Zambretti algorithms

[jim-easterbrook]

I've been comparing beteljuice's Zambretti forecaster (http://www.metserv.info/topic/38205-zam ... orecaster/) with another interpretation of the Zambretti forecaster (http://www.meteormetrics.com/zambretti.htm). I suddenly realised this morning that they have different understandings of how the thing works! On the original forecaster there are three scales - rising, falling and steady pressure. The rising and falling scales are accompanied by letters W & S. Beteljuice interprets these as summer and winter, whereas Meteormetrics interpret them as 'weak' and 'strong' rising/falling.

Anyone got (a copy of) the original instructions?

[steve]

Hmm, interesting. I think you could be right. One for the beteljuice, I think...

[beteljuice]

The beteljuice has the brass Zambretti forecast 'wheel', with instructions.

Very definately Winter and Summer (which are only used if barometer trend is NOT steady)

From the instruction card:

"The letters 'S' (Summer) and 'W' (Winter) are for the time of year, the former being taken from April to September inclusive, and the latter from October to March inclusive.

From the link you gave. I assume that this the 'questionable' paragraph.

"The Forecaster makes a crude adjustment for the rate of rising or falling pressure A weak falling generally results in a Z value lower by 1 unit compared with a rapid falling pressure. Similarly a strong rising, improves the prospects by 1 unit over a weak rising. A rising or falling rate of 0.5mB/hr or larger might be taken as a rapid change, and less as a weak change. The modification of the Z value accordingly would be relatively simple, in a post-processing step following the main calculation"

EDIT: They are refering to their own calculated 'Z' values, ergo their own forecaster.

This does seem to be incorrect as I can determine no interest in rate of change, just Rising, Steady and Falling.

The S/W 'differences' can be as much as 4 Zambretti code letters, and the Summer / Winter variations effectively 'reverse' the effect of any wind decision as prescribed by the original navy / meto forecast guide created by Admiral Fitzroy.

[jim-easterbrook]

Thanks for that - just the definitive information I was looking for. Would you like to verify (or not!) any of the rows in the table down the left side of the Meteormetrics web page?

[beteljuice]

I am not at all sure what they mean by PD (Deriviative of Pressure), and the table describes it in two columns both by text and a digital value. (I assume the 'P' used in the formulae)

eg. Pressure trend rising in Summer is described as R-S and 2

Summer = +2 -2
Winter = +1 -1
Steady = 0

I haven't cross-checked all the entries , but a few random samples seem OK.

Anyone who has a FO type station would disagree with their statement that wind direction makes little difference to the forecast, and they admit that their output should be adjusted to reflect this, nor do they say how you would rework their tables for the Southern hemisphere. (It would mean switching the 'P' value in their equations ie. +1 becomes -2 [ I think])

Also they suggest the Zambretti forecast is for upto 4 hours, rather than 'variable' between soon and 24+ hours (dependent upon rate of change)

BTW. Before the beteljuices mud patch had a make-over and my wx kit was dismantled. I was experimenting with rate of pressure change.

Mixed results !
I could make a VERY good 'now' caster, but the best other options were very accurate when they were right and very wrong when they were bad ......

[jim-easterbrook]

I am not at all sure what they mean by PD (Deriviative of Pressure), and the table describes it in two columns both by text and a digital value. (I assume the 'P' used in the formulae)

eg. Pressure trend rising in Summer is described as R-S and 2

Summer = +2 -2
Winter = +1 -1
Steady = 0

I haven't cross-checked all the entries , but a few random samples seem OK.

I assumed their table was of output of an actual Zambretti, rather than of their algorithm. The PD is just their misunderstanding of W & S, so 2 is rising strongly, 1 is rising weakly and so on. Derivative is just a fancy way of saying rate of change, i.e. dP/dt for the mathematicians amongst us.

One more question, if I may. Do the Zambretti instructions say what change of pressure, over what period, is the threshold for rising/falling rather than steady?

[beteljuice]

Summer = +2 -2
Winter = +1 -1
Steady = 0

I assumed their table was of output of an actual Zambretti, rather than of their algorithm. The PD is just their misunderstanding of W & S, so 2 is rising strongly, 1 is rising weakly and so on. Derivative is just a fancy way of saying rate of change, i.e. dP/dt for the mathematicians amongst us.

The left hand table IS the Zambretti output BUT your belief of rate of change does not follow. ie. Summer is ONLY +2 or -2, and Winter is ONLY +1 or -1 (except 0 = steady)

One more question, if I may. Do the Zambretti instructions say what change of pressure, over what period, is the threshold for rising/falling rather than steady?

No guidance is given, but should be a fairly obvious change I would have thought as It is based on 'old' weather lore. ie. Weather glasses and observation.

I can't remember atm what Steve has set the threshold rate to, but I think it's tweakable in the .ini somewhere.

[steve]

I can't remember atm what Steve has set the threshold rate to, but I think it's tweakable in the .ini somewhere.

The default is 0.1 mb/hr averaged over the previous three hours, configurable as you say via FCPressureThreshold in the ini file.

[wd40]

Here in the SE USA, moisture pulled up from the Gulf brings rain. If the low is far enough away so we are in the outer rain bands, the forecaster misses the rain event since the pressure change is so small.


I found this in USA Today and they source our National Weather Service. Some what different from the Cumulus forecaster.

The following table generally summarizes wind and barometer indications in the United States. The amateur forecaster should modify the table in accordance with his or her own observations. The following show the wind direction, the barometer reduced to sea level and the character of the weather indicated:

* SW to NW, 30.10 to 30.20 and steady - Fair with slight temperature change for 1 to 2 days.
* SW to NW, 30.10 to 30.20 and rising rapidly - Fair, followed within 2 days by rain.
* SW to NW, 30.20 and above and stationary - Continued fair, with no decided temperature change.
* SW to NW, 30.20 and above and falling slowly - Slowly rising temperature and fair for 2 days.
* S to SE, 30.10 to 30.20 and falling slowly - Rain within 24 hours.
* S to SE, 30.10 to 30.20 and falling rapidly - Wind increasing in force, with rain within 12 to 24 hours.
* SE to NE, 30.10 to 30.20 and falling slowly - Rain in 12 to 18 hours.
* SE to NE, 30.10 to 30.20 and falling rapidly - Increasing wind, and rain within 12 hours.
* E to NE, 30.10 and above and falling slowly - In summer, with light winds, rain may not fall for several days. In winter, rain within 24 hours.
* E to NE, 30.10 and above and falling rapidly - In summer, rain probably within 12 to 24 hours. In winter, rain or snow, with increasing winds, will often set in when the barometer begins to fall and the wind sets in from the NE.
* SE to NE, 30.00 or below and falling slowly - Rain will continue 1 to 2 days. SE to NE, 30.00 or below and falling rapidly - Rain, with high wind, followed, within 36 hours by clearing, and in winter by colder.
* S to SW, 30.00 or below and rising slowly - Clearing within a few hours, and fair for several days.
* S to E, 29.80 or below and falling rapidly - Severe storm imminent, followed within 24 hours, by clearing, and in winter by colder.
* E to N, 29.80 or below and falling rapidly - Severe northeast gale and heavy precipitation; in winter, heavy snow, followed by a cold wave.
* Going to W, 29.80 or below and rising rapidly - Clearing and colder.

Source: National Weather Service

Randy

[jim-easterbrook]

[quote="beteljuice"]The left hand table IS the Zambretti output BUT your belief of rate of change does not follow. ie. Summer is ONLY +2 or -2, and Winter is ONLY +1 or -1 (except 0 = steady)
[quote]
I don't have any belief of rate of change - I just observe that they've misunderstood the Zambretti's W & S letters, interpreting them as rate of change rather than winter and summer. Hence they make no reference to winter and summer at all.

[beteljuice]

Re-reading the page (carefully) again I believe your first interpretation was the correct one. ie. They have mistakenly interpreted Summer / Winter as Strong / Weak.

This does seem a rather basic error, and without writing a bit of comparative code I can't relate under which circumstances it would most be in error. It does go someway perhaps to explaining their statement that wind direction makes little difference.

It may be worthwhile emailing them to clarify their assumptions / wording.

[nking]

Slightly off topic but I came across this patent GB191506276 for an Apparatus for Applying Meteorological Observations to the Forecasting of Weather dated 27-01-1916

http://v3.espacenet.com/publicationDeta ... cale=en_gb