[1] Rime (both soft and hard) is technically not a type of ground frost.
Ground frost may also refer to the condition when the temperature of the upper layer of the soil falls below the freezing point of water.
[1] From 1906 to 1960 the Met Office practice was to base the number of days of ground frost on this criterion: a day with a minimum temperature reaching 30 °F (−1 °C), probably because 32 °F (0 °C) was not considered enough cold to cause damage to growing plants.
Since 1961 the statistics have referred to the number of days with grass minimum temperature below 0°C.
Occasionally, the term ground frost can still be seen, but it means simply a minimum temperature below 0 °C.