What Is Thundersnow?

Jessica Kourkounis, Getty Images
Jessica Kourkounis, Getty Images

The northeastern United States is dealing with its second major nor'easter in a week, with rain and heavy snow—and the associated power outages—cutting a path across the Mid-Atlantic and New England. But news of the adverse impacts of the snowstorm is being accompanied by an unusual buzzword: thundersnow. Thundersnow occurs during a thunderstorm that produces snow instead of rain. The mechanisms that produce rainy thunderstorms and snowy thunderstorms are largely the same, even if the air temperature is below freezing.

A band of snow can become strong enough to produce lightning through two processes known as convection and forcing. Convection occurs when an area of warm air quickly rises through cooler air above it. Convective snow is most common during lake effect snow events like those you’d find on Lake Ontario or Lake Erie, since the process requires extreme vertical temperature gradients that can result from bitterly cold air flowing over a warm body of water.

Forcing is slightly different. A strengthening low-pressure system involves fast, dynamic changes in the atmosphere, especially when one of these storm systems quickly gains strength. Such a fast-developing storm can cause large amounts of lift in the atmosphere, a process that forces air to swiftly rise like you’d see during convection. This creates intense bands of snow that can grow so strong that they produce thunder and lightning. This process is responsible for the thundersnow that occurs during blizzards and nor’easters, those powerful storms that regularly hit the eastern coast of the U.S. during the winter. Thundersnow can be pretty exciting—just ask The Weather Channel's Jim Cantore:

The name “thundersnow” can be a bit misleading. One of the most enjoyable things about a snowfall is how silent it is outside when there’s a thick blanket of snow on the ground. Snow absorbs sound waves so efficiently that you can usually only hear ambient noises immediately around where you’re standing. Snow muffles the sound of thunder for the same reason. Thunder that might be audible for many miles during a rainy thunderstorm might only be audible for a few thousand feet away from where the lightning struck. Unless the lightning strikes very close to where you are, you might only see a bright flash during thundersnow without ever hearing the thunder.

While thundersnow is a fascinating phenomenon to encounter, it does involve lightning, after all, and it’s just as dangerous as any other lightning bolt you’d see in a rainy thunderstorm. If you’re ever lucky enough to experience thundersnow, the event is best enjoyed indoors and out of harm’s way.

This piece originally ran in 2017.

Website Lets You Report Individuals Affected by Hurricane Michael to Search-and-Rescue Teams

Brendan Smialowski, AFP/Getty Images
Brendan Smialowski, AFP/Getty Images

When Hurricane Michael made landfall in Florida as a Category 4 hurricane on October 10, it became the strongest storm to hit the continental U.S. since 1992. Homes from Florida to Virginia have since been leveled and at least 11 people have died. With internet and phone lines down across the disaster zone, many people are desperate to know if their loved ones are safe—now there's an online tool that can help them.

If you're having trouble getting in touch with someone who was in the hurricane's path, you can report them through a new website set up by the Florida National Guard, First Coast News reports. The site asks for the person's name, gender, age, and address, as well as any life-threatening issues they may be facing, such as low oxygen or medication supplies. After you submit their information, the State Emergency Operations Center forwards it to the relevant local agency doing recovery work.

Michael moved back over the Atlantic as a post-tropical storm Friday morning following its rampage through the southeastern U.S. More than 1000 search-and-rescue workers have already been deployed in Florida alone, and the death toll is expected to rise as clean-up efforts continue across the region.

[h/t First Coast News]

How Are Hurricane Categories Determined?

NOAA via Getty Images
NOAA via Getty Images

Residents of Panama City and other areas in the Florida Panhandle are in the midst of Hurricane Michael, a Category 4 storm that Governor Rick Scott warned is the "worst storm" to hit the area "in a century."

Given that North Carolina is still battling the effects of Hurricane Florence, which made landfall less than a month ago, we've become accustomed to hearing about hurricanes, and to predicting what sort of damage they might cause based on their category number. But how do meteorologists categorize these often-deadly storms, and how does that scale work?

First, a quick primer: Hurricanes are tropical cyclones that occur in the Atlantic Ocean and have winds with a sustained speed of at least 74 mph. A tropical cyclone, in turn, is a storm system that develops in the tropics and is characterized by a low pressure center and thunderstorms that produce strong winds, rain, and storm surges. Tropical cyclone is a generic name that refers to the storms' geographic origin and cyclonic rotation around a central eye. Depending on their location and strength, the storms are called different things. What gets dubbed a hurricane in the Atlantic, for example, would be called a typhoon if it happened in the northwestern Pacific.

WHAT’S THE DIFFERENCE BETWEEN A HURRICANE AND A TROPICAL STORM?

Simply put: Wind speed. When tropical cyclones are just starting out as general areas of low pressure with the potential to strengthen, they’re called tropical depressions. They’re given sequential numbers as they form during a storm season so the National Hurricane Center (NHC) can keep tabs on them.

Once a cyclone’s winds kick up to 39 miles per hour and sustain that speed for 10 minutes, it becomes a tropical storm and the NHC gives it a name. If the cyclone keeps growing and sustains 74 mph winds, it graduates to hurricane.

ONCE WE CALL IT A HURRICANE, HOW DO WE CATEGORIZE IT?

In order to assign a numeric category value to a hurricane, meteorologists look to the Saffir-Simpson Hurricane Wind Scale, which was developed as a classification system for Western Hemisphere tropical cyclones in the late 1960s and early '70s by structural engineer Herbert Saffir and his friend, meteorologist Robert Simpson, who was the director of the NHC at the time.

When Saffir was working on a United Nations project to study low-cost housing in hurricane-prone areas, it struck him that there was no simple, standardized way of describing hurricanes and their damaging effects, like the way the Richter scale is used to describe earthquakes. He created a five-level scale based on wind speed and sent it off to Simpson, who expanded on it to include the effects on storm surge and flooding. Simpson began using it internally at the NHC, and then in reports shared with emergency agencies. It proved useful, so others began adopting it and it quickly spread.

HOW DOES THE SCALE WORK?

According to the NHC, the scale breaks down like this:

Category 1 storms have sustained winds of 74 to 95 mph. These “very dangerous winds will produce some damage: Well-constructed frame homes could have damage to roof, shingles, vinyl siding, and gutters. Large branches of trees will snap and shallowly rooted trees may be toppled. Extensive damage to power lines and poles likely will result in power outages that could last a few to several days."

Category 2 storms have sustained winds of 96 to 110 mph. These “extremely dangerous winds will cause extensive damage: Well-constructed frame homes could sustain major roof and siding damage. Many shallowly rooted trees will be snapped or uprooted and block numerous roads. Near-total power loss is expected with outages that could last from several days to weeks."

Category 3 storms have sustained winds of 111 to 129 mph. This is the first category that qualifies as a “major storm” and “devastating damage will occur: Well-built framed homes may incur major damage or removal of roof decking and gable ends. Many trees will be snapped or uprooted, blocking numerous roads. Electricity and water will be unavailable for several days to weeks after the storm passes."

Category 4 storms have sustained winds of 130 to 156 mph. These storms are “catastrophicand damage includes: “Well-built framed homes can sustain severe damage with loss of most of the roof structure and/or some exterior walls. Most trees will be snapped or uprooted and power poles downed. Fallen trees and power poles will isolate residential areas. Power outages will last weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

Category 5 storms have sustained winds of 157 mph or higher. The catastrophic damage entailed here includes: “A high percentage of framed homes will be destroyed, with total roof failure and wall collapse. Fallen trees and power poles will isolate residential areas. Power outages will last for weeks to possibly months. Most of the area will be uninhabitable for weeks or months."

While the Saffir-Simpson scale is useful, it isn’t the be-all and end-all for measuring storms, as the National Oceanic and Atmospheric Administration (NOAA) pointed out on Twitter in 2013:

IS THERE ANYTHING WORSE THAN A CATEGORY 5?

Not on paper, but there have been hurricanes that have gone beyond the upper bounds of the scale. Hypothetically, hurricanes could up the ante beyond Category 5 more regularly. The storms use warm water to fuel themselves and as ocean temperatures rise, climatologists predict that potential hurricane intensity will increase.

Both Saffir and Simpson have said that there’s no need to add more categories because once things go beyond 157 mph, the damage all looks the same: really, really bad. Still, that hasn't stopped several scientists from suggesting that maybe the time has come to consider a Category 6 addition.

Timothy Hall, a senior scientist at NASA's Goddard Institute for Space Studies, recently told the Los Angeles Times that if the current global warming trends continue, he can foresee a time—likely by the end of the century—where wind speeds could blow past 230 mph, which could create conditions similar to a F-4 tornado (which has the power to lift cars off the ground and send them hurtling through the air with relative ease).

“If we had twice as many Category 5s—at some point, several decades down the line—if that seems to be the new norm, then yes, we’d want to have more partitioning at the upper part of the scale,” Hall said. “At that point, a Category 6 would be a reasonable thing to do."

An earlier version of this article appeared in 2013.

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