Pressure question on FineOffset WH-1091

[Darknstormy]

Hi everyone,

Been two months now with me FineOffset WH-1091, The instructions are quite funny and kinda not explain things really well. So hopefull some one can tell me a few pointers.

My questions

1. Why can i change the pressure manually (does this mean i should set it to the same as a offical weather station nearby?)
2. Tendancy, also why can i change this, do i set that as well? For eg look out side set it to sunny if it is, and then set the pressure to match a nearby weather station?

cheers

Lyle

[steve]

1. Why can i change the pressure manually (does this mean i should set it to the same as a offical weather station nearby?)

Atmospheric pressure varies with altitude - this makes complete sense if you think about it, as pressure is due to the column of air above you. Higher altitude = less air above you = lower pressure. So it's hard to compare pressure between sites at different altitudes. For this reason, meteorologists always use pressure corrected to sea-level. Your station measures "absolute" pressure, and for it to be useful, it allows you to add an offset to that to correct it to sea-level - the "relative" pressure. So, yes, you should set your relative pressure to match the published pressure of a nearby station, preferably during a period of stable high pressure.

2. Tendancy, also why can i change this, do i set that as well? For eg look out side set it to sunny if it is, and then set the pressure to match a nearby weather station?

This one baffles me. I assume you can change it for calibration purposes, but as it's supposed to be a forecast, I don't know how you can do that.

[Darknstormy]

Thankyou Steve, I Set the pressure to a nearby station about 13 days ago as a test and things have been working very well Eg forecast. Yeah with the WH-1091 you press the screen once on the Tendency and you can select the desired forecast.. At the time of setting the pressure it was a sunny day and had been for a few days so I set it to sunny. Things seem to be working fine I just wanted to get the answers because I wasn't sure if I did it correctly

[Keyz]

The tendency on the actual unit can be frustrating.

I use mine seperate from the computer the majority of the time (98% I reckon), so it would be interesting to me if it was accurate.

Problem is in NZ the weather is so changeable, it can be sunny, rainey, cloudy all in one day, without great pressure variances it seems.

I would assume the display unit uses some kind of logic based on pressure change, but I think cumulus' forecast is much more accurate.

I have set the tendency on the display at level 4, storm at about 9, and after setting the graphic to fine on a nice day it seems to work ok lately.

[dougie6653]

anyone have any more thoughts on this? I cant understand why you should be able to set the tendency, and what difference different settings make. My last in/out oregon thermometer had a tendency window, set from out of the box and it was fairly accurate. any help would be appreciated.

thanks

[Big Daddy]

Hello everybody,
I am new here and new into weather stations and I have just bought what I think is a similar unit to this from Maplin (£69.99 for a grey one and £99 for a black one and as far as I can see they are the same?????) and was wondering about the tendancy feature myself. I have been playing around with it and here is the theory I have come up with.

Set the icon when the weather is changing, lets say from cloudy to sunny intervals and set it to the weather state its changing to. From what I can work out it then uses the current pressure level as a marker. Leaving the "change" unit at defaults i.e. 2, when the pressure increases by 2 hpa then the icon will change to the next icon up and if it falls by 2hpa then the icon goes down. In each step when the pressure is hitting near the level then it shows either an upward or downward arrow indicating the weather is about to change.

As an example (pressure measurements are for illustration only and not real readings)

Current weather is cloudy but is starting to break up and give some sunny intervals, pressure is 1000. Set the icon to "sunny intervals". As the pressure starts to reach 1002 an upward arrow will display and when the pressure hits 1002 it will change to "sunny". Hopefully the real weather has changed as well. If the pressure continues to rise it will stay "sunny". As pressure comes back down again when it nears 1002 a downward arrow will display and when it hits 1002 (possibly 1001.9) it will show "sunny intervals", as it drops to near 1000 again a downward arrow and when it falls below 1000 the "cloudy" icon appears. I havent worked out what the "storm" setting of 4 is for as we havent had much bad weather recently to test it but I am assuming that possibly a greter differential in pressure between "cloudy" and "rain" in meteorogigal terms needs to happen before it actually starts raining so between 1000 and 996 it stays "cloudy" and if it falls below 996 its time to call off the barbeque and get the umbrellas out. The BBC have predicted a drop in pressure over the next couple of days and also some heavy rain so I will get chance to see if this works and also how my rain gauge fairs as it hasn't had its initial wetting yet.

As for relative and absolute pressure, I had to research this as well so hopefully someone can confirm what I have found and that I have done this correctly.

Absolute pressure is the pressure at sea level. Relative pressure is the pressure where you are and the higher you go the less it gets. I read that for every 9 metres in height you lose around 1 hpa. I used the following link to find out my altitude. Zoom in on the map and click on your location and it will give your altitude.

http://www.daftlogic.com/sandbox-google ... titude.htm

I am roughly 21m above sea level so I set my relative pressure to 2.5 pha below the shown absolute pressure. (settings only steps in +/- .5). So as I type, my absolute pressure is 1014.8 and my relative pressure is 1012.3. However, do we need to know relative or absolute pressure, I just want to know if its gardening and BBQ weather or Sofa and TV weather.

If anybody has any further ideas on the tendancy feature then I would be greatful to hear from you.

[steve]

Absolute pressure is the pressure at sea level. Relative pressure is the pressure where you are and the higher you go the less it gets.

It's the other way around. Absolute Pressure, often called Station Pressure, is the pressure at your location as measured by your station. Relative Pressure is the pressure adjusted to sea-level.

I am roughly 21m above sea level so I set my relative pressure to 2.5 pha below the shown absolute pressure. (settings only steps in +/- .5). So as I type, my absolute pressure is 1014.8 and my relative pressure is 1012.3.

Your relative/sea-level pressure would normally be higher than your absolute pressure if you are above sea level, as absolute pressure decreases with altitude. You should set your station's relative pressure to match a reliable reading from somewhere not far from you (e.g. Met Office station).

However, do we need to know relative or absolute pressure, I just want to know if its gardening and BBQ weather or Sofa and TV weather.

It's important if you are quoting your pressure readings to someone else (e.g. on the web) to quote relative/sea-level pressure, so everyone is using the same measurement. But yes, it is often the rate of change of pressure which is of interest rather than the actual current value itself.

[Big Daddy]

Steve,
Thanks for correcting me. I am new to this and based the relative / absolute pressures on a couple of articles I read which I obviously mis-interpreted so now I have my pressures the right way round both on my weather station and in my head.

As for the tendancy theory, we have had a big drop in pressure today and steadily the icon on my station has changed from sunny intervals this morning down to the storm icon so hopefully my theory on this is there or there abouts.

Thanks for your help