Snow Viewer Guide
NSIDC’s Daily Snow Viewer provides daily maps and time series plots of snow cover, including percent, albedo, snow-covered days, and radiative forcing. (See the Snow Today glossary of terms in the About Snow Today page for definitions.) The Daily Snow Viewer also provides data overlays for snow water equivalent (SWE), including changes in SWE, and percentage of median SWE.
Using the Daily Snow Viewer does not require any special software or training. All you need is a web browser. The viewer works best on a desktop computer; using the viewer with a smaller screen may limit the display to a single column of maps and/or plots.
Each map displays the most recent full day of data, and each time series ends on the most recent full day of data. The most recent full day of data is generally from the previous day, but the exact date is listed as a subtitle on the plots. The Daily Snow Viewer is usually updated at 8 a.m. Mountain Time, although occasional processing delays may occur.
Using the controls
Buttons across the top of the Daily Snow Viewer let you change the display, and controls can be used independently of each other. You can use these controls to change the display as often as you like. Clicking the Reset button on the far right of the top menu restores the default settings.
Clicking this button opens a dropdown control panel that allows you to choose the layout of the map and time series graph tiles, as well as the variables shown in each. The minimum option is one tile in a single row and column, and the maximum option is eight tiles in four rows and two columns. The default display is two tiles in one row and two columns, showing the snow cover percent map and total snow cover area time series.
You can select the variables for the maps and plots independently of each other, but each map has a logical variable match for a plot. These are the intended map-plot correlations:
|Map Variable||Plot Variable|
|Snow Cover Percent||Total Snow Cover Area|
|Snow Albedo||Average Snow Albedo|
|Snow Radiative Forcing||Average Snow Radiative Forcing|
|Snow Cover Days||Average Snow Cover Days|
The default display places the map tile on the left and the plot tile on the right, but you can choose any configuration of tiles you like by updating the Tile Type dropdown.
The display will update immediately as you select new items in this control panel. Once you have chosen the desired rows, columns, maps, plots, and variables, click anywhere outside the Configure Variables dropdown panel to close it.
Select a Basemap
Clicking this button opens a dropdown control panel to select the basemap that displays behind the snow map. The default is the USGS Topographic map, but you can choose from several options, including maps provided by the US Geological Survey (USGS) or ArcGIS (Esri 2022).
Once you have chosen the desired basemap, the display will update immediately and the dropdown will automatically close.
Select a Region
Clicking this button opens a dropdown control panel to filter by region.
As of December 2022, the only region covered by the Daily Snow Viewer is the western United States, specifically these states: Arizona, California, Colorado, Idaho, Montana, Nebraska, New Mexico, Nevada, Oregon, South Dakota, Utah, Washington, and Wyoming. A red border outlines the states included.
You can focus on a smaller portion of the western United States, filtered by state or Hydrologic Unit Code (HUC). If you zoom in on a region, and if you have opted to display a corresponding plot, the plot will update to reflect data for just the selected region, with some exceptions (see note below).
Note about HUC plot availability: The daily Snow Viewer offers regional filtering for two kinds of HUCs: two-digit (HUC2) and four-digit (HUC4). Two-digit HUCs encompass larger regions, e.g., HUC 14: Upper Colorado. Four-digit HUCs are subsets of two-digit HUC regions, e.g., HUC 1402: Gunnison. Time-series plots are available for two-digit HUCs, but not four-digit HUCs.
The display will update immediately as you select new regions in this control panel. Once you have chosen the desired region, click anywhere outside the Select a Region dropdown panel to close it. To resume displaying maps and plots for the entire western United States region, click the radio button for None.
Select SWE Overlay
Clicking this button opens a dropdown control panel to add an overlay. Options include Snow Water Equivalent, Change in Snow Water Equivalent, and Percentage of Median Snow Water Equivalent.
Note: This option adds an overlay to all the maps currently displayed.
Display not-processed area
Checking this box adds an overlay showing where data have not been processed. Within the western United States, portions of Montana, South Dakota, and Colorado are not processed for the Daily Snow Viewer.
Moving this slider to the left adds transparency to the data on the maps chosen via Configure Variables. The more transparent those raster maps are, the more of the basemap shows through. The default is 100 percent opacity for the raster maps.
Clicking this button restores the default display.
Working with the maps
Map zoom controls
You can zoom in and out of any map using the + and - buttons in the top left corner of the map, or by using the scroll wheel on your mouse.
Legend placement and resizing
Within each map, you can move the legend (located by default at the bottom of the map) to the optimal location by clicking on it and dragging. When you hover over the legend, your cursor will change to a + sign with an arrow on each end, indicating that you can move the legend. You can also resize the legend by placing your cursor at any of the corners of the legend. Your mouse will change to a diagonal arrow indicating that you can click and drag to the desired size. There is no button option to restore the default location and size of the legend, but the default size and location will return when you reload the Daily Snow Viewer.
Snow Water Equivalent overlays
These overlays, which appear when you select an option under the Select SWE Overlay dropdown, bring up specific data points where on-the-ground (in situ) SWE measurements have been taken. Clicking on any data point will bring up a small data display showing latitude and longitude, elevation, and value. Click the X to close this display.
Note: When you click on a SWE data point, the resulting popup box showing the data value sometimes appears in the top right corner of the map, and sometimes appears next to the data point.
Interpreting the maps
Each snow cover map is selected through the Configure Variables dropdown, and uses a sequential color palette, as follows:
- Snow Albedo: lowest albedo is dark blue, highest albedo is white.
- Snow Radiative Forcing: lowest forcing is yellow, highest forcing is dark red-brown.
- Snow Cover Days: lowest number of days is white, highest number of days is dark purple.
- Snow Cover Percent: lowest percent is dark blue, highest percent is light green-yellow.
Each overlay selected through the Select SWE Overlay dropdown also uses a color palette, either sequential or diverging, depending on the variable shown:
- Snow Water Equivalent: sequential; lowest is white and highest is dark blue.
- Change in Snow Water Equivalent: diverging; decrease in red-orange and increase in blue.
- Percentage of Median Snow Water Equivalent: diverging; below 100 percent in red-orange and above 100 percent in blue.
Interpreting the plots
The time series plot on the right side of the Daily Snow Viewer shows the “snow year.” In the Northern Hemisphere, the snow year starts in October and runs through the following September. Assuming the maps and plots displayed correlate logically with each other (see Configure Variables above), the plot shows snow variables specific to the region designated in the corresponding map. For the duration of the current snow season, each plot displays all of the following:
- Year to date (solid blue line): Current snow-year values up to the most recently observed (generally yesterday’s data)
- Median (dashed gray line): Middle value of observed values over the 2001 to 2021 baseline
- Maximum (charcoal dashed-dotted line): Highest of all observed values over the 2001 to 2021 baseline
- Minimum (charcoal dotted line): Lowest of all observed values over the 2001 to 2021 baseline
- Interquartile range (light gray band): Innermost 50 percent of all observed values over the 2001 to 2021 baseline
Moving the mouse over the plot brings up a display for the corresponding date in the snow year. Each value is highlighted on its plot line with a specific symbol shown in the legend.
Known data issues
This list reflects known issues as of December 16, 2022.
- Incorrect data when solar zenith angle is greater than 67.5°: Input data are not processed when the solar zenith angle is greater than 67.5°. The pixels where this occurs were not flagged as not-processed, so they appear as 0 (or no snow) in the data, which is incorrect. When time permits, a future version of this product will either be released with these pixels marked as not-processed or we will process the data. To determine which pixels have this limitation, you can get solar zenith angles from MOD09GA from LPDAAC or you can look this information up on NOAA’s Solar Position Calculator.
- Data outage for approximately two weeks starting October 10, 2022: The Terra satellite carrying the MODIS sensor was repositioned during this time. This caused a data outage for this time period. This outage affects time series plots in the Daily Snow Viewer, causing spikes in uncertain estimates in the gap-filling process.
- Missing/erroneous radiative forcing and albedo values: Every snow pixel should have a radiative forcing and an albedo value, however, in some instances one or both of these values is missing. In addition, sometimes radiative forcing and albedo values erroneously appear in pixels where there is no snow. The radiative forcing and albedo values are correct except for those that appear on non-snow covered pixels. This error will be fixed in a future version of these data.
- MODIS Terra data processing temporarily suspended: Since December 1, 2022, MODIS Terra has been experiencing geolocation issues resulting in incomplete data for December 1 to December 7 and no data since December 7.
- Issue with radiative forcing and albedo values in forested areas: In this new version of the data for forested areas only, a new interpolation routine has been implemented that has resulted in higher radiative forcing values and lower snow albedo values than would normally be expected. This issue will be fixed in a future version of these data.
- Intermittent data processing delays: Generally, the Snow Today data and SWE values are processed by 8 a.m. (Mountain Time Zone). However, intermittent delays are possible if input data do not arrive from Jet Propulsion Laboratory, or if there are long queues on the supercomputer resulting in slower processing than normal. Delayed data will usually arrive by mid-afternoon but can be delayed as long as a day.