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 prominent 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. The forecasts were proven to be 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 "the guide") is a valuable resource that describes the mechanics of the Zambretti Forecaster. The guide also outlines the relationship between the Forecast Code (called "forecast letter" in the guide), Forecast Index (called "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 we use in our implementation.

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

Independent Review

The guide includes details of the Forecast Codes for a range of pressures under varying pressure trends, seasons and wind regimes. We independently reviewed these published results and our analysis and conclusions are presented below. The review steps were as follows:

  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.

Conclusions

The following statements can be 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. Unfortunately the algorithms in the guide did not produce results that matched the published results. Further testing identified that a small correction to each algorithm was needed to mitigate the differences. The remaining versions required a translation of the code from the native python and javascript languages to XSL.

The three XSL adaptation of the Zambretti Forecaster are:

See the live testing of the ML, RPi and JS adaptations of the Zambretti Forecaster. By way of further comparison, the forecast icon (cloud, rain or sun) showing on the weather station console is also represented.

The evaluation, which for interest sake will also include the stand-alone javascript version (codenamed BJ), will comprise two daily recordings of the Forecast Codes coinciding with the morning and evening manual observations. The recorded values over an initial thirty day period will be assessed against the actual conditions to determine the best performing version. The evaluation will be subjective and based on the points system outlined below:

  1. 0 = Forecast was very poor
  2. 1 = Forecast was poor.
  3. 2 = Forecast was good.
  4. 3 = Forecast was very good.