Here in a metric country (Australia) they use words like cold, cool, warm, hot.

Otherwise people turn to BOM (Bureau of Meteorology) hour by hour / day by day records and forecasts of temp. and rain with radar cloud coverage - these just use nearest whole Celsius numbers, eg: https://www.bom.gov.au/location/australia/western-australia/...

Almost everything you've conjectured about "how they do it in a metric country" has been wrong; builders don't use a third of a metre, for example, everything is typically in millimetres - 1200, 1600, etc (aka, 1.2 metres, 1.6 metres) along side usages of 1 mm and 5 mm for small tolerances, expansion gaps, etc.