Topic: Rate of Chnge of Pressure Graph. What does it tell me about the weather?

[jeraditto]

Hi! New to the forum.

 I have a WS2902 for 2 years now. Really like all the information, but I have just gotten around to learning more. I did some searching on this site to find the answer to my questions but didnt get to far.

 My question is about the "Rate of Change of Pressure Graph." From the Users Guide I understand the it reads " The difference between the daily average pressure and the 30 day average (in hPa)." HPa is HectoPascals so Pascals * 100. Ok, just scaling the pressure, and 30 day average pressure is fairly simple as is daily average pressure. If I am reading it right, and the gauge shows, say, -1 then the daily average pressure (of today) is 1 hPa lower than the average of the last 30 days. please correct me if that is incorrect.

Next question then, Is what does that tell me? I think that if pressure is dropping (on a short term basis) then storms may be approaching, and if it is rising, should be better weather. I dont understand how looking at the averages would be a lot of help.

any thoughts?Thanks!

[galfert]

I don't think it tells you much to just look at it one day as you've proposed. I think the purpose is to compare it day to day and then see the trend. Today -1 and yesterday -2 and the day before that -3 then that would show you that the trend is upwards even though for today it is -1 from the monthly average. Also as with any barometer (even an analog one) you need to make day to day comparisons every day at the same time of the day to get a better analysis.

[jeraditto]

With my limited knowledge that makes sense. I think the graphs on the Ambient web tool and WU also are far more intuitive! Thanks for replying!

[Old Tele man]

This is 'how' my ASI DBX2 electronic barometer interprets & displays pressure changes:


1) Rapid rise/drop alert occurs on +/- 0.03 inHg (1.02 mb) or greater change per hour. Rate of change indication is based on the following:

Steady is 0.00 inHg per hour (0.0 mb)

Slow is +/- 0.01 inHg per hour (0.34 mb)

Medium is +/- 0.02 inHg per hour (0.68 mb)

Rapid is +/- 0.03 inHg per hour (1.02 mb)

2) Storm watch is based on range and trend. Storm warnings is based on an active storm curve, range and rate of change.

Wind warnings generally occur on the rising side of an active storm curve.

Other warnings are based on multiple variables, including graph analysis, pressure range and rate of change.

3) The DBX2 can also detect gust fronts associated with thunderstorms and will often alert you with a rapid rise alert before impending storm. Tendency is based on these pressures:

CLEAR = Pressure greater than or equal to 30.48

DRY = Pressure 30.28 through 30.47

FAIR = Pressure 30.10 through 30.27

CHANGE = Pressure 29.90 through 30.09

PRECIP = Pressure 29.68 through 29.89

STORMY = Pressure 29.01 through 29.67

CYCLONE = Pressure less than or equal to 29.00

[jeraditto]

Wow, thanks for this! I am digesting what you wrote, but I think it makes sense!

[galfert]

Yeah but don't read too much into those ranges. It was 30.16 inHg the other day here and raining. It is just a good reference but doesn't always apply.

[Old Tele man]

Galfert is 100% correct...those are just empirical notions, ie: historically correct most of the time but (absolutely) NOT all of the time.