Weather
Forecasting
Weather forecasting, despite having massive computing power, access to thousands of weather stations, hundreds of satellites and weather radar, even large organisations such as the Met Office and NOAA still have trouble getting it right all of the time.
Most people only need to know if it is going to rain or if they need to pack the sun screen, at shed40 the almanac page shows the current weather for your location along with a simple weather forecast, the weather data comes from open-meteo.com
This gives a general overview of the upcoming 12 hours weather, it is not meant to compete with NOAA, it works on the simple technique of following the atmospheric pressure trend and the direction of the wind, this has been used effectively for hundreds of years, and generally it is almost 90% accurate.
Barometer
Studying the atmospheric pressure as displayed on a barometer can indicate a general weather trend as in 'Stormy', 'Rain', 'Change', 'Fair' and 'very dry' as the pressure increases from around 950milibars to 1050milibars.
It also shows falling pressure indicating deteriorating conditions and rising pressure as improving conditions, the small changes can be seen over time with the aid of a movable marker needle, just set it over the indicator needle and next time you look, you will see the amount of change and direction.
High pressure usually brings settled weather, cold and clear in the winter, warm and sunny in the summer, low pressure usually brings wet weather in the summer and storms in the winter and is associated with anticyclones and weather fronts.
Zambretti Forecaster
The forecast is based on the 'Zambretti Forecaster' which is a round slide rule type instrument whereby setting the wind and barometric pressure you are able to read a forecast from the reverse according to the letter shown in the falling, rising or steady windows .
This instrument was produced by Negretti and Zambra in around 1915 and is based on weather observations over the years, although i have not managed to find a definitive sauce for the data used.
A similar instrument and lookup table was produced by Raymond Sager in 1969, this was based on his own observations as a meteorologist and requires windspeed, wind direction, pressure, pressure trend and cloud amount to read the predictions, this produces an estimate of general weather, prediction of wind velocity and wind direction forecast.
The algorithms in the software have been determined by studying the geometry of the instrument to ascertain the various factors for the conversion of wind, pressure, season and pressure trend into a forecast letter, however as some of the factors are not exactly linear, the demarcation of some of the forecast letters may not be the same as on the mechanical disc.
I have tried to reproduce the output of the Zambretti Forecaster as closely as possible to the original, however all software implementations i have seen differ very slightly from the original disk, but they are all close enough to work as desired .
This is not intended to give an accurate weather forecast to today's standards, but it does show how even 100 years ago without the aid of computers, weather forecasting or predictions based entirely on previous experience could still be useful.
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