|
meteoptic-l [ät] ursa.fi
message archive
This is meteoptic-l [ät] ursa.fi message archive.
Note, your can reply the messages on this page only if your
are already subscribed the list.
» To the end of the list/message
Alister Ling: "Green skies" (May 16, 23:31):
> The association of green clouds with severe weather is a "solid" one.
I'd
> be suprised if there wasn't a comment on this topic in Weatherwise
> magazine, since they frequently deal with questions from readers.
>
> In general, my view is that the severe weather clouds that are green
(and
> I too have pictures) are quite low, and simply reflect (backscatter) the
> green vegetation at ground level. Anyone seen green clouds above yellow
> fields? I'm not sure I can buy the color via scattering off right sized
> droplets hypothesis, since droplets are vastly larger than the
wavelength
> of light, and we enter Mie scattering range. Typically large particles
> scatter white light. In a severe thunderstorm, the droplets are going to
> be sized all over a alrge range, tending to imply that the resulting
> scatter would be white. Of course, if the terrain below is green, then
the
> cloud will take on that sheen.
>
>
> Alister.
>
The explanation that the green colour would have its origin in the
reflection
from the ground below seems reasonable. In a scattering process the
wavelength
of incident light is unchanged, so the only way the (single) scattering of
light
could cause colours is that the refractive index of a scatterer is quite
strongly the function of wavelenght, so that different colours would tend
to scatter in different angles. This, of course, cannot explain large
regions of sky being green. Also, water droplets do not scatter light
this way.
Another explanation that might work is that there are inpurities in
droplets, so that they not only scatter, but also absorb light. Actually,
the
impurities doesn't have to be IN the droplets, it may also be within them.
Immediately comes into my mind if wind blown grass or green leaves could
do the job... However, scattering of light by water droplets cannot be
the key, be the droplets small or large, irregular or spherical.
What comes to Mie theory, it is valid for homogeneous and isotropic
spheres of any size. It is also one only ways to calculate the
scattering from particles that are neither very small or very large
compared to the wavelength.
To summarize, it seems to me that the colour must be due to absorbing
inpurities in droplets or with them, if the origin is not green incident
light, reflected from surface. Of course, all the factors may be
present. Again, correct me if I'm wrong.
|