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Generally express in figures, no space before or period after C or F; spell out the expressions "below zero" and "minus" except in conversions.  Use comma only with five digits or more:
              8°C (46°F)
            minus 102°C (-216°F)
            102° below zero Celsius
            It was 85°.
0°C (32°F) or zero degrees Celsius
20 degrees below zero Fahrenheit or 20° below zero Fahrenheit
2500°C, but 2,500 degrees

In scientific articles usually express temperatures in degrees Celsius or in kelvin, with Fahrenheit in parentheses.  In a more informal context Fahrenheit is acceptable.  Do not use the term centigrade.

C is the abbreviation for Celsius, cap C, no space or period.  When writing out, use degrees with zero or values greater than one:
            23 degrees Celsius, zero degrees Celsius; minus 4°C or -4°C

F is the abbreviation for Fahrenheit:
            32°F (no spaces, no period); 0°C (32°F).

In a temperature written with a degree symbol, use a comma only with five digits or more.

66°C (149°F)
minus 3°C (27°F)
Water boils at 100° Celsius
in the 70s
0°C (32°F) or zero degrees Celsius
105° heat or 105-degree heat

2,300 degrees
below zero (spell out)
minus 20°C (-4°F)
Do not use degree or degree symbol with kelvin: 3K or 3 kelvins.

Be careful when converting from one scale to another to distinguish between an actual temperature and a range of degrees:
            For instance, a temperature of 32 degrees Fahrenheit is equivalent
            to a temperature of zero degrees Celsius, but a change of 32 degrees Fahrenheit
            (say, from 10° to 42°F) equals a change of 17.7 degrees Celsius
            (one degree C equals 1.8 degrees F).