The phrase "snow extent" refers to the amount or area of land covered by snow. It tells us how much land is covered by snow during a specific time period or in a particular region. Full definition
The scientists used satellite data to measure snow extent over time and found that snow is melting 16 ± 10 days earlier in the 2000s compared with 1972 — 1999. (slate.com)
For example, we have seen the ice and snow extent greatly decrease throughout the world, reflecting unprecedented rates of melting in modern time. (realclimate.org)
Summer snow extent is defined by snow melting at high latitudes. (wattsupwiththat.com)