Most people enjoy watching a good snowfall. After all, there’s nothing like sitting around a fireplace with a cup of cocoa, watching through the window as the landscape is transformed into a beautiful white blanket. And many people like the excitement of a hailstorm—the thud of hailstones as they hit the ground. Unfortunately, both forms of precipitation have the potential to cause a good deal of damage and even death.
While heavy rain can limit visibility and soak you to the skin, a hailstorm is capable of breaking windshields, decimating crops, and even injuring livestock. If it wasn’t for updrafts, hail would never grow very large, and golf ball–sized and larger specimens would be unheard of. But as ice particles fall through a cumulonimbus cloud, they inevitably encounter strong vertical winds and get swirled skyward again, picking up extra layers of supercooled water droplets as they zoom above the freezing level.
If the updrafts are strong enough, like those in a supercell, the developing hailstones ride a wild roller coaster of wind as they spin up and around inside the storm, growing larger by the minute. Finally some become so big that they overcome the updraft’s power and begin to drop toward the ground. Falling at speeds of up to 120 miles an hour, they can dent cars and destroy crops, raising insurance rates wherever they strike.
Most hail is relatively small—around 2 inches in diameter or less—but on July 23, 2010, the great-granddaddy of all hailstones fell on Vivian, South Dakota. The hailstones measured 8 inches in diameter and weighed almost 2 pounds. Never mind an umbrella—with hail that size, you’d need a bomb shelter.
If you cut a hailstone in half, you can see the multiple layers of ice that mark its journey through the thunderstorm. Generally, the larger the hailstone, the more severe the updrafts were in the storm that it came from.
Snow forms from tiny particles of ice suspended in clouds up above the freezing level. As the particles form, they arrange themselves into hexagonal shapes due to the molecular structure of water, which is why simple snow crystals always have six points. Snowflakes that fall through a layer of slightly warmer air, however, can bind with other flakes to form very large, intricate structures that look like beautiful silver jewelry under a microscope.
Much of the rain that falls in the summer actually begins as snow and ice high in the tops of cumulonimbus clouds. In the wintertime the freezing level is much lower, and if you live in a snow-prone region, you’re aware that snowflakes can easily make it all the way to the ground, where they gather with billions of their friends for an impromptu party on your lawn.
Snow flurries usually fall from cumulus clouds and provide a light dusting of crystals that don’t cause much trouble for those below. Snow squalls, on the other hand, are brief but very intense snowstorms that are the equivalent of a summer downpour. They arrive with little warning, and their intense driving winds often create near-whiteout conditions in a matter of minutes.
A whiteout occurs when the clouds from which snow is falling take on a bright, uniformly white appearance. This happens when the light reflected off the snow is about the same as the light coming through the clouds, making objects in the storm very difficult to see.
Wind-driven snow officially becomes a blizzard when below-freezing temperatures are accompanied by winds of more than 35 miles per hour and visibility down to a quarter mile or less. In a severe blizzard, winds exceed 45 miles per hour and temperatures plunge to 10°F or lower. Blizzards can pile snow into gigantic drifts that can make travel impossible. During the great blizzard of 1888, known as the Great White Hurricane, some snowdrifts were measured as high as 50 feet.
The 1888 blizzard actually led directly to the creation of the New York subway system, as city leaders vowed to prevent the weather from ever bringing the city to such a standstill again. The entire East Coast, from Maine to the Chesapeake Bay, buried in up to 50 inches of snow, was cut off from the rest of the world as telegraph and telephone wires snapped like twigs under the crushing weight of snow and ice. Washington, New York, Philadelphia, and Boston were paralyzed for days. At least 100 sailors were lost at sea, 200 ships ran aground, and, with lifesaving water frozen in pipes and hydrants, raging fires caused more than $25 million in property losses. More than 400 people perished in what became known as the worst snowstorm in American history.
A frontal passage is the movement of the boundary between two air masses over a particular location. Frontal passages are usually accompanied by a change in wind speed and direction, humidity, cloud cover, precipitation, and temperature.
If you follow winter weather on TV, you’ve probably noticed that cities like Buffalo and Syracuse, New York, seem to get more than their share of snow. This is due to the “lake effect,” a condition that occurs when cold air moves over a warmer body of water, in this case the Great Lakes. Unlike the Great Plains, where snowstorms usually move through, release their quota of snow and leave, states to the south and east of the Great Lakes are often dumped on for days after a frontal passage, as cold air flowing south and east over the lakes picks up moisture and warmth from the water’s surface and carries it shoreward.
As damaging as snow can be, however, it has a gentler side. Since snow doesn’t conduct heat very well, dry snow can actually act as an insulator, protecting plants below its surface by preventing the ground from losing all of its warmth. Just as air spaces within a down jacket help insulate you from the cold, tiny gaps between dry snowflakes act as buffer zones against the cold air above. This same effect is what causes snow to absorb sound, making a walk through the woods after a snowfall a quiet, mesmerizing experience.
The Great Lakes, due to their size and depth, are able to retain much of their summer warmth well into fall and winter. When an air mass warmed by its passage over a lake reaches the shore, it is forced to rise rapidly—a process called orographic lifting—and heavy snow and snow squalls are often the result.
Have you ever heard someone say that it’s “too cold to snow”? Is it really possible for the temperature to drop so low that snow can no longer form? Well, no. It’s true that there may be a lack of snow on cold, still evenings, when high pressure drives away any snow-producing clouds. But while it’s true that cold, dry air can’t hold as much moisture as warmer air, there is always at least some water vapor present, and where there’s vapor, there can be precipitation.
On the flip side, you may have seen snow fall when the temperature at ground level is above freezing. For this to happen, the air aloft must be very dry. As snow begins to fall from clouds above the freezing level, it encounters warmer layers of the atmosphere and starts to melt. But because the air is dry, the melting snow evaporates quickly, cooling the air and making it possible for more flakes to penetrate downward. Eventually some of these flakes can make it to the surface, although they won’t last very long in a frozen state.