You fist need to determine a unique name/reference for your event, like ‘Temp Above 20’
Then your flow would be:
WHEN
- Temperature rises above 20 degees
AND
- Event ‘Temp Above 20’ did not happen within 1 hour
THEN (true/blue line)
- Trigger event ‘Temp Above 20’
- Send email
ELSE (false/yellow line)
- Trigger event ‘Temp Above 20’