This depends on whether he was talking about airframe icing or carb icing,
and that isn't at all clear from the original post. Carb ice is quite
possible without visible moisture, but it isn't a major risk, if the pilot
is alert for signs of slow, progressive power loss, the aircraft has an
effective carb heat system, and the pilot uses it properly.

Airframe ice is quite often not seen even if you are flying in visible
moisture, i.e. in a cloud or in precipitation. But there are so many
variables at play that it is not really practical to nail it down to a
simple, single number that says whether you will collect ice or not. You
would need to have instruments to determine at least two numbers - mean
droplet diameter and liquid water content. But even that doesn't tell the
whole story as the spectrum of droplet diameters may vary quite a bit, so
the single measurement of mean droplet diameter doesn't give the whole
story.

Once you had all this real-time data, you would need to spend a whole
bunch of mental energy putting it together with the ambient temperature
and total air temperature to figure out whether you were in the icing
envelope or not. And to make it all much worse the icing envelope will
vary with type of aircraft, as some aircraft are more efficient collectors
of ice than other types. It is easier to just look out the window and see
if you are collecting ice.

Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com

On Sat, 17 Jan 2004 22:22:40 -0800, Corrie wrote:

It's not humidity, per se, but visible moisture that's the risk factor.
You don't need a gague to detect that.

"karel adams" wrote in message
...
depending on where one flies, icing is a major concern the risk of
icing depends on outside air temperature and humidity

but this puzzles me:
it is quite normal for an aircraft to have an ourside air termometer
but i never heard of an outside air hygrometer gauge

i am wrong once again
or is there a good explanation?

grateful for all your patience,
karel