Tóm tắt
If the diffusivity K of a substance whose mass per volume of atmosphere is
χ
be defined by an equation of Fick’s type
ū
∂
χ
/∂
x
+
v
-
∂
χ
/∂
y
+
w
-
∂
χ
/∂
z
+ ∂
χ
/∂
t
= ∂/∂
x
(K ∂
χ
/∂
x
) + ∂/∂
y
(K ∂
χ
/∂
y
) ∂/∂
z
(K ∂
χ
/∂
z
), (1)
x
,
y
,
z
,
t
being Cartesian co-ordinates and time,
ū
,
v
-
,
w
-
being the components of mean velocity, then the measured values of K have been found to be 0·2 cm.
2
sec.
-1
in capillary tubes (Kaye and Laby’s Tables), 10
5
cm.
2
sec.
-1
when gusts are smoothed out of the mean wind (Akerblom, G. I. Taylor, Hesselberg, etc.), 10
8
cm.
2
sec.
-1
when the means extend over a time comparable with 4 hours (L. F. Richardson and D. Proctor), 10
11
cm.
2
sec.
-1
when the mean wind is taken to be the general circulation characteristic of the latitude (Defant). Thus the so-called constant K varies in a ratio of 2 to a billion. The present paper records an attempt to comprehend all this range of diffusivity in one coherent scheme. Lest the method which I shall adopt should strike the reader as queer and roundabout, I wish to justify it by showing first why some known methods are in difficulties.
