Forecast webtag

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The Cumulus forecast attempts to predict a weather forecast for the coming 12 hours.

It uses an algorithm based on the pressure and trend, wind speed, the month and your hemisphere. The initial formula was developed in the early 1900's and was based on years of observations and testing and proved to be around 91% accurate. It is said to have been extensively tested by NATO in the most extreme weather conditions. It has been demonstrated that the most accurate forecasts are based on readings taken at 9am however it apparently continues to give reliable results throughout the day. The algorithm is taken from a device known as a "Zambretti Forecaster".

The findings have since been coded and included in Cumulus.

Please note that the forecast is for 'novelty' use only. Do not rely on it, and please do not complain when it gets it wrong!


In order for the forecast webtag to be populated you must enable forecasting for your station. In Cumulus click the Configuration menu then Station and tick the box to the bottom right of the screen "Cumulus forecast". Note that if your weather station produces forecasts itself in a form that can be read by Cumulus (the Davis range, for example), and you want Cumulus to use the station-generated forecast, you should leave the box unticked.

The forecasting option was added in Version 1.8.3 and remains available in Cumulus 2 and MX.

Possible values

The <#forecast> web tag will return one of the following values, when the 'Cumulus Forecast' option is chosen. Otherwise, either a string supplied by your weather station will be returned, if it is able to supply one, or if not, 'Not available' will be returned.

The <#cumulusforecast> web tag will always return one of the following, and <#cumulusforecastenc> is available as an alternative with HTML encoding.

  • Settled fine
  • Fine weather
  • Becoming fine
  • Fine, becoming less settled
  • Fine, possible showers
  • Fairly fine, improving
  • Fairly fine, possible showers early
  • Fairly fine, showery later
  • Showery early, improving
  • Changeable, mending
  • Fairly fine, showers likely
  • Rather unsettled clearing later
  • Unsettled, probably improving
  • Showery, bright intervals
  • Showery, becoming less settled
  • Changeable, some precipitation
  • Unsettled, short fine intervals
  • Unsettled, precipitation later
  • Unsettled, some precipitation
  • Mostly very unsettled
  • Occasional precipitation, worsening
  • Precipitation at times, very unsettled
  • Precipitation at frequent intervals
  • Precipitation, very unsettled
  • Stormy, may improve
  • Stormy, much precipitation
  • Exceptional Weather, Stormy, much precipitation (outside of your 'normal' range)


The original formula was converted into a Javascript algorithm by 'Beteljuice' and kindly supplied to the Cumulus project for conversion into its codestream.