by Amanda Hagerty

Yellowstone in winter is a land of transformation. Snow blankets the valleys, steam rises from thermal features, and on the coldest mornings, trees, grasses, and even bison wear a glistening coat of frost. These icy formations, delicate yet fleeting, turn the park into an ephemeral sculpture garden. But look closely, and you will see that not all frost is the same.

Two of the most fascinating types of frost found in Yellowstone—hoar frost and rime ice—appear similar at first glance but form under entirely different conditions. Understanding them is more than just a science lesson; it is a glimpse into the dynamic interplay of temperature, moisture, and time—a reminder that even the smallest details of nature are shaped by precise conditions.

Hoar Frost

Hoar frost is winter’s most delicate and intricate ice formation. It forms when water vapor in the air undergoes sublimation, meaning it transforms directly from gas to solid without passing through a liquid phase. This occurs on clear, calm nights when air moisture levels are high and surfaces—blades of grass, tree branches, even fur—become colder than the surrounding air due to radiative cooling. As heat escapes from these surfaces into the open night sky, they drop below the dew point—the temperature at which moisture in the air condenses. However, instead of forming liquid dew, the moisture freezes immediately, creating ice crystals that attach and expand outward in intricate patterns.

Because there is no wind or external force shaping it, hoar frost grows freely, following the natural laws of crystallization. Its structure is influenced by temperature and humidity, forming feathery, hexagonal ice structures that resemble frost flowers or lace. The colder and more humid the air, the larger and more delicate the frost patterns become. But this beauty is fleeting—hoar frost is highly sensitive to changes in temperature and wind. A single gust of wind can shatter the fragile formations, and as soon as the sun rises, the warming air causes the ice to sublimate back into vapor, leaving little evidence of its existence.

Rime Ice

Where hoar frost thrives in calm conditions, rime ice is forged in movement. Rime ice forms when supercooled water droplets—tiny liquid particles that remain unfrozen even in sub-freezing temperatures—encounter a solid surface and freeze on impact. These droplets exist in clouds or fog when the air temperature is below freezing, but because they lack a nucleation site (a surface to begin forming ice crystals), they remain liquid until they touch something solid. The moment they collide with a tree branch, a rock, or even a bison’s fur, they flash-freeze, forming an opaque, rough-textured ice.

Unlike hoar frost, which develops in delicate crystalline patterns, rime ice is depositional, meaning it builds up gradually as new supercooled droplets continue to hit the same surface and freeze. The direction and intensity of the wind influences its formation, sculpting thick layers of ice into wind-swept ridges, jagged clusters, or smooth coatings.

You will often find rime ice in Yellowstone’s geothermal areas, where warm steam rises into frigid air and instantly freezes onto surrounding surfaces, coating boardwalk railings and trees in ghostly white formations. It is also common at higher elevations, where freezing fog—dense with supercooled droplets—blankets exposed ridges and creates windward-facing ice accumulations. The denser the freezing fog and the stronger the wind, the thicker the rime ice becomes, sometimes forming dramatic spiky formations that seem to stretch in the direction of the wind itself. Unlike hoar frost, which vanishes quickly in sunlight, rime ice is more durable, often lasting for days or weeks unless a warm front melts it away.

Where to See These Frosty Formations in Yellowstone

Yellowstone provides the perfect conditions for both hoar frost and rime ice, and each can be found in different places across the park.

• Hoar frost is most seen on cold, clear mornings in meadows, along riverbanks, and even on the fur of bison, turning them into ghostly white silhouettes.
• Rime ice thrives in geothermal areas where warm steam freezes instantly upon contact, often found coating trees and boardwalks in places like Mammoth Hot Springs or Norris Geyser Basin. It also appears on exposed ridges where freezing fog and wind create thick, icy layers on vegetation.

At their core, hoar frost and rime ice are a study in contrasts—one is delicate and fleeting, the other wind-forged and resilient. Hoar frost forms in perfect calm, while rime ice is born of movement and collision. One builds intricate lattices that vanish with the first touch of warmth; the other layers itself into thick, wind-carved ridges that linger long into the cold. Yet both are reminders of how nature works in balance, shaping the landscape through the quietest and most powerful forces. Their existence depends not on chance, but on a precise alignment of temperature, humidity, and time. They tell a story of Yellowstone in winter—not just of ice and cold, but of the conditions that shape the wild in ways both visible and unseen.

These frost formations may be temporary, but their beauty lies in their fleeting nature. Like a snowflake that lands on your glove, they are winter’s artistry at its most intricate—reminders that even in the harshest cold, nature finds a way to create something delicate, something fleeting, and something worth noticing.

So, next time you find yourself in Yellowstone on a frigid morning, take a moment to look beyond the obvious winter wonderland. The frost tells a story—one written in ice, shaped by wind, and fleeting as the morning sun.

 

Images:

Feature: Rime ice on the boardwalk of Mammoth Hot Springs Upper Terraces, YF / Matt Ludin
Top to bottom:
Hoar frost built up on grasses under standing dead lodgepole pine trees on Tangled Creek. NPS / Jacob W. Frank
Rime ice on trees on top of Palette Spring at Mammoth Hot Springs, YF / Matt Ludin
Hoar frost on cottonwood trees in Lamar Valley at -20 below zero F. YF / Matt Ludin
Bison covered in hoar frost near Old Faithful. YF / Matt Ludin