National Park Service

Southwest Alaska Network (SWAN)

About Lake Ice Data


The freeze and breakup dates of lake ice affect lake ecosystems, wildlife migration, wildlife habitat, and human subsistence and recreation. Here we use remotely sensed data to track “lake ice phenology” or the timing of ice formation and break-up on large lakes.

Data Collection

The images from the Moderate Resolution Imaging Spectroradiometer (MODIS), collected by the Aqua and Terra satellites, are used to monitor lake ice phenology at a number of large lakes in southwest Alaska. The daily images from these satellites make it possible for us to assign precise dates to the freeze and breakup, or make accurate estimates of them when the area is obscured by clouds. For water years from 2010 to present the percent of lake ice cover is quantified using 100 ha grid masked to lake margins, for the 2002‐2009 water years the percent ice cover was determined using ocular estimates.

Data Analysis

Once data for all lakes in a water year is quantified, the metric dates are calculated. These include:

  • freeze start = first time a lake has 10% or greater ice cover
  • freeze final = first time the lake has 91% or greater ice cover
  • breakup start = the last time the lake drops below 91% ice cover
  • breakup final = last time the lake drops below 10% ice cover

With all metric dates, we capture when the metric is meet plus the previous viewable day and report the date between the two. For example: freeze start, the lake is first seen to have 15% ice cover on Nov. 5th, the previous viewable day is Nov 1st with 4% ice cover, we would report the freeze start as Nov. 3rd.

Data Citation

To cite the data, please use:

Southwest Alaska Network, National Park Service. Year. Lake Ice Monitoring ( - Accessed Date).

Where Date is the date you downloaded the data and Year is the year of the download.

For more information please contact the SWAN Physical Scientist Peter Kirchner.

Note: The "Last Updated" date listed on the web pages is the last time the structure of the page was changed. The graph and table showing the data are pulled directly from the database and do not alter the page structure. When the data in the database is updated, it will show on the lake ice web pages without changing the "Last Updated" date.

Last Updated: February 07, 2017 Contact Webmaster