The active layer is not the permafrost

The active layer and permafrost are distinguished by their different seasonal and thermal definitions. This essay explains why thaw depth on one visit is not necessarily annual maximum active-layer thickness, and why neither measurement alone establishes the condition of deeper ground. Hypothetical sites with matching thaw depths but different temperature and ground-ice properties illustrate the limit. NSIDC definitions, the Circumpolar Active Layer Monitoring protocol, and NOAA's carbon assessment support a comparison of probing, frost tubes, and temperature records. Snow, vegetation, moisture, disturbance, and latent heat complicate simple links between air temperature and ground response. The practical reading method asks whether a claim concerns a thaw boundary, warming, ice loss, or permafrost disappearance before matching its depth, date, coverage, and method to a decision.