Re: reflectivity of clouds vs. oceans
[ Follow Ups ] [ Post Followup ] [ Class Discussion Groups ] Posted by James Risbey on September 16, 1997 at 18:48:21:
In Reply to: reflectivity of clouds vs. oceans posted by shane frederick on September 16, 1997 at 14:59:23:
: The earth is about 70% covered by oceans.
: When I look at those satellite pictures of the earth,
: it only looks like its usually about 30% cloudy, overall.: Both clouds and the oceans surface are liquid water.
: Why do the clouds reflect so much more sunlight,
: than the oceans do?Dear Shane,
Your estimate of cloud cover is close, but I think it is supposed to
be about 50%. Thus you could say that the surface (mostly ocean) and
clouds both intercept about half each of the incoming solar radiation.
Yet clouds scatter the majority, hence your question.
The intuition behind your question that scattering should increase in
the oceans (which are a film of water) over clouds (which are a
suspension of water droplets) is a good one. For instance, clouds
(water droplets) scatter more shortwave radiation than water vapour,
even though it is water molecules doing the scattering in both cases.
The difference is that water in droplet form scatters much more
coherently than water in vapour form. Similarly, water in a film
scatters more coherently than water in a droplet and so is a more
efficient scatterer.
So why is it that the ocean only scatters 5 units of incoming solar
radiation but clouds scatter 25 units? The answer I think lies in the
relative amount of scattering and absorption that occurs in each case.
As a suspension of droplets, clouds present a relatively high surface
area. This increases the efficiency of scattering relative to
absorption, and about half the solar radiation is scattered in a cloud
and about half is absorbed. As a film, the oceans present a relatively
small surface area to the incoming solar radiation and the efficiency
of scattering relative to absorption is decreased. In the oceans most
of the incoming solar radiation is absorbed. Once a photon has been
absorbed it is not available for scattering. Thus the oceans
scatter a much smaller fraction of the incoming solar radiation than
clouds do.