Snow Today updates daily images on snow conditions and relevant data, and also provides monthly scientific analyses from January to May, or more frequently as conditions warrant.
Snow Cover Percent
The areal extent of snow-covered ground, expressed as the mathematical percent of a region covered with snow. In the context of this website, the region refers to an Earth observing satellite’s smallest measurement area. We use data from the Moderate Resolution Imaging Spectroradiometer at ~463m spatial resolution. Note that the Earth’s surface is sometimes covered by clouds.
New snow that has fallen out of the atmosphere and accumulated on the ground since the previous day or since the previous observation.
Spatially and temporally complete snow cover percent
Snow cover percent observed with satellites but with gaps identified from missing data and clouds using spatial and temporal filters. Snow cover percent data is then interpolated to gap fill missing data. Interpolation weights nadir observations more than off-nadir observations. Satellite and forest impacts on the viewable portion of snow cover relative to the true on the ground snow cover are accounted for using forest height maps and vegetation cover percent.
Snow cover percent threshold
Snow cover percent at which we have less confidence below, generally due to bright soils or shallow water that cannot be easily separated using surface reflectance data.
Snow cover days
The number of days a region has been covered with snow as identified with snow cover percent greater than a snow cover percent threshold since a starting time. Our data uses 15% as the snow percent threshold and October 1 as the starting date for these maps.
The MODIS snow cover and grain size algorithm, a physically based model for estimating snow cover percent and snow grain size for clear sky surface reflectance using MOD09GA surface reflectance. MODSCAG has been processed by the SnowDS team at the Jet Propulsion Laboratory.
MOD09GA surface reflectance
Surface spectral reflectance of Terra MODIS Bands 1 through 7, corrected for atmospheric conditions such as gasses, aerosols, and Rayleigh scattering.
Snow water equivalent
The water content obtained if all snowpack at a location melted instantly. Because snow contains a mix of water (ice and liquid) and air, the snow water equivalent (SWE) is less than the depth of the snow on the ground.
A network of sites that measures SWE with snow pillows, as well as other variables of interest related to snow depth, weather, and soil moisture. These sites are maintained by the Natural Resources Conservation Service (NRCS), an agency within the United States Department of Agriculture (USDA).
A large, flat instrument that measures and reports the water weight of snowpack on the ground. The weight of water is the snow water equivalent (SWE).