meaning - What "subzero temperature" mean? Is it below zero F or below zero C?

What "subzero temperature" mean? Is it below zero F or below zero C? Is it different from "freezing"?

Answer

When dealing with a weather forecast in the U.S., subzero means below 0°F. It's a convenient way for the weatherman to say, "It's going to be dangerously cold!"

In a similar way, on a warmer day, you might hear a forecaster saying things like, "with lows in the teens" or "with highs in the upper 20s."

However, at a scientific conference, if I heard a researcher say, "The ion-doped material performed better in subzero temperatures," I'd probably assume she was talking about temperatures below 0°C, since scientists often use Celsius temperature scales, and because that's such a key benchmark temperature on the Celsius scale.