How to Make a Storm Glass and Use It to Predict the Weather

The crystals in a storm glass change, depending on temperature and other weather conditions.
The crystals in a storm glass change, depending on temperature and other weather conditions. (Anne Helmenstine)

A storm glass is an instrument used to predict the weather by observing the crystallization of liquid in a sealed container. While you’ll get a better weather forecast from a meteorologist, the storm glass does change over time, especially in response to temperature. Storm glasses are readily available online, but it’s easy to make one yourself.

Storm Glass Materials

The liquid inside a storm glass consists of water and alcohol. Camphor, potassium nitrate, and ammonium chloride are dissolved in the liquid and form various types of crystals. The ingredients you need are:

  • 2.5 g potassium nitrate
  • 2.5 g ammonium chloride
  • 33 mL distilled water
  • 40 mL ethanol
  • 10 g natural camphor
  • Small glass container that can be sealed (100 to 200 mL volume)
Storm Glass
Storm glass crystals can appear anywhere in the liquid. They display multiple crystal shapes, so even if you don’t use them to predict the weather, they are fun to watch. (Anne Helmenstine)

You can buy some of the chemicals in store or order them online. You can make the ammonium chloride, if you like. Be sure to use distilled water or water purified by reverse osmosis and not tap water or spring water. Avoid using water with added minerals. For ethanol, it’s easiest to use 99% alcohol from a drug store. Check to make sure it is denatured ethanol (ethyl alcohol) and not isopropyl alcohol (isopropanol). You can also use 70% alcohol, but then you need to use 16 mL water and 57 mL alcohol (because part of the product already contains water). Another option is to use 100-proof vodka (or other clear spirit). If you use vodka, use 73 mL to replace the alcohol and water.

While denatured alcohol sold in the United States is clear, it’s dyed in some other countries and may go by the name “methylated spirits.” Denatured alcohol sold within the European Union (EU) is ethanol, with smaller amounts of isopropyl alcohol, methyl ethyl ketone, denatonium benzonate (to make it taste bitter), and methyl violet (to dye it purple). It’s possible the colored product may work just fine in a storm glass, creating a glass with a lavender liquid. But, it’s easy to filter out the dye. Simply place some activated charcoal on a coffee filter and pour the alcohol through the charcoal. A water filter cartridge works, but if you use it then it will no longer be suitable for filtering drinking water!

How to Make a Storm Glass

Once you collect the chemicals, construction is easy:

  1. Dissolve the potassium nitrate and ammonium chloride in the water.
  2. Dissolve the camphor in the ethanol.
  3. Add the potassium nitrate and ammonium chloride solution to the camphor solution. You may need to warm the solutions to get them to mix.
  4. Either place the mixture in a corked test tube or seal it within glass. To seal glass, apply heat to the top of the tube until it softens, and tilt the tube so the glass edges melt together. If you use a cork, wrap it with parafilm or coat it with wax to ensure a good seal.

Ideally, your storm glass contains colorless, transparent liquid that forms crystals in response to the external environment. However, impurities in the ingredients may result in a colored liquid. A slight amber tint (usually from an impurity in the camphor) is common and won’t affect storm glass function. But, if the solution is cloudy, it’s likely the glass won’t function as intended. If that happens, check the purity of your chemicals and try again with better products.

Use a Storm Glass to Predict the Weather

Storm Glass Closeup
It rarely snows in South Carolina, but when it did, I saw the most beautiful crystals in my storm glass. (Anne Helmenstine)

Placement of the storm glass plays a large role in how well it works. You’ll get the best results placing the glass on a window sill. Ideally, choose a window that gets relatively constant light throughout the day. Otherwise, place the glass on a shelf or table near an outer wall.

Here is how to interpret a storm glass to predict the weather:

  • Clear liquid: bright and clear weather
  • Cloudy liquid: cloudy weather, perhaps with precipitation
  • Small dots in the liquid: potentially humid or foggy weather
  • Cloudy liquid with small stars: thunderstorms or snow, depending on the temperature
  • Large flakes scattered throughout the liquid: overcast skies, possibly with rain or snow
  • Crystals at the bottom: frost
  • Threads near the top: wind

Over time, you’ll get a sense for what the appearance of your storm glass means. Weather forecasting with a storm glass is not an exact science, but you’ll improve your predictions if you keep a log. Record your observations about the glass and the weather. In addition to the characteristics of the liquid (clear, cloudy, stars, threads, flakes, crystals, and the location of crystals), record as much data as possible about the weather. If possible, include temperature, barometer readings (pressure), and relative humidity. Over time, you’ll be able to predict the weather based on how your glass behaves. Keep in mind, a storm glass is more of a curiosity than a scientific instrument. It’s better to allow the weather service to make predictions.

A nice, easy-to-understand tutorial for making a storm glass using vodka.

How to Make a Color Change Storm Glass

Storm glasses that change color rest on a base that contains a color-changing LED light. These display bases are readily available online in a variety of sizes. While it may be possible to introduce an indicator into the storm glass liquid, there’s a good chance the pigment would disrupt crystal formation. However, if you decide to try to make the liquid change colors, I think your best chance of success is to add cobalt(II) chloride. It changes between pink and blue, depending on temperature.

How the Storm Glass Works

The premise of the functioning of the storm glass is that temperature and pressure affect solubility, sometimes resulting in a clear liquid and other times causing precipitants to form. In similar barometers, the liquid level moves up or down a tube in response to atmospheric pressure. 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.

But, research shows the crystals mainly form in response to temperature changes. The crystals change because solubility and crystal structure depend on both temperature and the rate of temperature change.

History of the Storm Glass

This type of storm glass was used by Robert FitzRoy, the captain of the HMS Beagle during Charles Darwin’s voyage. FitzRoy acted as meteorologist and hydrologist for the journey. FitzRoy stated “storm glasses” had been made in England for at least a century before his 1863 publication of “The Weather Book.” Storm glasses were also made in Italy, France, and Germany.

FitzRoy started to study the glasses in 1825. He described their properties and noted there was a wide variation in the functioning of the glasses, depending on the formula and method used to create them. The basic formula of the liquid of a good storm glass consisted of camphor, partially dissolved in alcohol; along with water; ethanol; and a bit of air space. FitzRoy emphasized the glass needed to be hermetically sealed, not open to the outside environment.

Modern storm glasses are widely available as curiosities. The reader may expect variation in their appearance and function, as the formula for making the glass is as much an art as a science.

Do Storm Glasses Work? A Personal Note

I first published instructions for making a storm glass circa 2001 or 2002 on the About Chemistry website. At that time, the site had a public forum, where chemists and enthusiasts gathered to discuss storm glass recipes, as well as tips and tricks for success. Many of us kept storm glasses and observed them. More recently, I purchased a storm glass from Amazon to see whether it worked. Because (let’s be honest), it’s much less expensive to buy a pre-made storm glass than it is to make one! I placed the glass in my kitchen window sill, which gets fairly constant indirect light throughout the day. I observed the glass for a year so I could comment on its function.

The storm glass works, up to a point. The crystals are markedly different in stormy weather (hurricanes, snow) compared with clear, sunny days. As far as predicting the weather goes, the glass indicates the weather is about to change, but that’s about it. Mostly, it reflects current conditions, so I use satellite radar if I really need to know what’s coming. Friends around the world who keep storm glasses have varying degrees of success with them. They are most interesting if kept in windows, garages, or outdoors (where temperature changes). Mostly, a storm glass is a fun science decoration.

Do you have a storm glass? Feel free to share your experiences in the comments!