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 1-to-1 ratio, so some temperatures are negative in Celsius but positive in Fahrenheit.

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.

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.

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 = 42.78

So, -45 degrees Fahrenheit are -42.78 degrees Celsius.