I wonder if someone here can help me. Ive just bought a very nice Suunto
Observer watch which is great and gadgety and does altimeter readings,
compass readings temperature, barometric readings and even tells the time.
Ive looked around the net, and read the instruction manual for the watch
loads of times, but cannot find the answers to my searching questions.
I went to my local beach and set the altimeter to zero. This apparently
automatically sets the sea level pressure in the watch also. The watch
displays in big bold type the current sea level pressure, which obviously
rises or falls. Underneath this, in smaller type, the watch displays
current atmospheric pressure which also rises or falls. My question is that
the atmospheric pressure is always 5 digits behind the sea level pressure,
always. The sea level pressure is currently recorded on the watch as 1015,
and the atmospheric pressure at 1010, 5 behind again. Is this always the
case or should it fluctuate a bit?
Also, fair enough the watch can say what the current atmospheric pressure
is, because it has a sensor, but how can it know what the sea level pressure
is if its not at sea level?
Also, as a rule of thumb, how much does pressure need to rise or fall in a 3
hour period to give a strong indication of changing weather?
It fell today (Cornwall UK) by about 20 hPa, which would seem to me to be a
big drop, but the weather just turned from cloudy and dry to cloudy and a
bit drizzly. I think I half expected such a big fall in pressure to
indicate at least a hurricane, but maybe 20 hPa isnt a big fall in your
eyes?
Sorry to ramble on, but I do find all this very interesting.
Thanks for any help,
CHEERS
Stuart

"Stuart" wrote in message
...
|I wonder if someone here can help me. Ive just bought a very nice Suunto
| Observer watch which is great and gadgety and does altimeter readings,
| compass readings temperature, barometric readings and even tells the time.
| Ive looked around the net, and read the instruction manual for the watch
| loads of times, but cannot find the answers to my searching questions.
| I went to my local beach and set the altimeter to zero. This apparently
| automatically sets the sea level pressure in the watch also. The watch
| displays in big bold type the current sea level pressure, which obviously
| rises or falls. Underneath this, in smaller type, the watch displays
| current atmospheric pressure which also rises or falls. My question is
that
| the atmospheric pressure is always 5 digits behind the sea level pressure,
| always. The sea level pressure is currently recorded on the watch as
1015,
| and the atmospheric pressure at 1010, 5 behind again. Is this always the
| case or should it fluctuate a bit?
| Also, fair enough the watch can say what the current atmospheric pressure
| is, because it has a sensor, but how can it know what the sea level
pressure
| is if its not at sea level?
| Also, as a rule of thumb, how much does pressure need to rise or fall in a
3
| hour period to give a strong indication of changing weather?
| It fell today (Cornwall UK) by about 20 hPa, which would seem to me to be
a
| big drop, but the weather just turned from cloudy and dry to cloudy and a
| bit drizzly. I think I half expected such a big fall in pressure to
| indicate at least a hurricane, but maybe 20 hPa isnt a big fall in your
| eyes?

I've always wondered how these watches separate pressure changes from
altitude changes. As you say, there is only one pressure sensor in the
watch and it is only capable of measuring the pressure where the watch is.

As I understand it, a pressure altimeter needs to be set to the appropriate
sea-level pressure for the desired location. This is one reason why
airports take weather observations - the corrected pressure reading is set
in an approaching (or passing) aircraft's altimeter so the pilot knows he
will encounter the runway when his altimeter shows the height of the airport
above sea level.

The electronic barometer I have invites me to input my altitude above sea
level and then gives me the corrected pressure. This is effectively the
opposite way round to the altimeter.

Presumably there are two modes for this watch - barometer and altimeter. If
you choose the first, it assumes you are remaining close to one place and
holds a constant correction between actual and sea level pressure - which
matches what you describe. If the second, it assumes you want to measure
variation from a standard pressure and convert this to altitude.

A 20hPa fall of pressure is not as dramatic in UK latitudes as it would be
in, say, the southern states of the USA because the coriolis force is higher
here and so a given pressure gradient produces a lower balancing windspeed.
Such a fall in a day would still indicate a disturbed situation with
probable strong winds and would be unlikely between the end of March and
mid-September when the reduced temperature gradient between poles and
tropics normally means less intense weather systems.

However, a 20hPa fall of pressure is consistent with a climb of only 200-300
metres. Cornwall is rather hilly...
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"Yokel" wrote in message
...
"Stuart" wrote in message
...

|I wonder if someone here can help me.

snip

I've always wondered how these watches separate pressure changes from
altitude changes. As you say, there is only one pressure sensor in the
watch and it is only capable of measuring the pressure where the watch is.

That is correct. It does say in the manual that the watch can only be used
as a barometer or altimeter at one time. As you say, this will be because
it will only measure pressure where the watch is. (Its a bit sad that I had
to have someone point that out to me!)


As I understand it, a pressure altimeter needs to be set to the
appropriate
sea-level pressure for the desired location. This is one reason why
airports take weather observations - the corrected pressure reading is set
in an approaching (or passing) aircraft's altimeter so the pilot knows he
will encounter the runway when his altimeter shows the height of the
airport
above sea level.

Yes, the watch automatically sets the sea level pressure when you input the
current height above sea level. I didn't understand why it was so important
to know the pressure at sea level if I wasn't AT sea level, but I take it
that you need to have a 'yardstick' to measure against? What I don't
understand is, with a barometer one needle stays the same showing the
current pressure, and the other needle moves showing the amount of pressure
falling or rising. My watch shows sea level pressure and current air
pressure (in barometer mode, nothing to do with altimeter) but both readings
move up or down together. What would be the point of showing both readings
if they always move the same amount together?

The electronic barometer I have invites me to input my altitude above sea
level and then gives me the corrected pressure. This is effectively the
opposite way round to the altimeter.


Presumably there are two modes for this watch - barometer and altimeter.
If
you choose the first, it assumes you are remaining close to one place and
holds a constant correction between actual and sea level pressure - which
matches what you describe. If the second, it assumes you want to measure
variation from a standard pressure and convert this to altitude.

Bang on. The manual advises if you are hiking for instance, to use the
altimeter while walking, and when camping (staying put) to switch to
barometer mode.

A 20hPa fall of pressure is not as dramatic in UK latitudes as it would be
in, say, the southern states of the USA because the coriolis force is
higher
here and so a given pressure gradient produces a lower balancing
windspeed.
Such a fall in a day would still indicate a disturbed situation with
probable strong winds and would be unlikely between the end of March and
mid-September when the reduced temperature gradient between poles and
tropics normally means less intense weather systems.

However, a 20hPa fall of pressure is consistent with a climb of only
200-300
metres. Cornwall is rather hilly...

I understand what your saying, if I had the watch in barometer mode and
walked up a hill it would show a drop in pressure falsely indicacting a
weather change. I was actually in the same place for the day (working on a
roof, which is why I think its important to know when that rains coming!)
but that is a very good point that I didnt realise and will now take into
account.

Thanks again for the time you've taken to help me out, its most appreciated.
CHEERS
Stuart


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OO 0 OO
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) ( /\ ) (

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"Stuart" wrote in message
...
|
|...
|
| Yes, the watch automatically sets the sea level pressure when you input
the
| current height above sea level. I didn't understand why it was so
important
| to know the pressure at sea level if I wasn't AT sea level, but I take it
| that you need to have a 'yardstick' to measure against? What I don't
| understand is, with a barometer one needle stays the same showing the
| current pressure, and the other needle moves showing the amount of
pressure
| falling or rising. My watch shows sea level pressure and current air
| pressure (in barometer mode, nothing to do with altimeter) but both
readings
| move up or down together. What would be the point of showing both
readings
| if they always move the same amount together?
|
|...
|

I suspect the correction for altitude is just a "standard". With my
electronic barometer, it knocks about 1hPa (or "millibar") off for every 10m
of height you tell it you are above sea level. In "barometer" mode, these
readings will always change together, because that correction is fixed, but
having *both* readings available enables you to both set the barometer and
determine your altitude above sea level. Choose a quiet spell of weather
(so the pressure is only changing slowly in time and space) and get the
sea-level pressure as reported from the nearest weather station. Adjust the
altitude setting on your watch until the "sea level" setting matches the
reported setting. You will then also have the true pressure and an estimate
of your altitude.

The reason I say "an estimate" is that the change of pressure with height
depends on a number of factors and the fixed scale used by your barometer
watch cannot take all these factors into account. If you take your watch to
the "high plains" and set it you will find that your indicated "sea level"
pressure will vary from that reported by your reference station from day to
day, even if you remain in the same place, because your watch is not taking
these factors into account whilst (to a certain extent) the reporting
weather station is.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
) ( /\ ) (

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Thanks Yokel, the mist is beginning to clear...
Stuart
"Yokel" wrote in message
...
"Stuart" wrote in message
...
|
|...
|
| Yes, the watch automatically sets the sea level pressure when you input
the
| current height above sea level. I didn't understand why it was so
important
| to know the pressure at sea level if I wasn't AT sea level, but I take
it
| that you need to have a 'yardstick' to measure against? What I don't
| understand is, with a barometer one needle stays the same showing the
| current pressure, and the other needle moves showing the amount of
pressure
| falling or rising. My watch shows sea level pressure and current air
| pressure (in barometer mode, nothing to do with altimeter) but both
readings
| move up or down together. What would be the point of showing both
readings
| if they always move the same amount together?
|
|...
|

I suspect the correction for altitude is just a "standard". With my
electronic barometer, it knocks about 1hPa (or "millibar") off for every
10m
of height you tell it you are above sea level. In "barometer" mode, these
readings will always change together, because that correction is fixed,
but
having *both* readings available enables you to both set the barometer and
determine your altitude above sea level. Choose a quiet spell of weather
(so the pressure is only changing slowly in time and space) and get the
sea-level pressure as reported from the nearest weather station. Adjust
the
altitude setting on your watch until the "sea level" setting matches the
reported setting. You will then also have the true pressure and an
estimate
of your altitude.

The reason I say "an estimate" is that the change of pressure with height
depends on a number of factors and the fixed scale used by your barometer
watch cannot take all these factors into account. If you take your watch
to
the "high plains" and set it you will find that your indicated "sea level"
pressure will vary from that reported by your reference station from day
to
day, even if you remain in the same place, because your watch is not
taking
these factors into account whilst (to a certain extent) the reporting
weather station is.
--
- Yokel -
oo oo
OOO OOO
OO 0 OO
) ( I ) (
) ( /\ ) (

"Yokel" now posts via a spam-trap account.
Replace my alias with stevejudd to reply.