Most people love a good snowstorm. After all, what’s better than a day off from school or work to sip warm cocoa as you wait for a chance to later explore the imminent winter wonderland? But just as no two snowflakes are alike, neither are any two snowstorms.
Many conditions give rise to snow. How and where they develop can make the difference between whether they drop a quiet dusting or the proverbial Snowmageddon.
Explainer: The making of a snowflake
Consider a late January 2016 storm that hit the U.S. East Coast from the mid-Atlantic states up to New England. In and around the nation’s capital, Washington, D.C., it dropped some 61 centimeters (24 inches) to more than 102 centimeters (40 inches). The storm also blanketed many New Jersey cities with 76.2 centimeters (30 inches) or so.
All snowstorms require the same ingredients: cold air, moisture and an unstable atmosphere. But winter air tends to be dry. It usually stores little moisture, the main ingredient in snow. That’s why living near a body of water — such as a lake, river or the ocean — can boost the chances that some regions will regularly get blanketed with mountains of flakes.
Explainer: What is thundersnow?
And while most snowstorms are relatively quiet, there are the occasional boomers. Scientists refer to these as thundersnows. Rare conditions can cause static electricity to build up within snow clouds and nearby structures. If a discharge occurs, the lightning can trigger a rumbling thunderclap.
The role of moisture
In some cases, one town might be buried beneath snow while the next neighborhood over remains dry. This often happens where the source of moisture for a winter storm is very localized — such as a lake. No surprise, such storms deliver what is known as lake-effect snow.
As winter approaches, chilly air can blow over water that still is fairly warm. This often occurs in November and December at sites where northern states border the U.S. Great Lakes. As streams of cold air flow in, lake water can heat up pockets of air near the surface. That air rises to form clouds. The phenomenon is similar to why you see your breath on cold days. The air you exhale is relatively warm and humid, so it briefly forms a cloud.
Eventually this air will cool, allowing its moisture to condense . Suddenly, flakes can begin to fly fast and heavy — and not let up for hours, days or even a week.
Lake-effect snow can dump 30 centimeters (one foot) or more of snow in less than a day. But the big totals tend to be quite localized. One area may see a lot, and a town just a short distance away may see few flakes.PaaschPhotography/iStockphoto
For maximum snow, the wind has to be just right. If it blows lengthwise along the lake, it ups how long a cloud can build, sopping up moisture. Once that cloud moves inland, it loses its fuel source (the lake’s water) and disintegrates. That’s why the affected communities may lie no more than 24 kilometers (15 miles) from a lake’s shore. Areas farther inland might see no more than a few flurries.
Compared to the monster winter storms that can spin up off the U.S. East Coast, bands of lake-effect snow tend to be quite small. Most are the size of a typical summer thunderstorm — only 10 to 20 kilometers (6.2 to 12.4 miles) across.
But lake-effect storms can be intense, dropping up to 15 centimeters (6 inches) of snow per hour. If the clouds tower high enough, thunder and lightning may develop. This thundersnow can be quite common in parts of upper New York, along the edges of Lakes Erie and Ontario. Once in a while, these tall wintertime clouds even drop small hail amidst the snow and thunder. Usually, the hail stones are smaller than the size of a pea.
Lake-effect snows, there, have racked up mind-boggling totals. From November 17 to 19 in 2014, a persistent lake-effect snowstorm settled over the southern suburbs of Buffalo, N.Y. It dropped 1.52 meters (5 feet) of snow. This storm led to 13 deaths, not to mention hundreds of collapsed roofs. The National Weather Service described the prolonged storm as one that just “didn’t budge.”
Equally impressive was how localized the precipitation was by November 18, midway through the storm. “The wall of snow was still quite apparent with blue skies to the north and zero visibility on the other side,” reported the National Weather Service office in Buffalo. “[T]here were only a few inches on the ground at Genessee Street, but several feet of snow . . . less than two miles [3.2 kilometers] south.”
Impressive, localized snows — in some cases surpassing 1.27 meters (50 inches) — were graphed for the first stage of a November 2014 storm near Buffalo, N.Y.NOAA, NWS, adapted by L. Steenblik Hwang
One day later, another storm just 16 kilometers (10 miles) to the south dropped more than a meter (4 feet) of snow on neighboring communities. Some sites in between were hit by both storms and ended up trapped beneath more than 2 meters (7 feet) of snow.
Storms that line up along a front are known as snow squalls. These may form just about anywhere. All they need is a strong temperature gradient — variation in temperatures — near the ground along some broad mass of cold air. This encroaching cold front brings cool, dense air. The incoming cold air thrusts upward the slightly warmer and moister air in front of it. This can set up a line of brief but heavy snows along the front edge of the incoming cold front.
Explainer: Winds and where they come from
Boundaries between air masses having different temperatures or humidity are a great source of lift — upwardly moving air. Any snow storms that develop here can now tap into strong winds high above the ground. A sudden squall could now sweep in and catch towns off-guard with briefly heavy snow and powerful wind gusts. Such squalls have been to blame for many large-scale traffic snarl-ups.
One noteworthy example occurred near Climax, Mich., on January 9, 2015. A quick incoming squall blew through a stretch of Interstate 94. It left a 193-car pileup in its wake. The wreckage was strewn along a 400-meter (quarter-mile) path. The accident spurred a fuel leak in a tractor-trailer. When it caught fire, the scene lit up with fireworks. Literally. The truck had been hauling a 18,140-kilogram (40,000-pound) payload of firecrackers.
In 2019, the National Weather Service developed and implemented a new “Snow Squall Warning.” It’s issued for short-triggered events like this and covers very localized areas. It preempts radio coverage, activating the Emergency Alert System to make sure everyone in the path is notified. Such alerts have been issued several times already this year.
The scariest of winter storms is the blizzard . These howling monsters are defined by their heavy, unremitting winds. To qualify as a blizzard, a snowstorm must blow with sustained winds of 56.3 kilometers (35 miles) per hour or deliver frequent gusts of that intensity. Such conditions also must last for at least three hours, according to the National Weather Service.
Snows can fall fast or slowly. The storm that brings them can fly through a region quickly — or stall over an area and dump huge totals.Dreef/iStockphoto
Blizzards develop when several different weather systems “stack” atop each other.
First, a zone of low pressure starts to become organized near to the ground. This must occur just in front of an upper-level dip in the jet stream — a fast river of air that flows high above Earth’s surface. This mix of conditions helps spin up a storm by causing the air ahead of the upper-level dip to rotate. A stronger area of low pressure above, meanwhile, acts as a vacuum to remove air from above. This helps the surface storm intensify. As the two weather systems approach each other, the surface storm intensifies until the two systems merge into one ferocious beast. Once the storm systems are “vertically stacked,” they will have reached peak intensity.
The lower the air pressure, the more intense the storm. That’s because the lack of air density draws in more nearby air. This speeds up the wind. (It’s also the explanation for why hurricanes have a clear eye and a staggeringly low air pressure.)
What makes a cyclone or blizzard so special is how rapidly a region’s air pressure drops. At sea level, the air pressure tends to hover around 1,015 millibars. A drop of a few millibars can signal bad weather is on its way. Some blizzards undergo a process called bombogenesis. This refers to a startling, one-day fall of 24 millibars in the storm’s central air pressure.
On December 9, 2005, a whopper of a storm developed in New York off the coast of Long Island. As it moved north toward Cape Cod, Mass., the storm strengthened. At one point, the local air pressure dropped an amazing 13 millibars in just three hours.
Such a sharp drop in air pressure reflects the movement of air up and out of the storm’s center. With a diminished column of air above the ground, that mass of air now weighs less. And that’s why the pressure (or force of air on the ground) drops.
An infrared camera aboard a NASA satellite shows the 1993 “Storm of the Century” slamming the eastern third of the United States. Heavy snows fell as far south as Alabama in the wrap-around “comma head” of the storm. The blue cloud tops in the far South indicate damaging thunderstorms. These thunderstorms produced tornadoes that killed several in Florida.NASA/Wikimedia Commons
The massive pressure drop revved this storm into a monster. It unleashed “microbursts” — winds that gusted to 161 kilometers (100 miles) per hour. There also was a barrage of winter waterspouts and thundersnow. A plane landing at Boston’s Logan Airport was even struck by the storm’s lightning.
At coastal sites, a blizzard’s twirling winds can pull in warmer air from the ocean. What later falls out in areas near to the coast may be rain, freezing rain, sleet — or an ugly mix of them. Indeed, that ocean layer makes it difficult to forecast what the precipitation here will be.
Blizzards often feature a warm side to their south. Here, a slug of moisture can create a line of damaging showers and thunderstorms. One massive system went down in the books as the “Storm of the Century” on March 13, 1993. On the north side, snow fell. But to the south, a damaging thunderstorm line developed — one that spawned 11 tornadoes that savaged parts of Florida.
When these sprawling storm systems develop off the U.S. East Coast, meteorologists will refer to them as “nor’easters.” Much of their strengths comes from the warmer air over the Gulf Stream’s lukewarm waters. That’s because the wind starts off blowing in from the northeast. Later, if storm races on into Canada’s Maritime provinces, the winds can abruptly pivot. They now can come in from the northwest. This switcheroo draws in much colder, drier air — sometimes even spurring a “flash freeze.” Most nor’easters occur in the cold season and produce snow, frequently leading to blockbuster storms.
Winter can wallop communities with surprising weather. Understanding the science behind snowstorms helps to explain why each one challenges the ability of forecasters to tell us what to expect.
air masses Large volumes of air, sometimes covering many hundreds or thousands of square kilometers (square miles), that typically have a consistent temperature or water-vapor content. Air masses are often classified by their source, such as continental, arctic or tropical. Air masses and other weather systems are steered across Earth’s surface by jet streams and by differences in atmospheric pressure.
air pressure The force exerted by the weight of air molecules.
atmosphere The envelope of gases surrounding Earth or another planet.
blizzard An intense snowstorm that is accompanied by sustained winds (or frequent gusts) of 56.3 kilometers (35 miles) per hour and that lasts a minimum of three hours.
bombogenesis Meaning weather bomb, this is a sudden, intense drop in air pressure that signals the potential arrival of a massively strong storm.
cloud (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.
condense To become thicker and denser. 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.
cyclone A strong, rotating vortex, usually made of wind. Notable examples include a tornado or hurricane.
density The measure of how condensed some object is, found by dividing its mass by its volume.
develop To emerge or come into being, either naturally or through human intervention, such as by manufacturing. are cut down and replaced with structures or landscaped yards and parks.
eye (in atmospheric sciences) The roughly circular area of comparatively light winds that encompasses the center of a severe tropical cyclone. The eye is either completely or partially surrounded by the eyewall cloud.
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.
Great Lakes A system of five interconnected lakes — Superior, Michigan, Huron, Erie and Ontario — the Great Lakes constitute the largest freshwater source in the world (based on surface area). They hold an estimated 6 quadrillion gallons of water, or about a fifth of the world’s fresh surface water. To give some perspective on that amount, the lakes’ water would, if spread evenly, cover the 48 touching U.S. states to a depth of about 2.9 meters (9.5 feet) deep.
hurricane A tropical cyclone that occurs in the Atlantic Ocean and has winds of 119 kilometers (74 miles) per hour or greater. When such a storm occurs in the Pacific Ocean, people refer to it as a typhoon.
jet stream A fast-flowing, high-altitude air current. On Earth, the major jet streams flow from west to east in the mid-latitude regions of the Northern and Southern Hemispheres.
lake effect snow Intense and/or long-duration snowstorms that are fueled by moisture picked up from relatively nearby warm surface waters, such as lakes, streams or bays. These storms tend to be quite localized, dropping most of their snow within just a few miles (kilometers) from the water source.
lift An upward force on an object. It may occur when an object (such as a balloon) is filled with a gas that weighs less than air; it can also result when a low-pressure area occurs above an object (such as an airplane wing).
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.
localized An adjective for something that has a very local impact. (antonym: broad or far-reaching)
marine Having to do with the ocean world or environment.
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.
meteorologist Someone who studies weather and climate events.
millibar A unit of atmospheric pressure. The average pressure at sea level is around 1,000 millibars, or not quite 15 pounds per square inch (or roughly 1 kilogram-force per square centimeter).
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.
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.
payload The weight that some vehicle (such as a truck or spacecraft) can carry, including people, scientific instruments and other equipment. It is usually important to keep the payload low. This will cut the amount of fuel needed to power the vehicle, maximizing how far it can travel.
persistent An adjective for something that is long-lasting.
phenomenon Something that is surprising or unusual.
precipitation (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.
sea level The overall level of the ocean over the entire globe when all tides and other short-term changes are averaged out.
squall A sudden, violent gale of wind and usually rain (or other precipitation).
thrust A force that makes an object move forward.
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.
wake An area of disturbed air or water left behind an object (such as a boat or animal) moving through it.
waterspout A rotating whirlwind of misty air. These can form either of two ways. Some are true tornadoes that emerge from rotating clouds in a thunderstorm. These either form over water or travel from land to a nearby body of water. Another type can form on non-stormy days. Unlike tornadoes that come down from a cloud, these whirlwinds tend to form above water and then rise toward clouds. They tend to develop where winds are light and they travel little.
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.