Similar statistical properties may be applied to
the index of refraction and one may define a
structure coefficient of the index of refraction 
,
related to 
by:
where 
is the wavelength.
Normally one considers as a reference the wavelength
= 500 nm and the previous equation becomes:
The seeing effect through an atmospheric layer of height
H can then be expressed an integral function of the index
of refraction structure coefficient 
,
whereby the Fried parameter 
is given by
where 
is the zenithal angle of the direction of
observation
.
Recalling equation (
), the FWHM of the seeing disk 
in arcsec is then given by:
where 
is the zenithal angle of the direction of observation.
For a vertical direction and 
= 500 nm, the
FWHM is expressed as:
Combining with equation (), for typical conditions of
astronomical mountain sites
(pressure 770 mb, temperature 10
) one obtains:
The diagram at fig. illustrates the order
of magnitude of the seeing effect with respect to a mean value of
and the integration distance.
Depending on the geometric scale of
the phenomenon causing seeing, the critical values of
will
be very different: for instance, if we set at 
0.1 arcsec an
arbitrary threshold for "bad" seeing from a single cause,
the corresponding critical (mean) value of 
will be
10 m): 
10
K
m
30 m):
K
m
1-40 mm) the
critical 
values are much higher, of
the order of 10 K
m
.
) and following show that the overall seeing
effects may be considered a 5/3-exponent sum of different terms
corresponding to the different atmospheric layers. Thus if 
is natural seeing FWHM, caused by the boundary layer and upward, and
is the local seeing, the overall seeing is:
