2.89 2.92<br />
18 2.79 2.82 2.85 2.88 2.91 2.94 2.97 3.00 3.03 3.06 3.09<br />
19 2.95 2.98 3.01 3.04 3.07 3.10 3.14 3.17 3.20 3.23 3.26<br />
20 3.10 3.14 3.17 3.20 3.24 3.27 3.30 3.33 3.37 3.40 3.43<br />
21 3.26 3.29 3.33 3.36 3.40 3.43 3.47 3.50 3.53 3.57 3.60<br />
22 3.41 3.45 3.49 3.52 3.56 3.59 3.63 3.67 3.70 3.74 3.77<br />
23 3.57 3.61 3.65 3.68 3.72 3.76 3.80 3.83 3.87 3.91 3.95<br />
24 3.73 3.76 3.80 3.84 3.88 3.92 3.96 4.00 4.04 4.08 4.12<br />
25 3.88 3.92 3.96 4.00 4.04 4.08 4.13 4.17 4.21 4.25 4.29<br />
26 4.04 4.08 4.12 4.16 4.21 4.25 4.29 4.33 4.38 4.42 4.46<br />
27 4.19 4.24 4.28 4.32 4.37 4.41 4.46 4.50 4.54 4.59 4.63<br />
28 4.35 4.39 4.44 4.48 4.53 4.57 4.62 4.67 4.71 4.76 4.80<br />
29 4.50 4.55 4.60 4.64 4.69 4.74 4.79 4.83 4.88 4.93 4.98<br />
30 4.66 4.71 4.75 4.80 4.85 4.90 4.95 5.00 5.05 5.10 5.15<br />
31 4.81 4.86 4.91 4.96 5.01 5.07 5.12 5.17 5.22 5.27 5.32<br />
32 4.97 5.02 5.07 5.12 5.18 5.23 5.28 5.33 5.39 5.44 5.49<br />
33 5.12 5.18 5.23 5.28 5.34 5.39 5.45 5.50 5.55 5.61 5.66<br />
34 5.28 5.33 5.39 5.44 5.50 5.56 5.61 5.67 5.72 5.78 5.83<br />
35 5.43 5.49 5.55 5.60 5.66 5.72 5.78 5.83 5.89 5.95 6.00<br />
36 5.59 5.65 5.71 5.76 5.82 5.88 5.94 6.00 6.06 6.12 6.18<br />
37 5.74 5.80 5.86 5.92 5.98 6.05 6.11 6.17 6.23 6.29 6.35<br />
38 5.90 5.96 6.02 6.08 6.15 6.21 6.27 6.33 6.40 6.46 6.52<br />
39 6.05 6.12 6.18 6.24 6.31 6.37 6.44 6.50 6.56 6.63 6.69<br />
40 6.21 6.27 6.34 6.40 6.47 6.54 6.60 6.67 6.73 6.80 6.86<br />
TABLE 1. Corrections to reduce mercury barometer readings (950-1050) at given instrument<br />
temperature (1-40 °C) to the 0 °C scale.