The storm alarm notifies you that a pressure drop of 4 hPa / 0.12 inHg or more has occurred during a 3-hour period. Suunto Essential will activate an alarm and flash an alarm symbol on the display for 20 seconds. The storm alarm only works when you have activated the barometer profile in the Alti & Baro mode.
To activate the storm alarm:
You can stop the storm alarm by pressing any button.
You are hiking in a dense forest when your activates the storm alarm. The weather has taken a turn for the worse in the last 3 hours - the sky is getting dark. Good thing your warned you, because you need to find shelter from the hard rain that may soon fall.