One type of barometer used by Fitzroy was a storm glass. Observing the liquid in the storm glass was supposed to indicate changes in the weather. If the liquid in the glass was clear, the weather would be bright and clear. If the liquid was cloudy, the weather would be cloudy as well, perhaps with precipitation. If there were small dots in the liquid, humid or foggy weather could be expected. A cloudy glass with small stars indicated thunderstorms. If the liquid contained small stars on sunny winter days, then snow was coming. If there were large flakes throughout the liquid, it would be overcast in temperate seasons or snowy in the winter. Crystals at the bottom indicated frost. Threads near the top meant it would be windy.
Here are instructions for constructing a storm glass, described by Pete Borrows in response to a question posted on NewScientist.com, attributed to a letter published in the June 1997 School Science Review.
Ingredients for Storm Glass
- 2.5 g potassium nitrate
- 2.5 g ammonium chloride
- 33 mL distilled water
- 40 mL ethanol
- 10 g camphor
Dissolve the potassium nitrate and ammonium chloride in the water; add the ethanol; add the camphor. Place in corked test tube.
Mark Ford, who has been making storm glasses for years, e-mailed me to add that man-made camphor, while very pure, does contain borneol as a by product of the manufacturing process. His experience is that the synthetic camphor doesn't work as well as natural camphor, perhaps because of the borneol.
Mr. Ford advises dissolving the nitrate and ammonium chloride in the water, then the camphor in the ethanol. Next, slowly mix the two solutions (adding the nitrate & ammonium solution to the ethanol solution works best). It also helps to warm the solution to ensure complete mixing. Mr. Ford never uses a cork, preferring to seal the mixture in small glass tubes.
No matter what method is selected to construct a storm glass, the reader is advised to use proper care in handling the chemicals.
The premise of the functioning of the storm glass is that temperature and pressure affect solubility, sometimes resulting in clear liquid; other times causing precipitants to form. The functioning of this type of storm glass is not fully understood. In similar barometers, the liquid level, generally brightly colored, moves up or down a tube in response to atmospheric pressure. Certainly temperature affects solubility, but sealed glasses are not exposed to the pressure changes that would account for much of the observed behavior. Some people have proposed that surface interactions between the glass wall of the barometer and the liquid contents account for the crystals. Explanations sometimes include effects of electricity or quantum tunneling across the glass.
Italian mathematician/physicist Evangelista Torricelli, a student of Galileo, invented the barometer in 1643. Torricelli used a column of water in a tube 34 ft (10.4 m) long. Storm glasses available today are less cumbersome, easily mounted on a wall.