Both the Celsius and Fahrenheit scales include temperatures that are below zero degrees. However, the 0 degree point on the two scales do not line up on a 1to1 ratio, so some temperatures are negative in Celsius but positive in Fahrenheit.
How Do Fahrenheit and Celsius Match Up?

On the Celsius scale, water freezes at 0 degrees  in Fahrenheit, water freezes at 32 degrees. Because of this, all temperatures between 0 and 32 degrees Fahrenheit are positive on the Fahrenheit scale and negative in Celsius. Zero degrees Fahrenheit lines up with 17.778 degrees Celsius, so all temperatures below 17.778 degrees Celsius are negative in both Celsius and Fahrenheit. Also, at 40 degrees Celsius and Fahrenheit the temperatures match up. So, 40 degrees Celsius and 40 degrees Fahrenheit are the same temperature.
Converting From Celsius to Fahrenheit

To convert from degrees Celsius – C  to degrees Fahrenheit – F  use the following equation:
F = 1.8 x C + 32
So, to convert a temperature from Celsius to Fahrenheit, you multiply it by 1.8, then add 32. Say you want to find what 10 degrees C is in Fahrenheit. First, multiply 10 by 1.8. Remember that a negative number multiplied by a positive number is a negative number.
10 x 1.8 = 18
Add 32 to 18 to get the temperature in Fahrenheit:
18 + 32 = 14
So, 10 degrees C is 14 degrees F.
Convert From Fahrenheit to Celsius

You can also convert a negative Fahrenheit temperature to a Celsius temperature.
To convert Fahrenheit to Celsius, use the following formula:
C = (F  32)/1.8
For instance, if you wanted to convert 45 degrees Fahrenheit to Celsius, first subtract 32 from 45. This results in in 77. Then, divide 77 by 1.8:
C = 77/1.8
C = 42.78So, 45 degrees Fahrenheit are 42.78 degrees Celsius.
References
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