Graeme Smith wrote:
Hi,

If I take 5 minute readings from a barometer what is the preferred/usual
method for calculating the rate of change over an hour.

Thanks,

Graeme

It would be very unsound to extrapolate a 5-minute change to
one hour because the barometer may only change by 0.1 or 0.2 mb in that
time. Can you read it that accurately, and is it that accurate anyway?
The only proper way to do it is to measure the pressure at the
beginning and end of the hour in question.

Tudor Hughes, Warlingham, Surrey.

"Graeme Smith" wrote in message
. uk...
Tudor Hughes wrote:
Graeme Smith wrote:
Hi,

If I take 5 minute readings from a barometer what is the preferred/usual
method for calculating the rate of change over an hour.

Thanks,

Graeme

It would be very unsound to extrapolate a 5-minute change to
one hour because the barometer may only change by 0.1 or 0.2 mb in that
time. Can you read it that accurately, and is it that accurate anyway?
The only proper way to do it is to measure the pressure at the
beginning and end of the hour in question.

Tudor Hughes, Warlingham, Surrey.


Sorry, I should have been clearer. I have continuous readings at 12 per
hour.

If I have a set of data like this (in 15 minute intervals rather than
the 5 I have for the sake of clarity):

Time Pressure
1000 1000
1015 1005
1030 1005
1045 1005
1100 1005
1115 1005
1130 1010

At 1100 if I take the 1000 and 1100 I can say that the pressure changed
5mb in one hour. At 1115 doing the same, except with 1015 and 1115, I
would have to say that the pressure is steady. But at 1130 I am back to
a change of 5mb.

So what I would like to know is given this sort of situation, how is the
rate of change normally calculated?

You should take your sensor back to the manufacturer. A change
of 5mbar in 15 minutes is well outside the bounds of experience,
at least in non-artificial conditions, at a fixed site in the UK ...
save, perhaps, for a tornado passing overhead. Although gravity
waves may, exceedingly rarely, approach it.

Pressure changes are normally smooth enough for an hourly value,
recorded on the hour, to provide more than enough information
about them. Shorter fluctuations associated with mesoscale
phenomena (e.g. thunderstorms) often merit closer inspection,
but then an hourly rate-of-change would not be relevant for them.

Philip

You should take your sensor back to the manufacturer. A change
of 5mbar in 15 minutes is well outside the bounds of experience,
at least in non-artificial conditions, at a fixed site in the UK ...
save, perhaps, for a tornado passing overhead. Although gravity
waves may, exceedingly rarely, approach it.

Pressure changes are normally smooth enough for an hourly value,
recorded on the hour, to provide more than enough information
about them. Shorter fluctuations associated with mesoscale
phenomena (e.g. thunderstorms) often merit closer inspection,
but then an hourly rate-of-change would not be relevant for them.

Philip

It was a gravity wave that produced the fastest pressure change I
have ever seen.. In January 1977 I was working for BP in Sunbury and
happened during a slack moment (there were many) to look at the lab
barometer. The mercury surface was visibly moving and the pressure
fell about 8 mb in 5 minutes. Not being aware of gravity waves I
thought the instrument had sprung a leak. There was no extraordinary
weather, a dull, drizzly breezy morning becoming for a few minutes
quite windy (gusts to force 7). The pressure stopped falling and
recovered at a more decorous rate. The time was about 10 a.m. and the
date (I think) the 19th. This must have been written up somewhere but
I forget where.

Tudor Hughes, Warlingham, Surrey.

Paul Hyett wrote:
In uk.sci.weather on Sat, 25 Mar 2006, Tudor Hughes
wrote :
It was a gravity wave that produced the fastest pressure change I
have ever seen.. In January 1977 I was working for BP in Sunbury and
happened during a slack moment (there were many) to look at the lab
barometer. The mercury surface was visibly moving and the pressure
fell about 8 mb in 5 minutes. Not being aware of gravity waves I
thought the instrument had sprung a leak.

So what *are* gravity waves?

Not exactly sure, but perhaps it's do to with a sudden displacement of
one surface relative to another (say a sudden depression in the sealevel
caused by an underwater earthquake or something). Are tsunamis a
particular type of gravity waves? I could imagine that the passage of
fronts might cause similar effects in the upper atmosphere.

--
Jonathan Stott
Canterbury Weather: http://www.canterburyweather.co.uk/
Reverse my e-mail address to reply by e-mail

In uk.sci.weather on Sun, 26 Mar 2006, Jonathan Stott
wrote :
Paul Hyett wrote:
In uk.sci.weather on Sat, 25 Mar 2006, Tudor Hughes
wrote :
It was a gravity wave that produced the fastest pressure change I
have ever seen.. In January 1977 I was working for BP in Sunbury and
happened during a slack moment (there were many) to look at the lab
barometer. The mercury surface was visibly moving and the pressure
fell about 8 mb in 5 minutes. Not being aware of gravity waves I
thought the instrument had sprung a leak.

So what *are* gravity waves?

Not exactly sure, but perhaps it's do to with a sudden displacement of
one surface relative to another (say a sudden depression in the sealevel
caused by an underwater earthquake or something).

Not much of that in Sunbury, I suspect...
--
Paul Hyett, Cheltenham