The Science of Winter Weather and its Strange Phenomena

Is the weather outside frightful, fascinating, or both? The drama of winter weather comes alive with howling snow squalls, sudden hard freezes, and the rare thrill of thundersnow. From the everyday challenges of snow and ice to the wonder of odd phenomena unique to the cold, winter’s weather offers us an opportunity to learn and appreciate the forces of nature we experience when temperatures drop. The National Weather Service (NWS) defines the climatological winter period as October 15th to April 15th, when winter weather events most commonly occur (1). These events take on various forms, featuring a mix of precipitation types, unusual phenomena, and their subsequent impacts.

The Varieties of Winter Precipitation

Snow

Water can take on several forms of precipitation in freezing conditions. Snow falls when a sufficiently deep layer of air at or below freezing exists in the troposphere, allowing ice crystals to remain frozen as they grow in size and fall to the ground as snowflakes. Snow can appear as a gentle flurry, a brief but hefty squall, or even a dangerous blizzard that, in extreme cases, can persist for several days. Snow transforms from ornamental to treacherous as its accumulation grows, with heavy snowfall causing road closures, power outages, roof collapses and other significant impacts.

One phenomenon that regularly produces heavy snow accumulation is lake-effect snow. Lake-effect snow occurs when a freezing air mass passes over a large lake. Because water has a higher specific heat capacity than air, the lake maintains some warmth even as the air and land around it plunge deep into winter (2). When a mass of cold air passes over the unfrozen lake, water from the lake’s surface evaporates into the air and the (relatively) warm lake heats the cold air mass from below. The air in contact with the lake’s surface becomes warmer and moister than the air above it and begins to rise. As it rises, the air expands and cools, and the moisture picked up from the lake condenses to form clouds and, eventually, snow.

The greater the temperature difference between the air and the water, the more moisture the air can gather, and the heavier the snow can fall. If the wind pushes these narrow squalls onto land, snow can fall at a rate of 2-3 inches or higher, per hour! On November 29th, 2024, Erie, Pennsylvania— situated on the south shore of Lake Erie— received 22.6 inches of lake-effect snow, which became their single snowiest day since records began in 1893 (3).

Sleet and Graupel

When a shallow layer of warm air is wedged between two freezing layers, snow that forms aloft will partially or completely melt into raindrops as it falls toward the surface. When these raindrops then encounter a final layer of freezing temperatures near the surface, they refreeze into small, icy pellets called sleet before they reach the ground. Sleet does not cling to surfaces the way snow can, but a heavy accumulation of sleet can still cause hazardous road conditions and damage to homes and infrastructure.

A similar type of winter precipitation is called graupel. While they may appear similar, sleet and graupel form in different ways: precipitation becomes graupel through a process called “riming,” where supercooled water droplets freeze onto snowflakes, and they fall to the surface as soft ice pellets (4).

Some people refer to graupel as “soft hail,” but hail and graupel form under different conditions. Whereas graupel forms around a snowflake and occurs during the cold season, hail forms around a frozen raindrop and is produced during thunderstorms, which occur most often during the warm season. Although graupel may look like cloudy, small hailstones, unlike hail, graupel tends to be fragile and quickly melts or disintegrates when handled.

Freezing Rain

Freezing rain is a particularly destructive form of winter precipitation. It occurs when snowflakes pass through a deep layer of warm air in the atmosphere above a very shallow layer of freezing air at the surface. As the snowflakes pass through the warm layer, they melt completely into raindrops. These raindrops don’t have time to refreeze as they pass through the shallow cold layer near the surface, so they hit the ground as liquid, spreading out into a layer of clear ice before refreezing. Freezing rain is heavy and sticks perniciously to trees and power lines. It can cause significant damage to property and infrastructure and create overall perilous conditions for the communities that experience it. In fact, some of the most damaging and costliest winter storms have been mainly due to freezing rain (5).

Mixed Precipitation

When two or more of these winter precipitation types fall over the same place simultaneously or over a short period of time, we call it mixed precipitation (6). This wintry combination can amplify the dangers and challenges posed by each individual precipitation type, and lead to even more damage and loss.

The Oddities of Winter Weather

Some consider the wintertime to be a predictable season marked by cold temperatures, snow, and ice. However, a variety of unusual phenomena that only occur with freezing temperatures can captivate meteorologists and the public alike. Thundersnow, produced by thunderstorms where snow falls as the primary form of precipitation, is a rare phenomenon because a cold atmosphere is often too dry and stable for a turbulent thunderstorm to form. However, with abundant moisture and strong instability, like that of some lake-effect snow events or ahead of the warm sector within a winter storm (i.e., extratropical cyclone), thundersnow can occur. Although thundersnow happens from the same mechanisms of a typical warm-season thunderstorm, the contrast of lightning and thunder amid the muffling snowfall and bitterly cold wind can create a surreal experience (7).

Another peculiar winter phenomenon takes on the form of miniature hay bales made of snow. Known as snow rollers, or snow bales, these rare formations require a light layer of snow that clings to itself but doesn’t stick to the ground, paired with gentle winds strong enough to curl the snow into cylindrical spirals without tearing them apart. Alternatively, snow rollers can form by gravity on a slight downhill slope (8). As the wind or gravity carry the snow roller along, it gathers layers of snow and grows, and while they are often small and short-lived, snow rollers can become several feet large under the right conditions. Regardless of their size, snow rollers can easily transform an ordinary snow-covered field into a whimsical landscape.

Among winter’s most dreamlike sights are ice pillars, also known as light pillars. This phenomenon occurs when tiny, flat ice crystals suspended in the atmosphere reflect strong natural or artificial light sources and create vertical columns of light that seem to stretch endlessly into the night sky. The ice crystals, like little hexagonal mirrors, drift horizontally through the air, exposing their widest surfaces to the light. As these surfaces catch the light, they reflect it onto the ice crystals above or below. The atmosphere must be still and frigid for ice pillars to form, allowing the ice crystals to remain frozen and suspended near the ground, undisturbed by wind. The result is a magical display of radiant towers that take on the color of the light source they reflect. Incandescent street or city lights cast a warm, yellow-orange glow while LEDs create a bright, almost bluish white. When the primary light comes from a natural source, such as the moon or the early rays of sunrise, the ice pillars can shimmer with hues of pink, purple, and green.

Final Thoughts

Winter weather presents a mix of challenges and wonders, each shaped by the unique and sometimes rare conditions of the season. Winter can remind us of the power and unpredictability of weather while offering a chance to marvel at its remarkable creations. From the dangers of freezing rain to the breathtaking spectacle of ice pillars, there is always something worth our attention when the temperature drops. ‘Tis the season to respect the elements and take a moment to appreciate the strange yet beautiful phenomena that only winter brings us.