Zambretti Forecast Project

The aim of this project was to develop a web-based version of the Zambretti Forecaster, similar to what appears on many weather station consoles and in some weather station software. The real-time weather data would be supplied in an XML document and then processed and displayed by XSLT.

The original Zambretti Forecaster was developed in 1915 by precision instrument makers Negretti and Zambra of London. It was a hand-held disc calculator, specifically designed for the UK, that provided on-demand short-term forecasts of up to 12 hours. It is reported that the forecasts are most accurate when generated at 9.00 am local solar time.

The calculator uses the mean sea level pressure (MSLP) and pressure trend (steady, rising or falling) as the primary input. The season (Summer or Winter) and the wind regime both play a minor role in determining the forecast.

Zambretti ForecasterZambretti Forecaster
Fig. 1. The original Zambretti Forecaster.
(left) front: overlaid discs for applying the prevailing weather conditions and reading the forecast letter
(right) rear: forecast text corresponding to the forecast letters displayed in the cut-outs in the top front disc

The Meteor2000D Implementation Guide (hereafter referred to as "the guide") is a valuable resource that describes the mechanics of the Zambretti Forecaster. The guide also outlines the relationship between the Forecast Code (described as "forecast letter" in the guide), Forecast Index ("forecast number" in the guide) and the Forecast Text. These relationships are presented in the following table, together with the modified forecast text and forecast icons used in our implementation of the Zambretti Forecaster.

Relationship between Forecast Codes, Forecast Index and Forecast Text
Forecast
Code
Forecast Index Forecast Text TrugWX Zambretti Forecast
Steady Rising Falling Forecast Text Forecast Icon
A 1 1 1 Settled fine. Settled. Fine. forecast icon forecast icon
Day / Night
B 2 2 2 Fine weather. Fine. forecast icon forecast icon
Day / Night
C 3 Becoming fine. Becoming fine. forecast icon forecast icon
Day / Night
D 3 Fine, becoming less settled. Fine. Becoming less settled. forecast icon forecast icon
Day / Night
E 3 Fine, possibly showers. Mostly fine. Chance of showers. forecast icon forecast icon
Day / Night
F 4 Fairly fine. Improving. Becoming fine. Showers easing. forecast icon forecast icon
Day / Night
G 5 Fairly fine. Possibly showers, early. Mostly fine. Possible early showers. forecast icon forecast icon
Day / Night
H 4 Fairly fine. Showery later. Mostly fine. Showers developing later. forecast icon forecast icon
Day / Night
I 6 Showery early. Improving. Mostly fine. Showers clearing. forecast icon forecast icon
Day / Night
J 7 Changeable mending. Changeable. Improving. forecast icon forecast icon
Day / Night
K 4 Fairly fine, showers likely. Mostly fine. Few showers likely. forecast icon forecast icon
Day / Night
L 8 Rather unsettled. Clearing later. Rather unsettled. Clearing later. forecast icon forecast icon
Day / Night
M 9 Unsettled, probably improving. Unsettled. Probably improving. forecast icon forecast icon
Day / Night
N 5 Showery. Bright intervals. Showers. Bright periods. forecast icon forecast icon
Day / Night
O 5 Showery. Becoming more unsettled. Showers. Becoming more unsettled. forecast icon
Day + Night
P 6 Changeable. Some rain. Changeable. Some rain. forecast icon
Day + Night
Q 10 Unsettled. Short fine intervals. Unsettled. Short fine periods. forecast icon
Day + Night
R 6 Unsettled. Rain later. Unsettled. Rain developing later. forecast icon
Day + Night
S 7 Unsettled. Rain at times. Unsettled. Periods of rain. forecast icon
Day + Night
T 11 Very unsettled. Finer at times. Very unsettled. Short fine breaks. forecast icon
Day + Night
U 7 Rain at times. Worse later. Rain at times. Increasing later. forecast icon
Day + Night
V 8 Rain at times. Becoming very unsettled. Rain at times. Becoming very unsettled. forecast icon
Day + Night
W 8 Rain at frequent intervals. Periods of rain. forecast icon
Day + Night
X 9 9 Very unsettled. Rain. Very unsettled. Periods of Rain. forecast icon
Day + Night
Y 12 Stormy. Possibly improving. Stormy. Possibly improving. forecast icon
Day + Night
Z 10 13 Stormy. Much rain. Stormy. Much rain. forecast icon
Day + Night

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Independent Review

The guide includes details about the Forecast Codes for a range of pressures under varying pressure trends, seasons and wind regimes. Using the methodology described points 1 - 5 below, we reviewed these published results and our analysis and conclusions are presented further down the page.

  1. Tabulate the Forecast Codes and Forecast Index by pressure, wind and season based on whether the pressure is steady (Fig. 2), rising (Fig. 3) or falling (Fig. 4).
  2. Compute the seasonal differences in the Forecast Index by pressure and wind regime for a rising pressure (Fig. 5) and a falling pressure (Fig.6).
  3. Determine whether the seasonal difference is influenced by, or independent of the pressure trend.
  4. Compute the effect of each wind regime on the Forecast Index for a steady pressure (Fig. 7), a rising pressure (Fig. 8) and a falling pressure (Fig. 9).
  5. Determine whether the effect of each wind regime is influenced by, or independent of the pressure trend (Fig. 10).

zambretti code and index by mslpseasonal ratings for steady pressure
Fig. 2. The Forecast Code and Forecast Index by MSLP, wind regime and season for a steady pressure.

zambretti code and index by mslpseasonal ratings for rising pressure
Fig. 3. The Forecast Code and Forecast Index by MSLP, wind regime and season for a rising pressure.

zambretti code and index by mslpseasonal ratings for falling pressure
Fig. 4. The Forecast Code and Forecast Index by MSLP, wind regime and season for a falling pressure.

mslp valueseffect of season on the Forecast Index for a rising pressure
Fig. 5. Effect of season on the Forecast Index for a rising pressure.

mslp valueseffect of season on the Forecast Index for a falling pressure
Fig. 6. Effect of season on the Forecast Index for a falling pressure.

mslp valueseffect of wind on the Forecast Index for a steady pressure
Fig. 7. Effect of wind on the Forecast Index for a steady pressure.

mslp valueseffect of wind on the Forecast Index for a rising pressure
Fig. 8. Effect of wind on the Forecast Index for a rising pressure.

mslp valueseffect of wind on the Forecast Index for a falling pressure
Fig. 9. Effect of wind on the Forecast Index for a falling pressure.

average effect of wind on the Forecast Index
Fig. 10. Average effect of the wind on the Forecast Index, from Figs. 7, 8 & 9.

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Conclusions

The following statements are made on the basis of our review:

Development & Testing

Three different versions of the Zambretti Forecaster set of algorithms were found, including those in the guide. The algorithms in the guide did not produce results that matched the published results, however further testing identified that each algorithm needed a small correction to mitigate the differences. The two other versions required a translation of the programming code from the native python and javascript languages to XSL. The three XSL adaptations of the Zambretti Forecaster that were independently developed comprised: (1) Meteor2000D version (codename ML), (2) Raspberry Pi version (RPi), and (3) Javascript version (JS).

The evaluation of the three XSL adaptations comprised two daily recordings of the Forecast Codes coinciding with the morning and evening manual observations over a thirty day period. The recorded values were assessed on a scale from 0 (very poor) to 3 (very good) against the actual weather during the forecast period to determine the best performing version. Out of interest, the evaluation also included the stand-alone javascript version (BJ).

The results of the evaluation are presented below in the evaluation summary table and detailed evaluation results (Fig. 11). The evening forecast in each case proved to be more accurate than the morning forecast, although the difference was significantly less for the ML and RPi versions. Overall the ML and RPi versions were the best performing versions, with the former rated marginally more accurate. Based on these results the ML version will provide the Zambretti Forecast on this website.

Evaluation Summary
Version Morning
Rating
Evening
Rating
Overall
Rating
ML 86% 89% 87%
RPi 83% 88% 86%
JS 58% 70% 64%
BJ 62% 76% 69%

Zambretti test results
Fig. 11. Detailed evaluation results.

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