"Col" wrote in message
...

"GKN" wrote in message
news
Hi Will.
I am not sure as to its actual thickness, but it looked at times that it
should have been pouring with rain. Lets say I have seen heavier rain
with the sun shining through thinner cloud.


For proper rain as opposed to drizzle you need upward motion and the correct
microphysics within the cloud. Either enough moisture to produce raindrops
by coalescence or (more normally in UK) the presence of ice crystals.
Thinnish altostratus with sun visible will produce rain as the air is
ascending normally due frontal ascent and there are ice crystals allowing
rain production by the Bergeron Findeisen process, i.e. tiny water cloud
droplets evaporate and condense onto ice crystals eventually falling out as
snow then melting to give rain. In mountain terrain thinnish stratus or
stratocumulus cloud can produce copious rainfall if very moist air is
continually lifted over the high ground and the stratus clouds are seeded
with rainfall/snow from above. So I hope you can see that optical thickness
(darkness) of clouds is not always the best guide as to what clouds will
give a lot of rain, one has to understand the physics and cloud dynamics as
well!

HTH

Cheers,

Will
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