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Weather change thresholds (e.g. when is pressure considered rising rather than steady?)


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Posted
  • Location: Chelmsford, Essex
  • Location: Chelmsford, Essex

I've got a program on my weather station which uses a form of the old Zambretti forecaster to generate forecasts four times a day. It relies on wind speed/direction, current pressure, pressure trend and season.

I've got it set so that for pressure trend it looks back over the last 4 hours and a change of >1.5mb is rising, <1.5mb is falling and in-between is steady.

For wind, it looks at average speed and gusts over last 3 hours, with an average of less than 1.3mph considered to be "dead calm".

My questions are:

What pressure change, and over what period, would be considered to be falling/steady/rising?

What average of wind speed/gusts over a few hours would be 'dead calm'? Literally 0? Or would 0.1 be considered to be registered as wind?

Thanks in advance. Would be interested in people's opinions on this!

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Posted
  • Location: Dorset
  • Weather Preferences: warehamwx.co.uk
  • Location: Dorset

It all depends on the program you are using. Steve @ Cumulus for example, would usually add code that would conform to the UK standards of measuring certain weather types. As far as i know, pressure trends are derived over 3 hours, but the trend can be extracted for a period of an hour. For example, my trend is showing 'Falling slowly' due to a change of -0.08 hPa/hr - so it can be a very small movement up or down.

As for the wind, I think it is taken over a 10 minute period, and also, that would come down to how your station sends the data. For instance, Davis' stations send a rounded number, its either 0, 1 or 2.. etc
If either the gust or speed show 0mph for 10 minutes, then the 'Calm' tag is initiated.

Edited by Mapantz
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Posted
  • Location: Chelmsford, Essex
  • Location: Chelmsford, Essex

Thanks - do you know what the UK standards are called? I'll see if I can find them - would be interesting to see.

It'd also be good to see any other examples of what's considered to be falling/rising etc. in terms of pressure.

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