Quickie sketch by Roger Murray
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The ivorine weather forecaster (originals) can still be found at specialist instrument collectors outlets.

You can also buy a modern copy of the forecaster from Téméraire of Brixham

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Links & references:

Negretti & Zambra ~
http://www.negrettiandzambra.co.uk
Beteljuice's Website ~

http://www.beteljuice.co.uk/zambretti/forecast.html

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Tips in the use of the Zambretti Forecaster.

Apparently the reading is best done at 0900 hrs local time.
The pressure trend can influence the accuracy too. The data I supply gives not only the basic steady, rising and falling but in addition rising slowly etc. This is reflected in the the figure that follows the observed pressure i.e. 1002.5 (0.3), which indicates there has been a pressure difference of 0.3mbs during the last hour. In this case I would tend to report it for the sake of the forecast as being steady and not rising or falling as the case may be. However, if there is a howling gale outside then the isobars are obviously close together and the pressure gradient is steep. In which case I would use the rising or falling option.
The wind direction again affects the result. I report the average wind over the last 10 minutes, not the predominant wind direction.

Negretti & Zambra were renowned
scientific instrument makers in London. In
the early 1900's they produced a circular
slide rule dedicated to forecasting the short
term weather conditions. Using the barometer
reading, trend, wind direction and the time
of year, it was possible to predict general
weather conditions based on
previously
observed weather data.

The instrument's intended use was confined to northern
latitudes and specifically the UK. Another criterion was
that the reading should be taken as close as possible to 0900 hrs.

It is reputed to be over 90% accurate in providing a short term forecast.

The "electronic online" version has been converted to work via a javascript program by Beteljuice (see below).

It's a bit of fun, try it you may be surprised. 12 hr forecast

About the Zambretti Weather Forecast Instrument

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lat: 53° 17' 32" N

long: 02° 04' 09" W

elevation: 280 m

image © Rudi Hurzlmeier

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