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| Winter wonderland |
Snowflake Formation
Snowflakes begin high in the atmosphere inside clouds, where tiny droplets of water and invisible water vapor are constantly moving around. When the temperature in a cloud drops below freezing, water vapor sticks to microscopic particles such as dust or pollen. Instead of turning into liquid water first, the vapor changes directly into ice.
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| Nakaya diagram of snow crystal formation Source (snowflakes.com) |
As soon as that first bit of microscopic ice forms, it becomes the center of a tiny crystal. Because of the way water molecules bond when they freeze, the crystal naturally forms a six-sided, or hexagonal, shape. That basic six-sided structure is the foundation of every snowflake.
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| Snowflake images - W. A. Bentley |
As the crystal falls through the cloud, more water vapor freezes onto it. The temperature and humidity of the air determine how the snowflake grows. In some conditions, it develops flat plates; in others, it grows long arms or delicate branching patterns. Since each snowflake travels through slightly different layers of air, no two follow exactly the same path or experience the same conditions. That is why every snowflake has a unique design.
Once those flakes reach the ground, an entirely new process begins. Over time, layer upon layer of fallen snow transforms into something important known as snowpack.
Snowpack
Snowpack is simply the accumulation of snow that remains on the ground over time. It forms when repeated snowfalls occur without completely melting between storms. Instead of disappearing, each new snowfall settles on top of the previous one, creating distinct layers. In regions with consistent winter cold—whether in the mountains of the West or during a steady winter—this layered structure can persist for weeks or months.
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| Principal physical processes of a snowpack WSL - Institut für Schnee und Lawinenforschung SLF |
When snow accumulates deeply enough, the bottom layer—where snow meets the ground—begins to change. Heat rising from the soil slightly warms this lowest layer, while the upper layers remain cold. This temperature difference causes the snow crystals near the ground to become larger and more loosely packed, creating small air spaces. Over time, these spaces connect, forming a hidden network of tunnels and pockets.
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| Diagram of ecotones - State of Maine |
Subnivean Zone
This space is called the subnivean zone (from the Latin “sub” meaning under, and “nivean” relating to snow). It exists between the soil surface and the compacted snow above. While air temperatures above the snow may plunge well below freezing, the subnivean layer remains comparatively stable—often hovering around 32°F. The snowpack acts as insulation, trapping heat from the earth and buffering against wind.
Many of native pollinators spend the winter far closer to the ground than we might imagine. Unlike honey bees, which overwinter as a colony, most native bees are solitary. A large number nest in soil, creating small underground chambers where larvae develop and eventually overwinter as adults or pupae. Others overwinter in hollow stems, leaf litter, or just beneath the soil surface.
A consistent snowpack provides insulation that moderates extreme temperature swings. Even when air temperatures drop into the single digits, soil temperatures beneath steady snow often remain close to freezing but relatively stable. That stability can mean the difference between survival and winterkill for ground-nesting bees.
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| Polyester bee (Colletes spp.) Photo:Heather Holm |
The same is true for many butterflies and moths. Some species overwinter as chrysalides attached to stems or hidden in leaf litter. Others overwinter as caterpillars tucked into protected crevices at the soil surface. Snow cover acts as a thermal blanket, shielding them from desiccating winds and repeated freeze–thaw cycles.
Some common overwintering insects stage and site.
American dog tick (Dermacentor variablis) - adult, in soil - (actually an archanid)
Japanese beetle (Popillia japonica) - mature grub (larva), in soil
Squash vine borer (Mellitia cucumberitae) - larva or pupa, cocoon in soil
Bean leaf beetle ( Ceratoma trifurcate) - adult, in soil or under plant debris.
Ironically, winters with little snow can be harder on pollinators than colder winters with steady snow cover. Without insulation, soil temperatures fluctuate more dramatically. Repeated freezing and thawing can damage overwintering insects and expose them to predators.
This connection reinforces an important gardening principle: leaving plant stems standing and allowing leaf litter to remain through winter is not neglect—it is habitat stewardship. When snowpack settles over an undisturbed garden bed, it creates a layered system of protection: soil warmth rising from below, insulating snow above, and structural plant material in between.
By early spring, as snow gradually melts, moisture filters into the soil, and soil temperatures rise slowly. This gentle transition supports emerging pollinators and early-season flowering plants alike.
What looks like a quiet winter garden is actually a protected nursery beneath the snow. The subnivean space, shaped by snowpack, helps ensure that when spring returns, the bees, butterflies, and other beneficial insects return with it.
Additional Resources
How do snowflakes form? - National Oceanic and Atmosphere Administration
Spring reveals subnivean secrets - Maine Department of Inland Fisheries and Wildlife
The physics of snowflakes - American University, Washington, DC
Below the snow - Department of Fish, Wildlife and Parks, State of Montana
Subnivean - Department of Fish and Game, State of Idaho
How to help bees and butterflies survive the winter - University of New Hampshire
New Jersey Pollinators - NRCS - USDA
Snow crystal information - snowcrystals.com
Bees in winter - University of Wisconsin - Madison
Surviving winter: what heat-loving butterfly do during cold months - Xerces Society
How do insects survive winter? - University of Nebraska - Lincoln
Snow - WSL - Institute for Snow and Avalanche Research SLF
