Snow cover

Snow cover mapping answers where snow lies, what fraction of each pixel it covers and for how many days, expressed as binary snow cover extent, fractional snow cover, onset and melt-out dates, snow cover duration and a wet versus dry snow flag ^[1][2]. Optical sensors see canopy snow rather than ground snow, so fractional products can report top-of-canopy and on-ground fractions separately ^[2]. Snow water equivalent and snow depth are separate products and out of scope here ^[1]. The Global Climate Observing System (GCOS) lists area covered by snow as an essential climate variable product with horizontal resolution of 50 m (goal), 500 m (breakthrough) and 1000 m (threshold) ^[1].

Optical mapping relies on snow being bright in visible light and dark in the shortwave infrared, a contrast that also separates snow from most cloud types ^[3]. The Normalized Difference Snow Index (NDSI) from the Moderate Resolution Imaging Spectroradiometer (MODIS) pairs a visible band with band 6 at 1.628 to 1.652 micrometres ^[4]. Sentinel-2 carries band 11 at 1610 nanometres and 20 m, and snow is often mistaken for opaque cloud in its Level-1C cloud mask ^[5]. PlanetScope documentation lists blue to near-infrared bands and no shortwave infrared band ^[6]. PlanetScope therefore lacks the band near 1.6 micrometres that separates snow from cloud in standard NDSI mapping, which limits cloud discrimination rather than ruling out snow detection ^[3][6].

MODIS MOD10A1 provides daily global 500 m snow maps from 24 February 2000 ^[7]. Cloud is the main constraint, polar darkness blocks winter mapping and very thin or ephemeral snow may not be mapped ^[7]. Investigators report 88 to 93 percent accuracy under clear skies, near-ideal illumination and several centimetres of snow ^[3]. The Visible Infrared Imaging Radiometer Suite (VIIRS) product VNP10A1 gives daily 375 m maps from 19 January 2012 ^[8]. As of the NASA alert last updated 20 October 2026, Terra and Aqua are drifting out of their designed orbits and are to begin shutting down in late 2026 or early 2027, and delivery of new Suomi NPP data ceases on 2 November 2026; NASA encourages use of VIIRS products ^[9].

The Copernicus Land Monitoring Service High Resolution Snow and Ice (HR-S&I) products cover the European Environment Agency (EEA38) countries and the United Kingdom, with fractional snow cover at 20 m from Sentinel-2 and a gap-filled daily product at 60 m using observations up to 7 days old ^[2]. Snow detection agreed with in situ snow depth at 94 percent accuracy in cloud-free cases (1414 pairs) ^[2][10]. Sentinel-1 synthetic aperture radar (SAR) detects wet snow at C-band from a backscatter drop, while dry snow is transparent at C-band and wet snow retrieval is unreliable over forest ^[2]. A Copernicus daily global 1 km snow cover extent product, excluding Antarctica, has run since December 2025 from Sentinel-3 Sea and Land Surface Temperature Radiometer (SLSTR) data with NOAA-20 VIIRS as backup, validated with a limited reference data set ^[11][12].

NOAA's daily Northern Hemisphere snow and ice analysis has run since February 1997 from multiple satellites and in situ data ^[13]. A NOAA Northern Hemisphere snow cover extent record from visible-band analyses spans 1966 to the present ^[14]. Passive microwave snow maps date from 1978; in a 2000 North America comparison from the Special Sensor Microwave Imager (SSM/I) era, visible and near-infrared maps were more accurate than microwave maps, whose footprint of about 30 km produced snow area differences of up to 5.33 million square kilometres against MODIS ^[4].

Shaded slopes at low sun are missed by optical maps, and Antarctic cloud and snow confusion limits polar mapping ^[10][3].