When winter bears down, it can be hard to think of anything outside of how much you hate (or love!) the snow and ice. But there's more to winter weather than you might think! Here are a few of the words and phenomena that could define this season.
One of the first signs of winter is the hoarfrost of late autumn. Deriving its name from an Old English word (hoar), meaning "to appear old," this is the thin, crisp coating of ice that often forms on objects during cool nights with clear skies. The clear skies allow the ground to lose heat more quickly than the surrounding air, and the humidity in the atmosphere condenses and freezes solid. This frost can occur even when the air a few feet above ground is well above freezing, and usually melts within an hour or two of sunrise.
2. FROST FLOWER
Frost flowers (and the related "ice ribbons" and "ice beards") are very thin, spindly, unique formations of ice, seen in late autumn or early winter, when plants are first freezing.
When the water in the plant stem freezes, it expands to the point where the plant splits open along the side, and the frozen water is extruded from the split. More water is then drawn up from the ground via capillary action, and the flower continues to grow all night long.
Because of this water requirement, the ground must be damp, but not frozen, and because the frost flower splits the stem open, they only form once a year from any individual plant. These beautiful formations generally form during the first hoarfrosts, and are best found in unmowed areas with lots of weeds and brambles. Though not uncommon in certain areas, this ephemeral event is rarely seen before it sublimates or evaporates.
3. GLAZE ICE
When falling rain hits a roadway or surface that's below freezing, it instantly forms what's known as "glaze ice," a buildup of transparent ice that is an extreme danger to drivers. This is also one of the causes of black ice, which is actually clear, and thin enough that it appears to be the same color and texture as the asphalt and macadam it has accumulated on. (Black ice can also be caused by the freezing of standing water or compacted snow, in which case, it's not glaze ice.) Glaze ice has contributed to some of the costliest ice storms in history, such as the 1998 North American ice storm, and the 2010 New Year's storm in the United Kingdom.
4. HARD RIME
As the weather cools further, freezing fogs can occur, and when that fog is combined with wind, hard rime can form on windward (wind-facing) surfaces. "Rime" literally means "hoarfrost," and while slightly different on a meteorological level, "soft rime" is very similar to a thick hoarfrost. Hard rime, on the other hand, is much thicker and harder to remove, and consists of fairly dense pellets of irregular ice crystals.
Though it's generally seen at high elevations (mountain-based weather observatories often have serious problems with hard rime coating their instruments), this icy coating can form anywhere that a freezing fog and wind occur simultaneously. It's rarely seen below -8 C (17.6 F), because colder air cannot hold enough moisture to create freezing fogs.
The conditions required to create thundersnow are most common around lakes (it occasionally accompanies "lake-effect snow") and coastal areas. In these places, the sun is able to heat the ground and cause (relatively) warm and humid columns of unstable air to rise up, and form turbulent clouds.
But clouds alone don't make thundersnow. Only if the layer of air between the clouds and the ground is warmer than the cloud cover, but still cold enough to create snow, and the wind shear is pushing the warmer air slightly upwards, does thundersnow form. (In lake-effect thundersnow, the temperature has to be at least 45 degrees Fahrenheit higher below the clouds than at the cloud top for the phenomenon to occur.)
Most instances of thundersnow occur during extreme storms, with high-intensity wind and accumulations of between two and six inches of snow per hour. The lightning bolts are rarely visible as they are in summer thunderstorms, and the snow can often muffle the thunderclap, meaning that many instances of thundersnow go unnoticed.
When a place has an extended cold season — like the majority of Canada, for example — snow rarely melts between each subsequent snowfall. Some of it will sublimate — or transition directly from a solid to a gas — especially in areas with lots of sunlight and dry wind, but the majority of it will stay present on the ground. When fresh snow falls on top of the old snow, the crystals of the old snow get packed down under the weight of the new cover. Depending on the length of time, snowflake types, and weather conditions between snowfalls, each layer of the snowpack may have a different thickness and density; heavy snow on top of a loose, unstable layer is one of the ways avalanches form.
By the end of the season, the snow in undisturbed areas will be many layers thick, and the melting of this accumulated water is an important source of fresh water for streams and rivers in the spring. In some places, though, the snow doesn't melt completely, and another year's snow accumulates on top of the old snowpack.
When years and years of snowpack accumulate in an area, that buildup is called "firn." It's much denser than regular snowpack, because of two factors: The partial melting during warmer seasons creates smaller crystals, packed closer together; and the new snow falling on top of the ultra-condensed pack pushes the crystals together without melting during the cold season.
When firn becomes dense enough, it's considered to be ice, and the buildup of firn at a glacial head, high up in the mountains or near the poles, is what makes glaciers able to maintain their size (assuming a stable climate) despite constant melting and breaking at the glacier foot. The density of firn is between 550 kg/m³ and 830 kg/m³. To put that in perspective, the density of freshly-fallen "powder" snow is around 50-70 kg/m³, and the snow at the bottom of a seasonal snowpack is generally no more than 300 kg/m³.
The fraction of the sun's radiation reflected from the earth's surface is known as the "albedo," from the Latin albus, or "whiteness." This is particularly relevant during winter snowfalls, since fresh, clean snow has an albedo of up to 0.7-0.85, meaning up to 85 percent of the radiation from the sun (including the radiant heat) is reflected back into the atmosphere. This can create local cooling effects after a new snowfall, even on a very bright, sunny day. The temperature depression from the albedo can actually cause more snow to fall in an area, if there are other, slightly warmer areas nearby.
On a very small scale, albedo can be experienced just by changing your shirt from black to white. The darker colors absorb much more of the radiation from the sun, and are much better at keeping you warm, while bright whites can reflect almost all of the heat, and can help keep you cool in the summer.
Also known as a "halo" or "icebow," this optical phenomenon causes a bright circle or rainbow around the sun or moon, 22 degrees away from the center of the object. To differentiate between a gloriole and the related "corona" phenomenon (caused by water droplets, and much closer to the sun or moon), if you put your palm over the sun and extend your fingers, they should reach about 20 degrees from the center.
Glorioles can be seen year-round, but are always caused by ice crystals suspended in the atmosphere. When a gloriole is seen in Tallahassee, Florida, in the middle of June, that simply means that the ice crystals are suspended very high up in the atmosphere, where the temperature is much lower. However, during the coldest parts of winter, the ice crystals can be suspended throughout most of the atmosphere, creating glorioles on most bright, sunny days, or when the moon is full and bright.
Sometimes accompanying glorioles are the "sun dogs" — more technically known as parhelia (meaning "beside the sun"). These are bright spots occurring 22 degrees away and the same distance above the horizon as the sun. This phenomenon has been known since ancient times, and was sometimes thought to be "multiple suns" in the sky. When the ice crystals that form glorioles are oriented randomly, they act as prisms in all directions, and the rainbow halo is what becomes visible. As these ice crystals sink through the atmosphere, they tend to fall into a vertical alignment, which reflects the light horizontally, and the sun dogs are formed. While parhelia can potentially be created while the sun is at any position in the sky, they're most often seen when it's just above the horizon.
The night-time equivalent of the parhelia, "moondogs" are exactly analogous to the "sun dogs." They're bright spots on a moon ring (a night-time gloriole), caused by vertically-aligned ice crystals in the atmosphere refracting light horizontally. In folklore, moon rings are said to predict storms, and when moondogs are present, the storm is said to be even stronger. While the predictive power of the 22 degree rings is limited in the winter (as there are often ice crystals in the air that are unrelated to upper-atmosphere changes), the moon rings in warmer months are usually caused by the thin cirrus clouds that often precede a storm front by a few days.
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