Frost or freezing occurs when the temperature of air falls below the freezing point of water (0 °C, 32 °F, 273.15 K).This is usually measured at the height of 1.2 m above the ground surface. There is a rather subjective scale to show several degrees of frost severity: 1  slight frost: 0 to −3.5 °C (32.0 to 25.7 °F).2  moderate frost: −3.6 to −6.5 °C (25.6 to 20.3 °F). 3  severe frost: −6.6 to −11.5 °C (20.2 to 11.3 °F).here is a rather subjective scale to show several degrees of frost severity: 1  slight frost: 0 to −3.5 °C (32.0 to 25.7 °F). 2  moderate frost: −3.6 to −6.5 °C (25.6 to 20.3 °F). 3  severe frost: −6.6 to −11.5 °C (20.2 to 11.3 °F).