If you’ve ever stopped to ponder why exactly the freezing point on the
Fahrenheit scale is set at 32 degrees–a rather arbitrary number, all
things considered–you’ll need to ponder all the way back to the 18th
century.
It was then, in the 1720s, that Daniel Gabriel Fahrenheit was ironing
out zero point for his temperature scale. The coldest temperature that
Fahrenheit could produce in his laboratory was the result of mixing ice,
water, and ammonia chloride to make a frigid slurry. This measurement
became the zero-point for the scale. The second point of reference
was the temperature of water when chilled to the point that ice began
to form–pure water forming ice was assigned to 32 degrees. Fahrenheit
later assigned 212 degrees as the value for boiling water because the
180 degree difference between the two is 180 degrees because 180
can be easily and evenly divided by a variety of numbers.
After a long period of metrification starting in the 1960s, the only
countries that still use Fahrenheit for conventional measurements are
the United States, Belize, and the British territory of the Cayman
Islands.
http://www.howtogeek.com/trivia/the-zero-point-on-the-fahrenheit-scale-is-based-on-what/