Explore · Field Atlas
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Winter begins as a geometry problem: a hemisphere tilted away from the Sun receives shorter days and lower-angle sunlight than it does in its summer season.
This explains the seasonal energy pattern; it does not predict local weather.
The winter half of the annual cycle brings reduced daylight duration away from the equator, with the contrast strengthening toward higher latitudes.
Day length varies with latitude and date; this universe does not infer a visitor's location.
NOAA's U.S. climate products group Northern Hemisphere meteorological winter as December, January and February for consistent climate analysis.
This is a calendar convention, not a claim that every place experiences winter conditions during those months.
Snow begins with ice-crystal growth aloft; falling crystals can aggregate into snowflakes and arrive in many forms rather than one iconic six-armed silhouette.
The atlas intentionally avoids assigning a crystal habit from temperature alone because real cloud histories are more complicated.
Snow cover is a changing stack of new and older snow and ice, not a static white sheet; wind, compaction, melt and refreezing can alter its structure.
This is descriptive cryosphere context, not avalanche or travel-safety guidance.
Clean snow reflects much visible sunlight, which helps explain its bright appearance and why surface reflectivity matters in winter visual interpretation.
Brightness in a photograph is not a calibrated albedo measurement.
Sleet can form when falling snow partially melts in a warm layer and then refreezes in a sufficiently deep subfreezing layer before reaching the ground.
The simplified profile is conceptual; operational diagnosis uses observed and forecast atmospheric data.
Freezing rain can occur when snow melts aloft and liquid drops remain supercooled through a shallow cold layer before freezing on contact with subfreezing surfaces.
Rime is an opaque granular ice coating produced when supercooled droplets freeze rapidly on exposed objects.
Rime and hoarfrost can look similar at a glance but arise from different moisture pathways.
Hoarfrost forms when water vapor deposits as interlocking ice crystals on surfaces below freezing, producing a feathery structure distinct from rime.
A photograph without environmental context may not be enough to distinguish frost from rime reliably.
Cold air crossing relatively warmer unfrozen lake water can gain heat and moisture, supporting narrow snow bands where wind and geography align.
The ingredients shown here are explanatory, not a lake-effect forecast for any real place or time.
Wind can lift and transport existing snow after it has fallen; NSIDC distinguishes drifting snow near the surface from blowing snow raised higher into the air.
Visibility impacts depend on actual wind, snow condition and terrain; this atlas does not calculate them.
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