Why does thermal effusivity have a square root in it?

Thermal effusivity and thermal diffusivity are nice basic properties of materials. The diffusivity tells how quickly temperature change propagates through a material, and it’s equal to the conductivity divided by the volumetric heat capacity. That seems expected. Thermal effusivity tells how quickly a material gives or takes heat when it contacts another material at a different temperature. I’d think it would be the product of the conductivity and the volumetric heat capacity, but, no, it’s the square root of that. Why?

Obviously, either way it could capture the interesting part of the material behavior.

What’s the intent behind taking the root? The significance of it, I mean?