The Apple Watch and Stryd are both waterproof and can be used in the rain. Since the Stryd app for the Apple Watch relies on touch screen functionality, you need to be careful when finishing your run. Once you have ended your workout (which requires force touching the screen and tapping the "end" button), be sure dry your watch to have full control over the touchscreen so that you can properly save your run.
If you inadvertently discard the run on your Apple Watch, note that you can still import the data from Stryd with the app on your iPhone.
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