"Alan White" wrote in message
...
On Sat, 25 Mar 2006 10:56:40 +0000, Jonathan Stott
wrote:

I'd also be interested in this! At the moment I take the difference
between the reading 3 hours ago and now and use that as the rate of
change. I think that 3 hours is the preferred length of time for rate of
change of pressure, although I may be wrong!

It would be nice to know. ATM I'm using two hours.

The 3-hour unit, if that's not an oxymoron, has been used in
observations for decades, simply because that is the interval between
one main/intermediate synoptic observation and the next, and the
change in pressure is allied to a "tendency" description which
may possibly be in the FAQ.

Any bench forecaster, therefore, will have a much better feel
for a 3-hr change in pressure than a 1-hr change in pressu
e.g., we all recognise that a fall of 4.5mbar in 3 hours is quite
fast and is very likely to be synoptically significant, but we would
have to think about (and probably multiply it by 3) if we were
told that the pressure had fallen 1.5mbar in 1 hour.

That's the only advantage of using the 3-hour unit I can think of.
Otherwise 1 hour makes more sense.

Philip