Winter & High-Latitude Monitoring: Snow, Low Sun, and Aerosols

Why winter & high latitudes are hard

Above roughly 60–70°N in winter, the sun sits very low or doesn’t rise at all. Optical sensors depend on reflected sunlight, so signal-to-noise drops and long shadows dominate. Even when scenes are acquired, there may not be enough light for reliable surface reflectance (BOA) estimates. By contrast, C-band SAR is an active microwave system; it illuminates the surface itself and collects data regardless of cloud or darkness, which is why Sentinel-1 is used extensively for winter monitoring and sea-ice mapping. Learn more: Copernicus Sentinel-1 (all-weather, day/night) · ESA explainer video

There are limited nighttime optical exceptions worth noting. NASA’s VIIRS Day/Night Band can image clouds, snow and sea ice under moonlight or airglow during polar darkness, providing useful low-light context but it’s not the same as daylight surface reflectance for quantitative BOA analysis. Background: NASA Earth Observatory—monitoring the Arctic during polar darkness · Earthdata: VIIRS Day/Night Band

What actually fails for optical

Low illumination is the first barrier: too little reflected radiance means unstable BOA retrievals. Geometry is the second: low sun angles amplify BRDF effects, so the same field can look different from one date to the next. Aerosols add a third problem. Winter “Arctic haze” and long atmospheric path lengths make aerosol properties harder to estimate over snow/ice, and BOA corrections can swing between over- and under-correction. Peer-reviewed work documents the difficulty of aerosol retrieval over snow-covered regions and the seasonality of Arctic aerosol optical properties. For context: Aerosols in the Arctic—seasonality · AOD over snow/ice challenges

Snow itself is tricky. It’s bright and fast-changing (accumulation, melt, wet/dry transitions), and its spectrum can resemble clouds. Classic cloud masks tend to over-remove valid snow or under-remove thin cloud over snow, while dilation to avoid halos can erase good pixels along edges. The result for time-series is familiar: gaps when you’re strict, contamination when you’re permissive.

Why SAR keeps going (and what it tells you)

Because SAR is active and cloud-penetrating, it provides continuous winter coverage. Changes in backscatter reveal freeze/thaw, wet vs. dry snow, surface roughness and inundation. To compare dates fairly, apply standard processing, radiometric calibration, terrain flattening, and incidence-angle normalization. Then, when any usable optical data does arrive, combine it for spectral context.

ClearSKY in winter

ClearSKY runs SAR-optical fusion for daily continuity in northern winters. We fuse same-day passes across satellites and, when gaps remain, include prior-day observations but never future data. Snow is handled explicitly, low-sun geometry is normalized where feasible, and when ordered, outputs can include per-pixel source, date, and confidence so uncertainty is visible instead of hidden.