A storm glass is a type of weather forecasting device, composed of a sealed glass container, filled with liquid, that allows the user to forecast the weather by observing the appearance of the liquid in the glass.
The liquid within the glass is a mixture of several ingredients, most commonly distilled water, ethanol, potassium nitrate, ammonium chloride, and camphor. This specific mixture was developed by Admiral Robert FitzRoy and used on his voyage with Charles Darwin on the HMS Beagle.
During the historic voyage, FitzRoy carefully documented how the storm glass would predict the weather:
- If the liquid in the glass is clear, the weather will be bright and clear.
- If the liquid is cloudy, the weather will be cloudy as well, perhaps with precipitation.
- If there are small dots in the liquid, humid or foggy weather can be expected.
- A cloudy glass with small stars indicates thunderstorms.
- If the liquid contains small stars on sunny winter days, then snow is coming.
- If there are large flakes throughout the liquid, it will be overcast in temperate seasons or snowy in the winter.
- If there are crystals at the bottom, this indicates frost.
- If there are threads near the top, it will be windy.
In 1859, violent storms struck the British Isles. In response, the British Crown distributed storm glasses, then known as "FitzRoy's storm barometers," to many small fishing communities around the British Isles that were to be consulted by ships at port before setting sail.
Proposed mechanisms 
Anne Marie Helmenstine discusses how storm glasses work in an About.com article:
"A storm glass works on the premise that temperature and pressure affect solubility, sometimes resulting in clear liquid; other times causing precipitants to form. However, the method by which this works is not fully understood. Although it is well-established that temperature affects solubility, some studies have simultaneously observed several different storm glasses forming similar crystal patterns at different temperatures. In addition, sealed glasses are not exposed to atmospheric pressure changes and do not react to the pressure variations associated with weather systems. Some people have proposed that surface interactions between the glass wall of the storm glass and the liquid contents account for the crystals. Explanations sometimes include effects of electricity or quantum tunneling across the glass."
Cecil Adams performed informal experiments with a storm glass and found that the success of prediction was no better than random probability. However, there have been a few published studies in peer-reviewed journals testing the instrument's accuracy; one example is an article in the Journal of Crystal Growth, whose conclusion was that temperature change is the sole cause of crystal growth in storm glasses.
- 2.5 g potassium nitrate
- 2.5 g ammonium chloride
- 33 ml distilled water
- 40 ml ethanol
- 10 g camphor
Warm the water and dissolve the potassium nitrate and ammonium chloride; add the ethanol; add the camphor. Place in corked test tube.
See also 
- Tempest Prognosticator – an alternative to the storm glass that the British government investigated
|Wikimedia Commons has media related to: Storm Glass|