Complex sentence: Negative forty is the unique temperature (in degrees) at which the Celsius and Fahrenheit scales read the same; it is often referred to as 40 below, which is short for "40 degrees below zero".
Split into simpler sentences: Negative forty is the unique temperature (in degrees) at which the Celsius and Fahrenheit scales read the same. It is often referred to as 40 below, which is short for "40 degrees below zero".