If it's the same model I think it is, press reset briefly once, the display should show something about "since". This only shows briefly, while it is there press and hold reset again to go back to zero.
Ah,, the rest button does a hard reset,, was only trying to reset the rain part,, but the instructions todo this dont work..
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To reset the past 24 hours rainfall:
1. Press SELECT to navigate to the UVI / Barometer /
Rainfall Area. will show next to the Area.
2. Press MODE repeatedly to select Rainfall display.
3. Press and hold MIN / MAX to reset the rainfall to
‘0’.
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The only downside to my weather station (WMR80) is, my old station had both outside and inside temps on display, but this 1 you have to switch between the outside and inside temps. I like all the info to be displayed without pressing buttons.. Also you should be able to select if you want it to display the past hr or 24hrs of rainfall too, but when you change it to the past 24 hrs, about 5secs later it goes back to hourly rainfall.