Storm glass

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A storm glass is a type of barometer, 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.

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.

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.