"Frank Ch. Eigler" wrote in message
...

"Bob Gardner" writes:

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. [...]

Does that refer to surface or upper winds?

[...] I think your .5 inch figure is overstated.

Well, today it's not hard to find two places on the continent with a
0.50 difference in altimeter settings: any place under the big storm
in the north east, and another place far enough, like KORD. That's
only a couple hours' flight in our bugsmashers.



Yes, Detroit to JFK would be just about .5 this evening.

That translates to 500 feet down-error, even if you ignore the "fly toward
the low, look out below" mantra and never adjust the altimeter. Allowing
200 feet altitude-maintenance error, you will still have 300 feet buffer
from the adjacent flight level.

Unless, of course, someone is flying in the opposite direction ALSO ignoring
the mantra, in which case his error will be UP, while yours is DOWN, and
interesting things may result.

So know your weather situation before takeoff, and get QNH updates
frequently along the route in such extreme situations.