Topic: Temperature Difference

[openvista]

Yes, your temperature difference on clear, calm nights is almost certainly driven by inversion (higher temps in higher elevations to a point). For instance, last night (which was clear and calm), my low was 7.1 deg at 5ft and 8.2 deg at 20ft. That's a 1.1 degree difference in 15ft.

As far as pressure, klschmidt was right on. Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter) during stable weather (steady barometer and no cold/warm fronts in the area). If you don't know how to find the airport's SLP measurement, simply give me your airport's location or call sign and I can create a link for you. You don't want to sync to the altimeter in the winter because the further you are from about 60 degrees, the higher the SLP reading will be compared to the altimeter. At 20F, we're talking like 0.10" - not an insignificant amount.

[CW2274]

Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter)

That's a choice, not a must. Granted, MSLP is what it's designed for, but I never even came close to a xx.10" error using the altimeter as a baseline, probably .04", at most, and that's between say ~ 29.60" and 30.30".

[openvista]

Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter)

That's a choice, not a must. Granted, MSLP is what it's designed for, but I never even came close to a xx.10" error using the altimeter as a baseline, probably .04", at most, and that's between say ~ 29.60" and 30.30".

Correct, it's a choice. But he lives in Kansas where it gets cold in the winter.

For the record, I didn't pull that 0.10" out of a hat. That's the current difference (as of 5:47PM EST) between my local airport's (KSAW) sea level pressure and altimeter readings at 19F -- a not altogether uncommon temp in KS in the winter.

[CW2274]

Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter)

That's a choice, not a must. Granted, MSLP is what it's designed for, but I never even came close to a xx.10" error using the altimeter as a baseline, probably .04", at most, and that's between say ~ 29.60" and 30.30".

For the record, I didn't pull that 0.10" out of a hat.

Never said you did, merely relating my experience.
 
If you'd like, set your console to zero elevation, dial in the altimeter, and see how it behaves. I realize I don't see your temps, I'm curious how it would behave there.

[openvista]

Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter)

That's a choice, not a must. Granted, MSLP is what it's designed for, but I never even came close to a xx.10" error using the altimeter as a baseline, probably .04", at most, and that's between say ~ 29.60" and 30.30".

For the record, I didn't pull that 0.10" out of a hat.

Never said you did, merely relating my experience.
 
If you'd like, set your console to zero elevation, dial in the altimeter, and see how it behaves. I realize I don't see your temps, I'm curious how it would behave there.

I have a Vue console, so I already have real altimeter fed to my weather software. I don't need to perform any tricks.

Just to clarify, I'm not saying the console's altimeter will be off 0.10" from a reference altimeter. I'm saying that's the spread between altimeter (QNH) and SLP (QFF) at a common Midwest elevation (1200ft) and winter temp (~20F).

Since the OP is only 115ft or so below his local airport, he could absolutely set elevation to 0 and sync to the airport's altimeter, when conditions are favorable. In fact, if he reports to CWOP, he may be better off doing that since they assume the pressure being sent is in altimeter format.

But if he wants to stay with SLP, then he should sync to the reference SLP measurement instead.

[CW2274]

Given that you have a VP2 console, I'd sync to the airport's SLP reading (NOT altimeter)

That's a choice, not a must. Granted, MSLP is what it's designed for, but I never even came close to a xx.10" error using the altimeter as a baseline, probably .04", at most, and that's between say ~ 29.60" and 30.30".

For the record, I didn't pull that 0.10" out of a hat.

Never said you did, merely relating my experience.
 
If you'd like, set your console to zero elevation, dial in the altimeter, and see how it behaves. I realize I don't see your temps, I'm curious how it would behave there.

I have a Vue console, so I already have real altimeter fed to my weather software. I don't need to perform any tricks.

Tricks?? All I tried to do is get a comparable altimeter with a VP2 console, and it was successful enough until I wanted better, hence the Vue, which obviously you seem to want also.
Don't bash for the effort.

[klschmidt]

My experience has been very similar to what CW2274 mentioned. About a .03" discrepency during very high or low pressure events (with the elevation set to zero). While this is likely the best option for one who doesn't own a Vue, but wants the barometer to display the current altimeter setting, the result is only an approximation. If the data is being used by anyone other than the owner of the station, it would be nice to provide accurate altimeter, sea level or raw station pressure.

CW2274, I'm not arguing with your idea; the fact that I can speak of the discrepency I've observed strongly suggests I've been doing the same thing

[BigOkie]

Are the VP2 console barometers so wildly erratic between different consoles as to have this?  I set mine 5 years ago at the altitude my station is at (620' MSL) and it never deviates more than about .02 or .03.  Never has.  That difference can likely be attributed to a little of the difference in altitude between my station and the local reporting station (KTUL airport).  I guess I'm just lucky.

[openvista]

Tricks?? All I tried to do is get a comparable altimeter with a VP2 console, and it was successful enough until I wanted better, hence the Vue, which obviously you seem to want also.
Don't bash for the effort.

Again, it seems like there's been confusion. First, I'm not bashing anyone. Davis' flagship product should have a console that outputs in all 3 pressure formats (station/altimeter/SLP). People do what they gotta do to get around that fact if pressure matters to them. No judgement conferred.

Secondly, I don't doubt that the aforementioned workaround/Jerry rig/kludge/trick/fallback (however you choose to describe the unsupported behavior of outputting quasi-altimeter from the VP2 console) tracks true altimeter reasonably well most of the time. 

My point was that if the OP wants to sync his pressure to the airport he should be sure it's to the right format, particularly during the winter.

[openvista]

Are the VP2 console barometers so wildly erratic between different consoles as to have this?  I set mine 5 years ago at the altitude my station is at (620' MSL) and it never deviates more than about .02 or .03.  Never has.  That difference can likely be attributed to a little of the difference in altitude between my station and the local reporting station (KTUL airport).  I guess I'm just lucky.

I think the issue here, is that, assuming the OP has previously calibrated his barometer, he may have used the local airport's pressure during warmer conditions.  If so, he may not have realized that, by default, the airport transmits altimeter pressure which is a different format from VP2 console which reads in MSLP (which is what the NWS refers to as "SLP"). As the temps drop, the differences become apparent and he notices that he's "off". In fact, he might not be off. Once he can pull up the airport's SLP reading, convert it to imperial, and compare it to his reading he might find that he's still "on".

Again, I'm assuming he's previously calibrated the barometer.

I've had 2 VP2 consoles and both arrived from the factory about 0.04" off of true pressure. For some that wouldn't matter. But from the sounds of it, he wants to dial it in. So I'm just trying to make sure he has the information to do that.

[ValentineWeather]

Airport reports MSLP (mean) once an hour. The temperature used in the sea level correction is a twelve hour mean.

Reason I use a second (VUE) console for uploading is its ability to convert to altimeter accurately plus my website and CWOP require altimeter, it's pretty much the standard most all use TV, radio, pilots.
Some software including WL will convert SLP to altimeter while sending to places like CWOP.
My understanding SLP is used primarily by meteorologist but for my use all I want is altimeter.

[DRoberts]

I want to thank all on this page who replied and have given me advice.

I did calibrate the barometer on a very hot day when I set up my station. This is the first winter. I did not notice the difference in my barometer and the altimeter readings at KPHG until the cold weather hit last month.

Going to think about the responses here and decide after Thanksgiving what is best. Some kind of recalibration is necessary though. Knowing me, it would be best if that involved the simplest solution with the most accurate outcome. KISS. 

I plan to get my data online in the near future using the WifiLogger most likely. (Just need to dig up another mayonnaise jar of money first. ) So I want what I put out there for the NWS or whoever to be accurate within my knowledge and ability.

 Once again, thanks for all the advice here. This is a great forum for diverse views from knowledgeable people.

[CW2274]

Knowing me, it would be best if that involved the simplest solution with the most accurate outcome. KISS. 

That's easy, both are solved by buying a Vue console, entering your elevation MSL, select ALT SET, and your done. No endless monitoring, no worrying about the temp, wind, fronts coming, going, nothing but instant, accurate pressure. Now that's KISS.