The red dotted-to-solid line depicts the average temperature profile with depth of soil in a permafrost region. The trumpet-shaped lines at the top show seasonal maximum and minimum temperatures in the "active layer", which commences at the depth where the maximum annual temperature intersects 0 degrees C. The active layer is seasonally frozen. The middle zone is permanently frozen as "permafrost". And the bottom layer is where the geothermal temperature is above freezing. Note the importance of the vertical 0 degrees C line: It denotes the bottom of the active layer in the seasonally variable temperature zone and the bottom limit of permafrost as the temperature increases with depth.