Description of Soil Temperature Regimes
according to USDA Soil Taxonomy
8th edition of Keys

Classes of Soil Temperature Regimes

The following explanation is actually organised as a key, although that isn't stated explicitly. So, you should read from top to bottom, and the first description that fits a soil temperature regime is used.

Cryic (Gr. kryos, coldness; meaning very cold soils)

Soils in this temperature regime have a mean annual temperature lower than 8° C but do not have permafrost.

1. In mineral soils the mean summer soil temperature (June, July, and August in the Northern Hemisphere and December, January, and February in the Southern Hemisphere) either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower, is as follows:
   1. If the soil is not saturated with water during some part of the summer and
      1. If there is no O horizon: lower than 15° C; or
      2. If there is an O horizon: lower than 8° C; or
   2. If the soil is saturated with water during some part of the summer and
      1. If there is no O horizon: lower than 13° C; or
      2. If there is an O horizon or a histic epipedon: lower than 6° C.
2. In organic soils the mean annual soil temperature is lower than 6° C.

Cryic soils that have an aquic moisture regime commonly are churned by frost.

Isofrigid soils could also have a cryic temperature regime. A few with organic materials in the upper part are exceptions.

The concepts of the soil temperature regimes described below are used in defining classes of soils in the low categories.

Frigid

A soil with a frigid temperature regime is warmer in summer than a soil with a cryic regime, but its mean annual temperature is lower than 8° C and the difference between mean summer (June, July, and August) and mean winter (December, January, and February) soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower

Mesic

The mean annual soil temperature is 8° C or higher but lower than 15° C, and the difference between mean summer and mean winter soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Thermic

The mean annual soil temperature is 15° C or higher but lower than 22° C, and the difference between mean summer and mean winter soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower.

Hyperthermic

The mean annual soil temperature is 22° C or higher, and the difference between mean summer and mean winter soil temperatures is more than 6° C either at a depth of 50 cm from the soil surface or at a densic, lithic, or paralithic contact, whichever is shallower. If the name of a soil temperature regime has the prefix iso, the mean summer and mean winter soil temperatures differ by less than 6° C at a depth of 50 cm or at a densic, lithic, or paralithic contact, whichever is shallower.

Isofrigid

The mean annual soil temperature is lower than 8° C.

Isomesic

The mean annual soil temperature is 8° C or higher but lower than 15° C.

Isothermic

The mean annual soil temperature is 15° C or higher but lower than 22° C.

Isohyperthermic

The mean annual soil temperature is 22° C or higher.