"The freezers hover at –30 degrees Celsius—cold enough to make my pen dry up and to require that workers wear protective jumpsuits, hats, gloves and face masks."
Context helps. That dash means minus, also known as negative. Freezers start working for water below 0 degrees Celsius, also known as 0 degrees Centigrade, or 32 degrees Fahrenheit.
The dash used to seaparte, e.g., –30 degrees Celsius—, fooled me as I did not read it as a minus sign. I wish I had finished my degree in Thermal Dynamics.