Ice cover across a water body can be measured by either its area or its extent. Area is an attempt to quantify the total amount of a water surface covered by ice. Extent is the total area of the surface—broken up into grids on a map—covered by at least 15% ice. 

Scientists studying ice from space-based platforms often prefer using the extent measurement because their microwave instruments can see well through clouds and winter darkness but at times may have trouble discerning between ocean water and meltwater on the surface of ice in summer. This confusion between ocean and meltwater can lead to miscalculations of ice area. The scientific community also has greater confidence in measuring ice extent than it does area, and the way ice extent is counted in platform imagery reduces day-to-day variability.

NASA data can be used for measuring ice extent to aid in understanding weather and climate patterns, monitoring habitat health in the cryosphere, and for the safety and planning of human activities in polar regions. 

Learn more about near real-time ice extent data for shipping provided by NASA's Land, Atmosphere Near real-time Capability for Earth observation (LANCE).