by Bill Evans
3
Blizzards
How Bizarre! Blizzards, Snowstorms, and Ice, Oh My!
Blizzards are the worst weather that winter can dish out! Whenever you hear someone exclaim, “It looks like a blizzard outside!” they’re generally talking about the nastiest of days. Meteorologists and weather forecasters use the term blizzard to describe a day of heavy snow, biting winds, and low visibility. The term blizzard originally meant a cannon shot or a volley of musket fire until an Iowa newspaper used the term in 1870 to describe a snowstorm. Blizzards can occur in any location in the world where snow is naturally made.
National Weather Service, National Oceanic and Atmospheric Administration/Department of Commerce
What Makes a Snowstorm a Blizzard?
Officially, the National Weather Service defines a blizzard as a storm that contains large amounts of snow or blowing snow, has winds in excess of 35 mph and visibilities of less than a quarter mile, and lasts for at least 3 hours. When these conditions are expected, the National Weather Service will issue a blizzard warning. Blizzard conditions often develop on the northwest side of an intense storm system. The difference between the lower pressure in the storm and the higher pressure to the west creates a tight pressure gradient, or difference in pressure between two locations, which in turn results in very strong winds. These strong winds pick up any available snow from the ground, and/or blow about any snow which is falling, creating very low visibility and the potential for significant drifting of snow. Cold polar air that whips freshly fallen, fine, powdery snow off the ground creates what is called a ground blizzard.
Blizzards can create life-threatening conditions. Traveling by automobile can become difficult, or even impossible, due to whiteout conditions and drifting snow. Whiteout conditions are most often caused by major storms that produce a dry, very powdery snow. In this situation, it doesn’t even need to be snowing for there to be a whiteout. When the snow that is already on the ground is blown around, visibility can be reduced to near zero at times.
During a blizzard, gusty winds can knock down trees, bringing down power lines and cutting off electricity for extended periods of time. Often, it can be snowing so hard, at the rate of 2 to 4 inches per hour, that thunder can be heard and flashes of lightning seen. When that happens, it’s called thundersnow. The conditions for making thundersnow are the same as in a thunderstorm except the precipitation falls in the form of snow rather than rain. It’s a very rare phenomenon that happens when very cold air slams into warm air, along a cold or warm front, or a strong storm system that has a very strong upward motion of wind.
How Do You Make That Snowy Stuff?
To me, snow is by far the most beautiful of all of the various forms of precipitation. To put it simply, snow is cool! The way snow is made is really complex…and some parts of the process still remain a mystery. Snowflakes are made from the water droplets in a cloud, just like rain. But in winter, the water vapor is supercooled—cooled below freezing and frozen into ice crystals. That happens around 14ºF (-10ºC). Snowflakes are made from ice crystals, pure and simple. Sure, rain can freeze, but frozen rain is called sleet and it’s structurally very different from snow.
Do you know that most snowflakes have six sides? Do you know why? The water molecules in ice crystals form a hexagonal lattice—that is, they have six sides––so when a water droplet freezes, it forms the six-fold symmetry of snow crystals. The most basic snowflake shape is known as a hexagonal prism.
The word snowflake is a general term for many ice crystals that have come together. A snowflake can be made up of two crystals or hundreds. A flake’s six beautiful points are determined by the amount of ice crystals and the temperature of the atmosphere.
Snowflakes are mysterious things. Their sides and points are symmetrical, and their fundamental form derives from the arrangement of the water molecules in the ice crystal. When a liquid freezes, the molecules tend to settle in the lowest-energy state, and that almost always involves some form of symmetry. The higher the symmetry, the more stable the crystal is.
Dendrite snowflake
Water molecules floating freely in a vapor begin to arrange themselves into a crystalline solid when the temperature drops below freezing. The two hydrogen atoms of the molecules tend to attract neighboring water molecules. When the temperature is low enough, the molecules link together to form a solid, open framework that has a strict hexagonal symmetry.
Why are snowflake shapes so elaborate? Nobody has a good answer for that. The general explanation is that atmospheric conditions when snowflakes are forming are very complex and variable. A crystal might begin to grow in one manner and then minutes or even seconds later, something changes (temperature or humidity), so it starts to grow in another manner. The hexagonal symmetry is maintained, but the ice crystal may branch off in new directions.
These changes in environment take place over a large area compared with the size of a single snowflake, so all the flakes developing in the same region are similarly affected. In the end, all kinds of forms can arise: everything from prisms and needles to the familiar lacy snowflakes. Water is an amazing substance!
The Life of a Snowflake
The shape and size of snow crystals depend on the amount of water vapor in the air and the temperature of the atmosphere. Crystals take on different shapes depending on the amount of time they spend blowing around in the snow cloud and what the overall weather conditions are.
Snow Morphology
There are all kinds of snowflakes, many more types than most of us may have ever thought existed. The amount of moisture and the temperature of the air determine the size and shape of snowflakes. Some flakes are very fine and some are as big as pancakes.
Thin plates and stars grow around 28ºF (-2ºC), while columns and slender needles appear near 23ºF (-5ºC). Plates and stars again form near 5ºF (-15ºC), and a combination of plates and columns are made around -22ºF (-30ºC).
Frank Picini
Snow crystals tend to form simpler shapes when the humidity is low, while more complex shapes form at higher humidities. The most extreme shapes—long needles around 23ºF (-5ºC) and large, thin plates around -5ºF (-15ºC)—form when the humidity is especially high. Why snow crystal shapes change so much with temperature remains something of a scientific mystery. The growth of snowflakes depends on exactly how water vapor molecules are incorporated into the developing ice crystal, and the physics behind this is complex and not well understood. Most snowflakes are irregular and perfect ones are rare. Next time you are wearing gloves and it’s snowing, take a good look at the flakes in your hand. What are they? Are they plates, stars, or needles? Better yet, if you can, get a look at them under a microscope and you’ll see that no two snowflakes are alike.
Some people think that “No two snowflakes are alike” is just a folk saying, but it is a true statement and there are many reasons for it. The size and shape of snowflakes are influenced by many things: air currents (which direction the air is moving), humidity levels (the amount of water vapor in the air), wind speed, pressure from the weight of other crystals, the combination of one snow crystal with another, and how long it takes the crystal to fall. There is an infinite variety of snowflakes. You can spend your whole life looking at snowflakes and never find two that are alike!
Types of Snowflakes
Frank Picini
National Oceanic and Atmospheric Administration/Department of Commerce
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Simple Prisms
The examples at right show two stubby prisms (left and right) and one thin plate (center). Snow crystal facets are rarely perfectly flat, being more typically decorated with various indents, ridges, or other features.
A hexagonal prism is the most basic snow crystal. Depending on how fast the different facets grow, snow crystal prisms can appear as thin hexagonal plates, slender hexagonal columns (shaped a lot like wooden pencils), or anything in between. Simple prisms are usually so small they can barely be seen with the nake
d eye.
Stellar Plates
These common snowflakes are thin, platelike crystals with six broad arms that form a starlike shape. Their faces are often decorated with amazingly elaborate and symmetrical markings.
Platelike snowflakes form when the temperature is near 28ºF (-2º C) or near 5ºF (-15ºC).
Sectored Plates
The simplest sectored plates are hexagonal crystals that are divided into six equal pieces, like the slices of a hexagonal pie. More complex specimens show prominent ridges on broad, flat branches.
Stellar plates often show distinctive ridges that point to the corners between adjacent prism facets. When these ridges are especially prominent, the crystals are called sectored plates.
Stellar Dendrites
Judging by holiday and winter decorations, stellar dendrites are the most popular type of snow crystal. You can see these crystals for yourself quite well with just a simple magnifier.
Dendritic means treelike, so stellar dendrites are platelike snow crystals that have branches, and sometimes, side branches. These are fairly large crystals, typically 2 to 4 mm in diameter, that can be easily seen with the naked eye.
Fernlike Stellar Dendrites
Some snowfalls contain almost nothing but stellar dendrites and fernlike stellar dendrites. It’s quite a sight when they collect in vast numbers, blanketing everything in white.
Sometimes the branches of stellar crystals have so many side branches they look a bit like ferns, so we call them fernlike stellar dendrites. These are the largest snow crystals, often 5 mm or more in diameter. In spite of their large size, these are single crystals of ice—the water molecules are lined up from one end to the other.
The best powder snow, where you sink to your knees while skiing, is made of stellar dendrites. These crystals can be extremely thin and light, so they make a low-density snowpack.
Hollow Columns
Note how the two hollow regions are symmetrical in each column. Sometimes the ends grow over and enclose a pair of bubbles in the ice.
When hexagonal columns form with hollow conical regions at each end, such forms are called hollow columns. These crystals are small, so you need a good magnifier to see the hollow regions. Columns form near 23°F (-5°C), and near -22°F (-30°C).
Needles
A shift of just a few degrees in temperature changes the pattern of crystal growth from thin, flat plates to long, slender needles. Why this happens is still unknown.
Needles are slender, columnar ice crystals that grow when the temperature is around 23ºF (-5ºC). On your sleeve these snowflakes look like small bits of white hair.
Capped Columns
The three small illustrations at the right are three views of a capped column. The top view is from the side, showing the central column and the two plates edge-on. The other two views show the same crystal from one end, with the microscope focused separately on each of the two plates. The other three images are also of capped columns from the side.
These crystals first grow as stubby columns which are blown into a region of the clouds where conditions cause crystals to grow into plates. The result is two thin, platelike crystals growing on the ends of an ice column. Capped columns don’t appear in every snowfall, but you can find them if you look for them.
Double Plates
The picture at the far right shows a double plate from the side. The center picture shows a double plate with the microscope focused on the smaller plate. In the picture on the left, note the slightly out-of-focus hexagon that is about one-sixth as large as the main crystal. This hexagon is the second side of a double plate, connected to the main plate by a small axle.
A double plate is basically a capped column with an especially short central column. One plate grows more quickly than the other, and because the two plates are so close together, the larger one shields the smaller from some water vapor. These crystals are common—many snowflakes that look like ordinary stellar plates are actually double plates if you look closely.
Split Plates and Stars
You may have to stare at these pictures a bit to see how the two distinct pieces fit together. Note how in each case the crystals are connected in the center with short axles.
These are forms of double plates, except that part of one plate grows large along with part of the other plate. Split plates and stars, like double plates, are common but often go unnoticed.
Triangular Crystals
Surprisingly, no one knows why snow crystals grow into these threefold symmetrical shapes. (Note, however, that the molecular structure of triangular crystals is no different from ordinary six-sided crystals. The facet angles are all the same.)
Plates sometimes grow as truncated triangles when the temperature is near 28ºF (2ºC). If the corners of the plates sprout arms, the result is an odd version of a stellar plate crystal. These crystals are relatively rare.
12-Sided Snowflakes
These crystals are very rare, but sometimes a snowfall will bring quite a few. The picture in the center shows a 12-sider where the two halves are widely separated.
Sometimes capped columns form with a twist, a 30-degree twist to be specific. The two end-plates are both six-branched crystals, but one is rotated 30 degrees relative to the other. This is a form of crystal twinning, in which two crystals grow joined in a specific orientation.
Bullet Rosettes
Sometimes a bullet rosette can become a capped rosette, as shown in the example in the middle.
The nucleation of an ice grain sometimes yields multiple linked crystals that grow together at random orientations. When the different pieces grow into columns, the result is called a bullet rosette. These polycrystals often break up into isolated bullet-shaped crystals.
Radiating Dendrites
The far example on the right shows radiating plates. The other example shows a fernlike stellar dendrite with two errant branches growing up out of the main plane of the crystal.
When the pieces of a polycrystal grow out into dendrites, the result is called a radiating dendrite or a spatial dendrite.
Rimed Crystals
The first two pictures at the right have relatively light rime coverage. The final example is completely covered with rime, but you can still see the six-fold symmetry of the underlying stellar crystal.
Clouds are made of countless water droplets, and sometimes these droplets collide with and stick to snow crystals. The frozen droplets are called rime. All the different types of snow crystals can be found decorated with rime. When the coverage is especially heavy, so that the assembly looks like a tiny snowball, the result is called graupel.
Irregular Crystals
The most common snow crystals by far are the irregular crystals. These are small, usually clumped together, and show little of the symmetry seen in stellar or columnar crystals.
Artificial Snow
You can see from the picture at the right that artificial snow is made of frozen water droplets, with none of the elaborate structure found in real snow crystals.
Snowmaking machines shoot a mixture of water and compressed air out of nozzles. The water comes out as fine droplets, and the air cools as it decompresses, causing the droplets to freeze. A fan blows the ice particles onto the slopes.
Man-made snow may look like real snow, but look at the difference between it and the real thing. Because the ice crystals of man-made snow are not born in the very cold reaches of Earth’s atmosphere, they do not get to form into beautiful flakes. It’s great to ski on, but it’s certainly not as pretty as the real thing!
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Tracking a Snowstorm
One of the most frequent questions I am asked by students is, “How do you know when a snowstorm is coming?” Let’s take a look at a storm as it develops and track it across the country.
Most big snowstorms need just two things to really make a lot of snow. They need lots of moisture and lots of cold, preferably arctic, air. It’s common for a low to develop over the Centr
al Plains and head southeastward. Let’s position that low in Texas near Dallas on a Monday at 12 P.M. It is heading east, through the southeastern United States. High pressure—our preferred arctic air mass—is sitting over the Great Lakes and sinking south from Canada.
used by permission of James Quirk; taken by his grandfather.
As the low moves eastward, a cold front trails south from it. Warm air is ahead of the front. As the low works eastward through the south, it produces soaking rains and thunderstorms while picking up tons of moisture from the Gulf of Mexico. When that moisture moves north and hits colder air, it produces snow through Tennessee, Kentucky, and the Ohio Valley.
By midnight Monday, the low is over Atlanta, and thunderstorms are dumping heavy rain over Georgia, South Carolina, and Florida. As the warm front pushes northward into the colder air, snow breaks out over the mid-Atlantic states, the Smokey Mountains, and the Virginias. By noon Tuesday, the low is over Washington, D.C., producing snow, and the arctic high is sliding eastward over New England.
Now the low is moving northeastward and out over the Atlantic Ocean, where conditions cause the low to strengthen. As the low becomes stronger it throws more moisture from the ocean back into the northeastern United States, creating a major snowstorm. In this scenario as much as 20 to 30 inches of snow can be produced throughout New England and the other northeastern states. By midnight Tuesday, the storm is just east of Boston.