In a 160 deep snowpack the gradient is isothermal for the first 120 cm (ground upwards). In the last 40 centimeters there is a 2.4 degree temperature gradient. How is this to be read? Do I consider the gradient in the entire snowpack, or just from the depth where the temperature begins to vary?

Consider the gradient from whenever there is a change. So you have a 6C/m temperature gradient in the upper half of the snowpack.

Ok. Thx.
erik