The storm glass was made popular by Admiral Robert Fitzroy after his voyage on the HMS Beagle. The electromagnetic changes in weather patterns activate the crystals inside, so when air pressure decreases the crystals form.
Clear - Bright and clear weather
Cloudy - Cloudy weather and often predicts thunderstorms and rain
Large flaky crystals - cloudy skies, snow in winter
Small stars - Humid or foggy
Crystals at the base - frost