- the maximum depth at which soil is frozen.
- the lower limit of permafrost.
- the deepest point in the ground to which frost will penetrate
- the limit towards the equator beyond which frosts do not occur
- In regions where there is no permafrost, the maximum depth to which frost penetrates the ground in the winter.
- The lower limit of permafrost.
- In tropical regions, the elevation below which frost never occurs.