Forty degrees below is where Fahrenheit and Celsius meet. Nowhere else are they equal. For instance, one hundred degrees Fahrenheit is about thirty-eight degrees Celsius. Going negative, one hundred degrees below zero on the Fahrenheit scale is approximately negative seventy-three degrees when we're talking Celsius. To further complicate things there's another scale known as Kelvin. Zero on the Kelvin scale is also known as absolute zero and expressed in terms of the other two scales, this would be -459.67F or -273.15C. Interestingly enough, two-hundred degrees Celsius is 473.15K and two-hundred degrees Fahrenheit is 366.483K. Of course, forty degrees below using either the Fahrenheit or Celsius scales equals the same in Kelvin and that would be 233.15K. When it comes to everyday usage, expressing temperature on the Kelvin scale would obviously be somewhat inconvenient!