Ken,

Visible moisture is not a necessary condition for Carb ice. Carb ice can occur in clear conditions - even very clear conditions.  The amount of moisture in the air will only affect the rate at which ice will form, and not whether it will form.  If the throat temperature in the carburator decreases below 32 degrees F ice will form at much lower relative humidities than required for visible moisture.  The higher the temperature the more moisture the air can hold.  So there is more water vapor per cubic foot of air at 70 degrees F and 75% relative humidty than at 35 degress F at 99% relative humidty.  (Don't quote me on the numbers.  I didn't check a psychometric chart, but you get the idea.)

The throat temperature decrease is due to the cooling caused by fuel evaporation and the decrease in air pressure as the air accelerates through the throat.  I would say the factors effecting these conditions are the geometry of the carborator, mixture setting and efficiency of the carborator jets to atomize the fuel.  There is an outside air temperature at which ice won't form for a given engine e.g. above maybe 85-90 degrees F.  At very low outside temperatures say for example 0-10 degrees F, there is not enough moisture in the air to matter.

my two cents