What indicates that the temperature is at or below zero Celsius in a METAR report?

The indicator for a temperature at or below zero Celsius in a METAR report is the negative sign that precedes the temperature value. METAR reports present temperatures in degrees Celsius, and any temperature expressed with a negative sign signifies values below freezing. For example, a temperature reading of "-5" indicates that the temperature is minus five degrees Celsius, clearly indicating that it is below zero.

Other choices do not directly indicate temperature. Mentions of frost or ice remarks can suggest low temperatures but do not provide a specific measurement. Lowercase letters are not used in METAR reports to signify temperature; rather, they serve different purposes in aviation context. Thus, the absolute presence of the negative sign is the definitive way to know the temperature is at or below zero Celsius.