Explainer: What is thundersnow? | Science News for Students

Explainer: What is thundersnow?

Emerging forensics suggests there may even be a human link to this bizarre phenomenon
Mar 9, 2018 — 12:19 pm EST
Manhattan snowstorm

Tall structures, such as skyscrapers, radio towers and wind turbines, may create conditions that greatly up the chance of cloud-to-ground lightning during windy snowstorms.

palinchakjr/iStockphoto

Heavy snow can often create a peaceful setting. A blissful quiet may develop as snowflakes drape the landscape beneath a blanket of white. But occasionally, a sky-wide flash can disrupt this tranquility with a deafening, ear-splitting crash. That sound can echo, briefly, like gunshots. The ground may even shudder.

This is thundersnow.

To occur, the circumstances have to be exceptional. And unless it occurs almost directly overhead, you may never know it. The reason: Snow acts as a sound suppressor, muffling thunder and limiting the sound’s ability to bounce and spread.

Yet thundersnow appears to be getting a bit less rare.

For instance, a huge March 7 nor'easter snowstorm hit the Northeast states and New England earlier this week. And it was accompanied by numerous cracks of thunder. One bolt even struck New York's tallest structure, the new 104-story World Trade Center building.

Two months earlier, another epidemic of thundersnow punctuated New England skies. It arrived shortly after daybreak on January 4, 2018. On that morning, a flurry of more than 30 flashes hit an otherwise quiet, wooded area in Montville, Conn. They occurred along a narrow band on the northwest side of rural Lake Konomoc.

Lightning mapping is accurate to within a few hundred meters (up to 1,000 feet). So it’s impossible to confirm with only those data what got struck. But there are two radio- and television-transmission towers in nearby Oakdale that soar some 316 and 367 meters (1,037 and 1,204 feet) into the sky. A limousine company — Liberty Limited — is right next to the property on which these towers sit. Angela Ried works for that limo company. And she confirms the towers were struck that day.

They “got struck at least four or five times,” she recalls. “It was pretty loud.” Though she knew at once it was lightning, she was surprised to hear it in winter. “I’ve worked here since ’93,” she notes, “and this is the first time I’ve ever seen thunder and lightning during a snowstorm.”

Her memory matches lightning reports logged by the National Weather Service.

Thundersnow also moved into Needham, Mass. Lightning was registered at sites near the WCVB-TV transmission towers. These structures rise some 395 meters (1,300 feet) into the air. They too sparked off about a dozen lightning strikes.

Prudential Tower
The 276 meter (906 foot) Prudential Tower in Boston, the top of which is show here, was hit by lightning during a snowstorm earlier this year. It’s one of the tallest buildings in the city and has an antenna that broadcasts radio signals.
bostux/flickr (CC BY-NC-ND 2.0)

In nearby Boston, only one building got struck. It was the Prudential Tower, a 52-floor skyscraper with a rooftop spire topping 276 meters (906 feet). The mast broadcasts signals for multiple radio stations. “I heard it,” said Owen Anastas, of Boston. This particular strike, he notes, “happened around 11:30 during an incredible snow.”

The storm dumped a foot (one-third meter) or more of snow throughout a wide swath. And an estimated nine in every 10 lightning strikes in that storm struck man-made structures more than 250 meters (820 feet) tall. That raises the question: Are human structures playing some role in fostering thundersnow?

What makes thundersnow?

Thunderstorms usually form when warm air near the ground rises (because it is less dense than nearby masses of cold air). It’s the same reason a hot air balloon soars. And these conditions are why most boomers are spawned during spring and summer months.

The climbing air will rise several kilometers (miles) up, to a height where the temperature is below freezing. This can trigger a phenomenon called triboelectrification (Try-bo-ee-lek-trih-fih-KAY-shun). This word describes friction among air particles that causes a separation of electrical charge. It’s somewhat like rubbing a balloon against fabric so that the separated charge now allows the balloon to temporarily “stick” to the wall.

Air within the thunderstorm is very turbulent. This causes ice crystals to bump into each other. Through this process, they can gain or lose electrons. Ice crystals lose electrons, leaving them positively charged. Wetter precipitation gains electrons, making it negatively charged. When the charges build up enough — ZAP! An electric spark, or lightning, jumps between the two regions to balance out the charge.

summer thunderstorm
In a normal thunderstorm, lightning can reach down from clouds having a low base. Strong temperature differentials between the stormy area and the region in front of the storm, can cause air to rise quickly, creating lots of friction and charge separation.
designer125/ iStockphoto

To get this in the wintertime, however, is challenging. In the summer, pockets of air rise vertically to produce thunderstorms. That doesn’t really happen in the winter. Frigid-weather storms develop differently.

Two conflicting forces wage a battle that sends air on a “slantwise” path high into the sky. That means the air isn’t rising straight up and down, as in most thunderstorms. Thundersnow storms also do not usually form on the warm side of large, spiraling cyclones, as thunderstorms typically do. Instead, they develop in a weird spot – the colder backside of the storm system.

Because large storm systems oftentimes look like commas, that aggressive backlash is called the “comma head.” This is where cold air wraps in from the north.

Snowstorms can become super windy. This will happen because the lowest air pressure will occur at the center of the storm. It mimics a vacuum, pulling in air from its surroundings. Air spirals into the middle of storm systems like water swirling down a drain.

Or this is what usually happens.

But the January 2018 storm threw a meteorological curveball. It brought with it an extremely strong temperature gradient. Over the ocean waters off of Cape Cod, Mass., air temperatures soared to near 13° Celsius (55.4° Fahrenheit). Just 330 kilometers (205 miles) to the west, over land in Connecticut, it was 18 degrees C (23 degrees F) colder.

That extreme temperature contrast over such a narrow region generated a thermal wind. That’s when air flows from warm to cold regions.

Since cold air is denser; it sinks to the ground. Warm air from the ocean gets pulled in to replace it. That surface-hugging cold air undercuts the encroaching warm air. So the warm air now sloshes up that cold “surface” of air.

That warm air goes on to climb into the atmosphere because it has so much momentum. It’s like rolling a ball up a slide. Here, the slide is the surface of cold air. And the warm air is that ball rolling up that surface. Normally, the air wouldn’t take this path. It’s like bowling the ball up the slide, against the force of gravity. 

It’s also fairly uncommon, which makes it tough to forecast. It is much easier to predict the conditions that tend to be associated with it, such as narrow bands of heavy snow.

Figuring out if, when and where lightning will strike within a snowstorm is a different story.

The National Lightning Detection Network is a commercial array of antennas across the United States. It monitors lightning strikes 24/7, all year long. But this network’s antennas will miss bolts that flash within clouds. That’s why the National Weather Service relies on public reports of thunder or lightning to track most thundersnow.

On rare occasions, as occurred earlier this winter, bolts may strike the ground. And when they do, these can be just as dangerous as strikes during a summer storm. They can cause damage, injury — even death. One bolt during a snowstorm on February 9, 2017, caused a house fire in Warwick, R.I. The bolt also zapped a nearby tree, blasting part of its trunk into the wall of that home,

The link to human activities

So what’s going on? Two Japanese researchers had some insights 24 years ago that they described in the Journal of Geophysical Research. Their paper reviewed decades’ worth of wintertime lightning strikes off the north coast of Japan. The pair used radar data and measurements from instruments used to measure electrical activity. From these data, clues emerged. It appeared that a key change takes place when low-topped winter thunderclouds mature.

Think of the cloud as a three-layer cake, with each layer having a different electrical charge. For shallow, low-topped winter thunderclouds, the charges in these layers are positive-negative-positive. The lower positive charge can appear at temperatures from 0 to -9° Celsius.

And where the lower layer has a net positive electric charge, that layer “is apparently capable of initiating ground flashes,” the paper noted.

So why did the 2018 New England storm clouds almost exclusively throw their lightning at tall towers?

It’s possible that these towers triggered the lightning by poking into the underside of clouds. In doing so, they take on this lower positive charge. They can now spark a bolt between the now-positive tower and the negative charge in the middle of the cloud above.

But that alone shouldn’t be enough to generate a bolt. After all, the electric fields in snowstorms are significantly smaller than those in summer thunderclouds.

However, those fields can be locally enhanced by pointy objects. Those points can focus a charge, boosting it 10-fold. And that may be enough to exceed the level required for an electric charge — or spark — to leap through the air. Once this happens, that spark can set off a rapid chain reaction.

With that, a lightning bolt is born.

The role of winds — high winds

But there’s a catch. Nature resists charges leaping through the air. So when a charge builds up on some object, the air tends to create a local region around it that has the opposite electric charge. This is known as a “space charge.”

Consider the tower. If a positive charge were to build up on the tip, a negative space charge should form around it. This would shield the tower tip from being struck by a bolt from a region in the middle layer of the cloud .

However, where the winds are strong enough, they can actually blow away this shielding space charge. That would leave the tower tip exposed, dramatically upping the odds of it triggering a lightning strike.

This was observed in 2011 during the Chicago thundersnow storm of February 1 and 2. Researchers Tom Warner, Timothy Lang and Walter Lyons observed winds of 29 kilometers per hour (18 miles per hour) during each cloud-to-ground lightning flash. They noted a whopping 93 percent of lightning strikes in the snowy region of the storm involved tall buildings or towers (including wind turbines).

During New England’s January thundersnow events, the top of each tower where a lightning bolt was recorded had also experienced high winds. Indeed, the minimum speed during every single flash exceeded 36 kilometers per hour (22.4 miles per hour). Moreover, the base of these storm clouds had been extremely low.

The minimum height at which moisture will condense, forming a cloud, is known as the “lifting condensation level.” In the case of the January storms, that level was around 275 meters (902 feet). And guess what: Each tower struck by lightning had been higher than that. So they would have poked into that lower positively charged region of the clouds.

And then there were the wind turbines

There was an exception, however. It was off the coast of Block Island, Rhode Island.

At first glance, it looked like 10 or so bolts of lightning had randomly struck the water. Data would later show that five wind turbines were out there. The turbines’ blades were mounted atop 30-meter (98.5-foot) pedestals. The turbines’ shafts were each 100 meters (328 feet) tall. And each turbine blade was 73.5 meters (214 feet) long. Their total height, then, would exceed 200 meters (656 feet) when a blade tip was pointed up.

That’s still, however, 75 meters (246 feet) short of the seemingly minimum height needed to pierce the cloud bottoms. But that doesn’t have to violate the rule because when clouds move over the ocean, they will encounter air with additional water vapor. And that will allow the minimum cloud-bottom height to fall somewhat. That means the blade tips indeed could have been immersed in the clouds' lower positive charge.

Making predictions

Knowing this, can meteorologists forecast thundersnow in advance?

It appears so.

They can scan for conditions that would make possible such electrical light shows. For instance, something known as “ice crystal canting” often precedes winter lightning strikes. This term refers to the orientation of snowflakes. Those flakes and other ice crystals normally fall flatly horizontal, like a pancake on a griddle. But as an electric field builds in the base of a cloud, it can tilt (or cant) ice crystals into a vertical (up and down) orientation.

This shows up on radar as confused-looking banding. Knowing how to spot that radar signature could give forecasters a heads up to a field strong enough to produce thundersnow.

Figuring out which towers are tall enough to scrape the cloud base also  could pinpoint likely strike candidates.

It’s entirely possible that without skyscrapers and other super-tall human-constructed towers, most thundersnow simply wouldn’t happen.

Using such knowledge, it might be possible one day soon to calculate the risk in a storm that any given tall structure will be struck by winter lightning.

A Weather Channel meteorologist is outside during a Boston snowstorm when thundersnow occurs.
The Weather Channel

Power Words

(for more about Power Words, click here)

air pressure     The force exerted by the weight of air molecules.

antenna     (in physics) Devices for picking up (receiving) electromagnetic energy.

archive     (adj. archival) To collect and store materials, including sounds, videos, data and objects, so that they can be found and used when they are needed. The term is also for the process of collecting and storing such things. People who perform this task are known as archivists.

array     A broad and organized group of objects. Sometimes they are instruments placed in a systematic fashion to collect information in a coordinated way. Other times, an array can refer to things that are laid out or displayed in a way that can make a broad range of related things, such as colors, visible at once. The term can even apply to a range of options or choices.

atmosphere     The envelope of gases surrounding Earth or another planet.

chain reaction     An event that once started continues to keep itself going.

cloud     A plume of molecules or particles, such as water droplets, that move under the action of an outside force, such as wind, radiation or water currents. (in atmospheric science) A mass of airborne water droplets and ice crystals that travel as a plume, usually high in Earth’s atmosphere. Its movement is driven by winds. 

commercial     (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

condense     To become thicker and more dense. This could occur, for instance, when moisture evaporates out of a liquid. Condense can also mean to change from a gas or a vapor into a liquid. This could occur, for instance, when water molecules in the air join together to become droplets of water.

constant     Continuous or uninterrupted.

crystal     (adj. crystalline) A solid consisting of a symmetrical, ordered, three-dimensional arrangement of atoms or molecules. It’s the organized structure taken by most minerals. Apatite, for example, forms six-sided crystals. The mineral crystals that make up rock are usually too small to be seen with the unaided eye.

cyclone     A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.

echo     To bounce back. For example, sound bouncing off walls of a tunnel, and returning to their source. Radio waves emitted above the surface can also bounce off the bedrock underneath an ice sheet — then return to the surface.

electric charge     The physical property responsible for electric force; it can be negative or positive.

electric field     A region around a charged particle or object within which a force would be exerted on other charged particles or objects.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

epidemic     A widespread outbreak of an infectious disease that sickens many people (or other organisms) in a community at the same time. The term also may be applied to non-infectious diseases or conditions that have spread in a similar way.

federal     Of or related to a country’s national government (not to any state or local government within that nation). For instance, the National Science Foundation and National Institutes of Health are both agencies of the U.S. federal government.

field      (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

force     Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.

friction     The resistance that one surface or object encounters when moving over or through another material (such as a fluid or a gas). Friction generally causes a heating, which can damage a surface of some material as it rubs against another.

gradient     From the word “grade,” it describes the incline, slope or degree of increase in some measure (such as temperature, pressure or even color) that develops as one moves in time, position or along some scale.

gravity     The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.

horizontal     A line or plane that runs left to right, much as the horizon appears to do when gazing into the distance.

lightning     A flash of light triggered by the discharge of electricity that occurs between clouds or between a cloud and something on Earth’s surface. The electrical current can cause a flash heating of the air, which can create a sharp crack of thunder.

literally     A term that the phrase that it modifies is precisely true. For instance, to say: "It's so cold that I'm literally dying," means that this person actually expects to soon be dead, the result of getting too cold.

magnet     A material that usually contains iron and whose atoms are arranged so they attract certain metals.

mass     A number that shows how much an object resists speeding up and slowing down — basically a measure of how much matter that object is made from.

mature      (verb) To develop toward — or into — a more complex and full-grown form or phase of something.

meteorologist     Someone who studies weather and climate events.

moisture     Small amounts of water present in the air, as vapor. It can also be present as a liquid, such as water droplets condensed on the inside of a window, or dampness present in clothing or soil.

momentum     A measure of the motion of something, made by multiplying its mass and velocity. Changing the speed or direction of an object will also alter its momentum.

National Weather Service     An agency of the National Oceanic and Atmospheric Administration. Created in 1870, its current role is to collect weather, precipitation and climate data. It also issues forecasts and warnings 24 hours a day for the entire United States, focusing on signs of possible conditions that could threaten lives and structures.

network     A group of interconnected people or things.

northeaster     Large, intense storms (known as nor'easters), theyspin up as cyclones over the waters of the Atlantic along the mid-latitudes. Their winds tend to blow in from the northeast, giving rise to the storms' name. Althugh they can occur at any time of year, but most commonly develop between September and April. They can dump lots of precipitation (snow or rain) accompanied by high winds.

phenomenon     Something that is surprising or unusual.

precipitation     (in chemistry) The creation of a solid from a solution. This can occur if there is too much of a chemical to dissolve completely into a solution. It also can be a sign that some chemical reaction is taking place. (in meteorology) A term for water falling from the sky. It can be in any form, from rain and sleet to snow or hail.

pressure     Force applied uniformly over a surface, measured as force per unit of area.

radar     A system for calculating the position, distance or other important characteristic of a distant object. It works by sending out periodic radio waves that bounce off of the object and then measuring how long it takes that bounced signal to return. Radar can detect moving objects, like airplanes. It also can be used to map the shape of land — even land covered by ice.

radio     To send and receive radio waves, or the device that receives these transmissions.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

skyscraper     A very tall building.

thermal     Of or relating to heat. (in meteorology) A relatively small-scale, rising air current produced when Earth’s surface is heated. Thermals are a common source of low level turbulence for aircraft.

transmission     Something that is conveyed or sent along. (in mechanics) In a liquid-fueled vehicle, the machinery used to transfer power from the engine to the drive wheels. (In medicine) To spread a disease or toxic agent.

turbine     A device with extended arm-like blades (often curved) to catch a moving fluid — anything from a gas or steam to water — and then convert the energy in that movement into rotary motion. Often that rotary motion will drive a system to generate electricity.

turbulent     (n. turbulence)  An adjective for the unpredictable fluctuation of a fluid (including air) in which its velocity varies irregularly instead of maintaining a steady or calm flow.

vacuum     Space with little or no matter in it. Laboratories or manufacturing plants may use vacuum equipment to pump out air, creating an area known as a vacuum chamber.

vertical     A term for the direction of a line or plane that runs up and down, as the vertical post for a streetlight does. It’s the opposite of horizontal, which would run parallel to the ground.

water vapor     Water in its gaseous state, capable of being suspended in the air.

weather     Conditions in the atmosphere at a localized place and a particular time. It is usually described in terms of particular features, such as air pressure, humidity, moisture, any precipitation (rain, snow or ice), temperature and wind speed. Weather constitutes the actual conditions that occur at any time and place. It’s different from climate, which is a description of the conditions that tend to occur in some general region during a particular month or season.

wind turbine     A wind-powered device — similar to the type used to mill grain (windmills) long ago — used to generate electricity.

NGSS: 

  • MS-ESS2-5
  • MS-ESS3-2
  • MS-ESS3-3
  • HS-ESS2-2
  • HS-ESS2-3
  • HS-ESS2-4
  • HS-ESS3-6

Citation

Journal: T.A. Warner, T.J. Lang and W.A. Lyons. Synoptic scale outbreak of self-initiated upward lightning (SIUL) from tall structures during the central U.S. blizzard of 1–2 February 2011. Journal of Geophysical Research: Atmospheres. Vol. 119, August 16, 2014, p. 9530. doi: 10.1002/2014JD021691.

Journal: N. Kitagawa and K. Michimoto.  Meteorological and electrical aspects of winter thunderclouds. Journal of Geophysical Research: Atmospheres. Vol. 99, May 20, 1994, p. 10713. doi: 10.1029/94JD00288.