What’s the deal with a storm glass? Hammacher Schlemmer sells one and says, “Although how it functions remains a mystery, the ability of the stormglass to predict atmospheric change is well documented.” Does it work? If so, how? Or is it just a crappy lava lamp? —Dan

Hey, don’t knock lava lamps. For $179.95, a storm glass from Hammacher Schlemmer gets you a weather forecast of dubious accuracy. Whereas a 25-buck lava lamp, aided by the right combination of tunes and substances, will let you see God.

A storm glass, also called a weather glass or camphor glass, is a glass tube containing a mixture of ammonium chloride, potassium nitrate, camphor, water, and alcohol, making a normally clear liquid in which different types of white crystals periodically grow and dissolve. The idea is that the mixture is so finely balanced that minor fluctuations in atmospheric conditions will change the solubility of the chemicals and produce a wide variety of crystal shapes, from tiny floating flakes to large masses of feathery fans. Each supposedly predicts a certain type of weather.

The inventor of the storm glass is unknown, but descriptions date to the late 18th century. Early theories held that the chemical blend inside was sensitive to light, heat, wind, atmospheric pressure, or even electrical charge. In some glasses the contents were exposed to atmospheric pressure via a flexible rubber cap, but other models were hermetically sealed. (The sealed version is standard nowadays, mainly because a whiff of the contents can bowl you over.)

Interest in storm glasses crested in the 1860s, when such scientific notables as Michael Faraday, Robert Fitzroy, and Charles Tomlinson investigated their properties. Fitzroy, meteorologist and captain of HMS Beagle (of Charles Darwin fame), touted the glasses’ accuracy in his Weather Book of 1863. Tomlinson, on the other hand, tested a glass for several months and found it was sensitive only to heat, calling it a “rude thermoscope.” Japanese research from 2008 backs this up, pointing to temperature change as the sole cause of crystal growth.

I decided we should check this out. However, no way was I shelling out $179.95. No problem, said my assistants Una and Fierra. We’ll make some storm glasses of our own.

They researched storm glass recipes and ordered the appropriate chemicals and laboratory equipment. A hitch: initially no scientific supply house would ship the goods to a private residence, doubtless seeing in the ominous-sounding chemicals the ingredients of a terrorist plot. Una eventually convinced one supplier to send the chemicals after producing her engineering license.

Toiling late one night at Straight Dope Labs, Una and Fierra made six storm glasses. Each consisted of a big test tube filled with the precisely measured chemical mixture, then capped. At first the experiment looked like a bust—the storm glasses became opaque with massed crystals. But after a few days the initial crystal growth settled to the bottom of the tubes, leaving the liquid above clear. Thereafter new crystals would grow or diminish in response to …well, that’s what we meant to find out.

Every day for 12 weeks, Una and Fierra recorded local weather conditions plus their observations of the crystals in each glass. Problems emerged. First, how do you read crystals? Previous researchers’ descriptions were vague, but this much seemed plain: clear liquid meant clear skies, while crystals or cloudiness meant precipitation, which we defined as rain. This gave us a couple simple tests: the storm glass was clear or it wasn’t; rain fell or it didn’t.

In the end, accuracy for individual glasses ranged from 45 to 54 percent, for an average of 49 percent. I’ve got a penny that can do as well as that.

Defenders of the storm glass may blame this on our simplistic scoring method. Suppose a storm glass develops crystals, indicating rain, and subsequently the weather is overcast and threatening—but no rain actually falls. Was the storm glass wrong?

To avoid such ambiguities, I had Una look strictly at days when it rained: did the storm glasses show crystals or not? Result: 53 percent accuracy, with a range of 38 to 62 percent.

Personally, I’m sticking with the lava lamp.

This article appears in Oct 13-19, 2010.

14 replies on “How do storm glasses work? Do they work at all?”

  3. Apparently these storm glasses predict based on barometric pressure (atmospheric pressure). And in my interpretation, for most accurate results you need to place them outside due to the fact that atmospheric pressure inside a home or building fluctuates with the temp of the building vs. outside temp. Higher temp inside (winter) = lower pressure inside / lower temp inside (summer) = higher pressure inside. (https://www.quora.com/Is-air-pressure-when-inside-a-building-lower-than-outside)

    Try putting it outside or next to open window (if you don’t pay utilities), and post review?

  5. Marcha, potassium nitrate is used in high explosives, but also used to fertilize crops, and is highly controlled federally. The first part is why the author needed to convince a chemical supplier. I myself needed some because I am a model rocketer hobbyist and wanted to try my own engines. It comes under different names. The only place I could get it was a pharmacist to which I got the one eye brow raised, qudstioned what I was doing, had to sign a waiver and had to wait almost a week. So nothing fishy there.

  6. I think these most function on the ability of the temperature to effect the solubility of the solvent. I love these glasses. The crystals take on different shapes under different conditions. Nothing is 100% with weather, not even tv weathermen. The instructions I have seen say they must be in a place with no temperature control. I keep mine outside.

  8. I just bought one on amazon for my 15 year old grandson. Turns out there are elaborate warnings in side the box about the “toxic” contents — hand washing and a trip to the emergency room are recommended if the tube is broken. I’ve been on customer service with amazon for 15 or 20 minutes to find out the contents. No one there knows. I’m pretty sure my grandson will break it, accidentally or not. Trying to gauge the downsides.

  9. I have had mine for several months and it changes in ways that sometimes make sense. I have noticed it changes more with the window open which would speak to changes in temperature. However, now it is on a table near a window open for 4 days and I have seen many changes. During the lead up to a severe storm, all I could see is crystals. Now, on warm clear day, I can’t even see any crystals at all. Today has been warm and clear with 80 degree temps that have not fluctuated more than 2 degrees all day. I get the feeling that something besides temperature drives these changes.

  10. Got Mine from Amazon, Top up with water from the tap, with some Food Colour mixed with the water. it worked fine, but the water started to get “Stuff in it” so replaced the Water. Kept it Clear. How much water do you use, with the “Storm Glass”

  11. Not sure if this is a farce or not.. I’ve had this for several weeks and t hasn’t done anything other than take up space on my bedside table Huge storm is heading our way, and I’ll be interested one more time to see if there’s any sign of a change with this glass!!

  12. I definitely dig your style, too. I’m wondering though: you said Una and Fierra made six storm glasses. Was each glass filled with a different mixture or were they all the same with the object being to see if they all reacted the same? Also, you mentioned that while today’s storm glasses are hermetically sealed, other styles exist/ed with the ability for exposure to the elements. Were all of Una and Fierra’s glasses sealed or were some top-poppable? It seems to me that most chemicals trapped inside a jar with no hope of escape WOULD only truly respond to fluctuations in temperature… barometric pressure cant even get a word in, edgewise. Just curious if there was more lengthy scientific stuff that got edited out in the spirit of shaping such a smashing article.

  14. Could the light flux from the window as well as eddy currents around the glass be affecting Peggy Bayliss’s stormglass in the post above? Does anyone know what wavelength changes on clear or rainy days might affect crystallisation/bond bending? 😀 interesting

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