Can this simple contraption predict the weather?
Can this simple contraption predict the weather? Courtesy of Tripoint Gifts/Used with Permission

Hanging outside the homes across Bermuda are little vials of fluid that are the locals’ secret to predicting the island weather. Or so they say.

Bermuda has a long tradition of using shark oil as a meteorological tool, but the true efficacy and mechanics of the predator’s oil is a matter of debate. According to the Bermuda Institute of Ocean Sciences (BIOS), the local tradition of using a vial of shark oil to predict the weather has been in place for up to 300 years.

“Now where they developed, I don’t know,” says Captain Alan Card, a lifelong Bermuda fisherman, now 69. “I can remember 65 years ago seeing a bottle of shark oil hanging down at the marina.” Card has been looking to the barometers for just about his entire career. “The best one that I’ve seen used to be down at Somerset Bridge,” he says. “Someone either broke it or stole it. You’d get to that barometer and think, ‘Huh. We’ve got some weather coming. It might not be today mind you, but we’ve got some weather coming.’”

The construction of the barometers is exceedingly simple—it’s just a small amount of oil extracted from a shark liver, sealed in glass. Sometimes that means a jar, an old wine bottle, or a more artful custom container. According believers in the barometers, like Card, the devices are able to predict oncoming weather with a fair degree of accuracy. When asked if they were effective, Card responded with a simple, “Hell, yeah.” 

An example of a dusky shark.
An example of a dusky shark. shishihenge/CC BY 2.0

Thatcher Adams, a local shark oil barometer expert who was referenced in a 1985 article in Yachting Magazine, said there are at least 40 different readable signals that can appear in a given oil barometer. Most of the readings have to do with the small amount of sediment contained in the liquid.

“We’ve got one batch in either a scotch bottle or a rum bottle, a Bacardi bottle, something like that,” says Card. “It’s got two inches of white sediment on the bottom of the bottle.” On days with perfect weather, the oil is clear. On overcast or rainy days, ”you’ll look at it and you’ll find a little spiral or a little peak in the sediment. Then other days you look at it and it’s absolutely cloudy. It looks like milk, you can’t see through it.”

The oil is said to begin swirling as hurricanes approach, and the way the sediment slopes can show which direction the wind is coming from. But the barometer is not just capable of predicting extreme weather—small variations in the peaks and valleys on the surface of the sediment, or crystals forming on the glass, are believed to portend more subtle weather situations.

An unsourced, but hand-drawn diagram of a shark oil barometer on shows a few of the basic signs to be read from shark oil, including summer rains, which are indicated by rising bits of sediment, called, “rain seeds.” This diagram suggests that the sea can be predicted by the oil, the weather by the exposed glass, and even the horizon based on the surface of the oil. Along the top, it is rather poetically noted, “As The Oil, So Is The Sea / As The Glass, So Is The Sky.” Card says that it can take years of practice to learn to read shark oil, getting used to comparing its behavior to the actual weather.  

While the construction of the barometer is simple, according to local lore, the collection of the main ingredient is surrounded by its own set of beliefs. In an article published by the BIOS, they not is believed that in order to extract the most accurate oil, the fluid must be taken from a small dusky shark (known locally as “puppy sharks”), ideally during a full moon. The relation between the phase of the moon and the efficacy of the oil seems to echo the general changes in the weather that accompany the shifting tides.

Card’s account of the liver collection process is a bit more utilitarian. “You take a piece of the liver and put it in a stocking, for argument’s sake,” he says. “You hang it up somewhere where it doesn’t get stinking in the bedroom. Outside somewhere. And over a period of days, the oil would leach out and fall in the bottle. You seal the bottle, and there you go, you’ve got your shark oil.”

In terms of the science behind the fluctuations in the oil, traditional barometers work by measuring changes in atmospheric pressure, but shark oil barometers don’t seem to have any scientifically observed correlation to the air pressure. The science behind them is as murky as shark oil on a stormy day.

One theory posits that the oil reacts to the atmosphere while it is still in the shark, giving them an internal warning to avoid stormy waters, but the extracted oil doesn’t seem to prove that. Another, seemingly more outlandish theory, posited by Adams in the article in Yachting, is that the oil reacts to infrared radiation, which subtly affects the temperature of the liquid. According to a study conducted by BIOS, the oil does seem particularly sensitive to temperature, so maybe Adams’ theory has some merit.

We may not know exactly what drives shark oil barometers, but several hundred Bermudians still hang them outside their houses and around marinas. And they’re not just for show. “We look at it,” says Card. “Not on a daily basis, but when we know the weather’s gonna get shitty, I’ll ask them, ‘What’s the shark oil doing today?’”