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# What Is the 'Cone of Uncertainty'?

Cyclone Catarina in the South Atlantic as photographed by astronauts on the International Space Station in 2004. Image credit: NASA via Wikimedia Commons // Public Domain

Weather forecasts are hardly ever perfect. Most of the forecasts you receive these days are pretty solid if you use the right source, but it’s not always a sure bet. Sometimes your friendly local meteorologist uses bad information to arrive at a bad forecast, and sometimes he or she just misses one. Predicting the future inherently involves some level of uncertainty, and some situations are more uncertain than others. Hurricanes are no different. When we look at a hurricane forecast map, the most striking feature you’ll see is the aptly named cone of uncertainty.

A hurricane forecast is conveyed to the public using a map that shows the forecast position of the very center of a tropical cyclone at seven different time steps up to five days out from the initial forecast. The cone of uncertainty, which looks like a quotation bubble emanating from the first point on a hurricane forecast map, is the historical margin of error in a meteorologist’s forecast track for the center of a tropical cyclone.

The forecast track and cone of uncertainty for Hurricane Igor in September 2010. The hurricane’s center stayed within the cone of uncertainty for almost its entire lifespan. Image: National Hurricane Center

At the end of every hurricane season, meteorologists at the National Hurricane Center look at all of the forecasts they made for storms in previous years and average out how far off their forecasts were from the actual location of the tropical cyclone. They average out and compile these errors for every time step in the forecast period all the way out to five days from the initial forecast. When you plot their average track errors on a map using a sequence of circles, the smoothed-out end result is the cone of uncertainty we’re all familiar with.

The center of a tropical cyclone typically stays within the cone of uncertainty 66 percent of the time, which also means that it strays outside of the cone 34 percent of the time. Tropical cyclone forecast tracks are much better today than they were in years past, but track error naturally grows with time as confidence in a forecast track decreases. The National Hurricane Center is only off by an average of about 35 miles when they predict the location of the center of a tropical cyclone 12 hours out. That average error grows to 97 miles two days out and goes all the way up to 273 miles five days out. To give you an idea of how wide the cone of uncertainty is five days out, it’s just roughly the distance you would cover traveling from Boston to Philadelphia.

The cone of uncertainty does not show you the confidence that forecasters have in that particular forecast. It shows you neither the size nor the total extent of the impacts a tropical cyclone will have. It doesn’t matter if it’s a 500-mile-wide hurricane or a 20-mile-wide tropical depression; the cone only applies to the very center of a hurricane, and it only tells you how far off forecasters were when they predicted the track of previous cyclones. You aren’t necessarily safe if your location falls outside of the cone of uncertainty, nor does it mean that areas inside the cone are definitely going to get blasted. Understanding the size and impacts of a tropical cyclone requires more context than one simple map can provide you.

Most tropical cyclones form and move along tracks that don’t stray too far from their predicted paths. A sudden jog to the west or east usually isn’t too much of an issue, but even a small deviation in course could be a big deal during complex or high-risk situations.

The forecast track and actual track of Hurricane Joaquin in October 2015. Map: Dennis Mersereau

The most common reasons storms deviate from their predicted paths are that they’re extremely weak (and subject to erratic movement) or they exist in a complicated setup with lots of moving parts. One of those latter situations occurred in 2015 during Hurricane Joaquin. The major hurricane formed over the Bahamas in a complex environment that would ultimately dictate whether it would make landfall in the United States or swirl harmlessly out to sea. On October 1, meteorologists issued a forecast that showed the storm approaching the Mid-Atlantic coast of the United States. Most of the models misread the weather pattern that week and the storm wound up trekking out to sea, causing Hurricane Joaquin’s ultimate path to fall almost entirely outside of the cone of uncertainty for that forecast.

If you find yourself in or near the cone of uncertainty four or five days out, that’s your cue to keep tabs on the situation and make preparations in case you have to secure your property or evacuate to safer ground. It’s time to act if you’re still in the cone a few days out. Watches and warnings will already be in effect by then and you’ll have plenty of information from the experts telling you what you should do in order to stay safe.

Jason English
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15 Surprising Facts About Winter Weather
Jason English

Whether you enjoy bundling up in your coziest gear or are already counting down the days until spring, here are 15 facts about what’s happening outdoors this time of year.

#### 1. IT SOMETIMES SNOWS WHERE YOU LEAST EXPECT IT.

You wouldn’t be shocked to see snow on the ground of Siberia or Minnesota when traveling to those places during the winter months. But northern areas don’t have a monopoly on snowfall—the white stuff has been known to touch down everywhere from the Sahara Desert to Hawaii. Even the driest place on Earth isn’t immune. In 2011, the Atacama Desert in Chile received nearly 32 inches of snow thanks to a rare cold front from Antarctica.

#### 2. SNOWFLAKES COME IN ALL SIZES.

The average snowflake ranges from a size slightly smaller than a penny to the width of a human hair. But according to some unverified sources they can grow much larger. Witnesses of a snowstorm in Fort Keogh, Montana in 1887 claimed to see milk-pan sized crystals fall from the sky. If true that would make them the largest snowflakes ever spotted, at around 15 inches wide.

#### 3. A LITTLE WATER CAN ADD UP TO A LOT OF SNOW.

The air doesn’t need to be super moist to produce impressive amounts of snow. Unlike plain rainfall, a bank of fluffy snow contains lots of air that adds to its bulk. That’s why what would have been an inch of rain in the summer equals about 10 inches of snow in the colder months.

#### 4. YOU CAN HEAR THUNDERSNOW WHEN THE CONDITIONS ARE RIGHT.

If you’ve ever heard the unmistakable rumble of thunder in the middle of a snowstorm, that’s not your ears playing tricks on you. It’s likely thundersnow, a rare winter weather phenomenon that’s most common near lakes. When relatively warm columns of air rise from the ground and form turbulent storm clouds in the sky in the winter, there’s potential for thundersnow. A few more factors are still necessary for it to occur, namely air that’s warmer than the cloud cover above it and wind that pushes the warm air upwards. Even then it’s entirely possible to miss thundersnow when it happens right over your head: Lightning is harder to see in the winter and the snow sometimes dampens the thunderous sound.

#### 5. SNOW FALLS AT 1 TO 6 FEET PER SECOND.

At least in the case of snowflakes with broad structures, which act as parachutes. Snow that falls in the form of pellet-like graupel travels to Earth at a much faster rate.

#### 6. IT DOESN’T TAKE LONG FOR THE TEMPERATURE TO DROP.

Don’t take mild conditions in the middle of January as an excuse to leave home without a jacket. Rapid City, South Dakota’s weather records from January 10, 1911, show just how fast temperatures can plummet. The day started out at a pleasant 55°F, then over the course of 15 minutes a wicked cold front brought the temperature down to 8 degrees. That day still holds the record for quickest cold snap in history.

#### 7. THE EARTH IS CLOSEST TO THE SUN DURING THE WINTER.

Every January (the start of the winter season in the northern hemisphere) the Earth reaches the point in its orbit that’s nearest to the Sun. Despite some common misconceptions, the seasonal drop in temperature has nothing to do with the distance of our planet to the Sun. It instead has everything to do with which direction the Earth’s axis is tilting, which is why the two hemispheres experience winter at different times of the year.

#### 8. MORE THAN 22 MILLION TONS OF SALT ARE USED ON U.S. ROADS EACH WINTER.

That comes out to about 137 pounds of salt per person.

#### 9. THE SNOWIEST CITY ON EARTH IS IN JAPAN.

Aomori City in northern Japan receives more snowfall than any major city on the planet. Each year citizens are pummeled with 312 inches, or about 26 feet, of snow on average.

#### 10. SOMETIMES SNOWBALLS FORM THEMSELVES.

Something strange happened earlier this year in northwest Siberia: Mysterious, giant snowballs began washing up on a beach along the Gulf of Ob. It turns out the ice orbs were formed naturally by the rolling motions of wind and water. With some spheres reaching nearly 3 feet in width, you wouldn’t want to use this frozen ammunition in a snowball fight.

#### 11. WIND CHILL IS CALCULATED USING A PRECISE FORMULA.

When the weatherman reports a “real feel” temperature of -10 degrees outside, it may sound like he’s coming up with that number on the spot. But wind chill is actually calculated using a complicated equation devised by meteorologists. For math nerds who’d like to test it at home, the formula reads: Wind Chill = 35.74 + 0.6215T – 35.75(V^0.16) + 0.4275T(V^0.16).

#### 12. CITIES ARE FORCED TO DISPOSE OF SNOW IN CREATIVE WAYS.

When snow piles up too high for cities to manage, it’s usually hauled away to parking lots or other wide-open spaces where it can sit until the weather warms up. During particularly snowy seasons, cities are sometimes forced to dump snow in the ocean, only to be met with criticism from environmental activists. Some cities employ snow melters that use hot water to melt 30 to 50 tons of snow an hour. This method is quick but costly—a single machine can cost \$200,000 and burn 60 gallons of fuel in an hour of use.

#### 13. WET SNOW IS BEST FOR SNOWMAN-BUILDING, ACCORDING TO SCIENCE.

Physics confirms what you’ve likely known since childhood: Snow on the wet or moist side is best for building your own backyard Frosty. One scientist pegs the perfect snow-to-water ratio at 5:1.

#### 14. SNOWFLAKES AREN’T ALWAYS UNIQUE.

Snow crystals usually form unique patterns, but there’s at least one instance of identical snowflakes in the record books. In 1988, two snowflakes collected from a Wisconsin storm were confirmed to be twins at an atmospheric research center in Colorado.

#### 15. THERE’S A DIFFERENCE BETWEEN FREEZING RAIN AND SLEET

Freezing rain and sleet can both have scary effects on driving conditions, but their formations differ in some key ways. Both types of precipitation occur when rain formed in warm air in the sky passes through a layer of cold air near the ground. Thicker layers of cold air create sleet, a slushy form of water that’s semi-frozen by the time it reaches the Earth. Thinner layers don’t give rain enough time to freeze until it hits the surface of the ground—it then forms a thin coat of ice wherever it lands.

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It Just Snowed In the Sahara for the Second Time In Less Than a Month
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The town of Aïn Séfra, Algeria might need to find a new nickname. Though it’s often referred to as “The Gateway to the Sahara,” the 137-year-old province in northwest Algeria is currently digging out from a rare—and unexpected—snowstorm that left the desert town covered in several inches of snow and battling sub-zero temperatures.

While the Daily Mail reported that “locals took to the nearby sand dunes to enjoy the unusual weather,” the strangest part of the story is that this is Aïn Séfra’s second snowfall in less than a month. On Sunday, January 7, a freak blizzard left parts of the Sahara blanketed in as much as 16 inches of snow.

This most recent storm marked the region’s fourth snowfall in nearly 40 years; in addition to January's dose of the white stuff, the area has been hit with other surprise wintry events in February 1979 and December 2016.

But North Africa isn’t the only area that’s seeing record-breaking weather events. On Saturday, February 3, 17 inches of snow fell on Moscow within 24 hours in what the country has dubbed “the snowfall of the century.” In mid-January, Oymyakon, Russia—a rural village in the Yakutia region, which is already well known as one of the coldest inhabited areas of the world—saw temperatures drop to -88.6°F, making it chilly enough to both bust thermometers and freeze people’s eyelashes. And you thought dealing with single-digit temperatures was tough!

[h/t: Daily Mail]

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