Here it is: a graph I was hoping to find on the 'net that shows the
temperature ranges and dewpoint/humidites at which icing can occur.
Note that the range extends from well below freezing to over 100
degrees F. This graph will apply to avgas; mogas has a higher
volatility (evaporation rate) and can cause icing outside these
parameters.

http://www.wsaa.net/icing.htm

Icing can also occur in very cold temps if carb heat IS used: ice
crystals in the air can melt as the incoming air is heated, collect on
the throttle plate and other parts, and freeze due to the pressure drop
and evaporative cooling.

Dan