Name: Simply Weather
Rating: 4.5 (33 reviews)
Author: simonletts

This is a short-term weather forecasting widget based only on the current air pressure, its trend and the wind direction. It does not need an internet connection. I’ve seen it claimed that a forecast made around 09:00 is greater than 90% accurate.
The app is a software implementation of the Zambretti Forecaster (https://en.wikipedia.org/wiki/Zambretti_Forecaster) produced at about the time of the 1st World War. There have been several software versions of the forecaster, this one is adapted from Beteljuice’s Javascript code (https://www.beteljuice.co.uk/zambretti/forecast.html).
Usage
First you need to set the wind direction. Face into the wind, hold your device in front of you and press Menu. You can swipe left (and maybe right) on touchscreen devices or press Select or Enter to cycle through all the wind directions. One “direction” is Calm (no wind). The only way to get there is by cycling through. It’s between N and NNW so you can set the approximate direction and then cycle. The widget will use the last direction you chose until you change it.
The original algorithm was designed to use barometric pressure adjusted to mean sea level. For this the widget needs a good GPS signal to obtain the altitude. Alternatively you can use the uncalibrated ambient air pressure. I don’t know how good the results will be if you use ambient. You might need to tweak some of the other settings as well.
The widget can optionally show the current temperature. If you have an external tempe sensor it will try to use it, otherwise it will use the watch’s internal sensor if available. Don’t forget this will be affected by your body. The tempe can take a very long time to connect to the watch, until it does the widget shows the internal temperature. The temperature units (C or F) will be what has been set globally for the watch. The widget doesn’t have its own setting.
On watches with at-a-glance carousel mode the widget shows the last forecast it calculated. This could, of course, be very wrong!
Settings
You can choose whether to use mean barometric pressure calibrated to sea level pressure or the ambient pressure. The default is adjusted for mean sea level.
Nearer the Poles a greater spread between lower and upper barometer limits and large trends are needed for weather conditions to change. Nearer the Equator a smaller spread and smaller trends are needed.
Initially the widget is set for the UK, 950 hPa to 1050 hPa. You can change the limits to suit your own area.
Different forecasters use different times when looking for pressure changes and different changes to indicate a Rising, Steady or Falling trend. I use 3 hours and 0.5hPa. You can set different values.
My watch barometer consistently seems to read too high. You can set a positive or negative correction value that is added to the raw pressure reading. N.B. I have changed the way of reading the sensor which seems to give better results.
By default the current temperature is shown.
If your watch supports glances you can set your own display name for the glance loop.
Icon design is by Freepik (https://www.flaticon.com/authors/freepik) from https://www.flaticon.com and licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0).
I created the widget for personal use – it's shared for anyone who would like to try it.
The widget has been tested on my vívoactive® HR, a vívoactive® 3 and other devices in the simulator.
Please contact me if you have any problems, comments or requests. Include your email address if you would like a reply.
Thank you Garmin user @Thomas for asking if this widget was possible and helping me test it. Thank you also to all the others who have made useful comments and suggestions.