Ice day
Temperature & humidity
On an ice day, air temperature remains below 0 °C throughout the day. The daily maximum defines it, not the presence of ice.
Not even the maximum reaches zero
An ice day occurs when the highest measured air temperature of the day remains below 0 °C. A maximum of exactly 0.0 °C does not qualify. The term describes temperature, not an ice cover.
How it differs from a frost day
A frost day only requires a minimum below zero. A day with −5 °C at night and +3 °C in the afternoon is a frost day but not an ice day. If its maximum stays at −1 °C, it meets both definitions.
Location and reference period
The frequency of ice days is evaluated for a particular weather station and reference period. An individual winter can differ substantially from the long-term mean. A station record does not automatically describe every courtyard and street.
Not a certificate for a frozen lake
Even several ice days do not reliably establish ice thickness. Water temperature, currents, snow cover and the preceding weeks matter. A weather statistic cannot replace local approval that an ice surface is safe to use.