According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. Increase the distance to 1000
miles and the windspeed will be 40 mph after three hours. Don't know about
you, but 80 mph surface winds are beyond my meager skills. I think your .5
inch figure is overstated.

Bob Gardner

"O. Sami Saydjari" wrote in message
...
Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point
substantially. Even a 0.25 inch change is substantial, and if there is
not a standard of when one changes settings, it seems like a risk, albeit
small.

My experience is that ATC does not give me settings of nearby airports.
For example, I recall entering Chicgo Center airspace from the south. I
was just north of Lafayette, Indiana. The controller gave me the
altimeter setting for Chicago Midway. That is a long way away. Lafayette
airport was at least 0.1 inches different. I used ATC's setting, because
I figure they must have some standard reference point in a region, or
sub-region, so that all the planes are at least on the same relative
setting.

-Sami
N2057M, Piper Turbo Arrow III

A Lieberman wrote:

On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote:


0.5 inches is a HUGE altimter setting change. Are you sure about
that?


I have to agree with Roy here. I just finished coming back from Ohio to
Mississippi, and I experienced a
very large altimeter setting adjustments in my journey southbound north
of
Bowling Green KY, but I sure did not experience .50 inches adjustments en
route. I did end up .25 inch less over the whole trip though.

If I remember correctly on one ATC hand off, I went from 29.96 to 29.88
which lowered my altitude by 80 feet. This really alarmed me as I sure
don't like deviating more then 20 feet from assigned altitude. Needless
to
say, I made a quick climb to my assigned altitude!

There was a rather steep pressure gradient even in my trip I just did
this
week, but as Roy says, .5 inch altimeter change in a flight range
distance
in planes I fly is probably indicative of weather I wouldn't / shouldn't
be
flying in anyway.

Allen