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.
1. Hoar frost
One of the first signs of winter is the hoar frost of late autumn. Deriving its name from an Old English word (hoar, meaning “to appear old”), this is the thin, feathery 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 into a solid when it makes contact with surfaces. 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 through the stem from the ground via capillary action, adding more ice crystals to the frost flower.
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 hoar frosts, and are best found in unmowed areas with lots of weeds and brambles.
3. Glaze Ice
When falling precipitation hits a surface that’s below freezing, it can instantly form what’s known as “glaze ice,” a buildup of smooth, clear, and transparent ice. It can be seen coating tree branches and plants following an ice storm. Because it can also coat roads, glaze ice 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 winter 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, freezing fogs can occur, and when that fog is combined with wind, hard rime can form on windward (wind-facing) surfaces. “Rime” literally means “hoar frost,” 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, and consists of fairly dense pellets of irregular ice crystals glommed together.
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 and the temperature falls between -2°C and -10°C (28°F and 14°F).
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.
Most instances of thundersnow occur during extreme storms, with high-intensity wind and accumulations of between 2 and 6 inches of snow per hour. The snow can often muffle the thunderclap, meaning that many instances of thundersnow probably go unnoticed.
When a place has an extended cold season, 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, that buildup is called “firn.” It’s much denser than regular snowpack, because of two factors: the partial melting during warmer seasons packs the snow mass closer together; and the new snow falling on top of the ultra-condensed pack pushes the crystals together without melting during the cold season.
The buildup of firn at high elevations eventually forms glaciers, and replenishes glacial mass to counteract the melting and calving at their lower elevations. The density of firn is between 550 830 kilograms per square meter. To put that in perspective, the density of freshly-fallen “powder” snow is around 50 to 70 kilograms per square meter, and the snow at the bottom of a seasonal snowpack is generally no more than 300 kilograms per square meter.
The fraction of the sun’s radiation reflected from the Earth’s surface is known as the “albedo,” from the Latin albus, or “whiteness.” A surface’s albedo is given as a figure between 0 (completely black and nonreflective) and 1 (completely white and reflective). This is particularly relevant during winter, 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.
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 makes a bright circle or rainbow to appear at a distance around the sun or moon. Glorioles can be seen year-round and are caused by ice crystals suspended in the atmosphere refracting sunlight or moonlight. 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 “sun dogs”—technically known as parhelia (meaning “beside the sun”). This phenomenon has been known since ancient times, and was sometimes thought to be “multiple suns” in the sky.
When hexagonal ice crystals in the atmosphere sink toward Earth, they tend to fall into a vertical alignment. The crystals refract sunlight horizontally, causing two extra “suns” to appear on other side of the real sun. 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 their predictive power is limited in the winter (because ice crystals in the air are often 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.
A version of this story originally ran in 2013; it has been updated for 2021.