Wayne B :

For example, a 12 ft length of #4 cable carrying 100 amps dissipates
about 35 watts total, approximately 3 watts per foot. That causes a
certain amount of heating. Assuming free space and decent air flow,
how warm is the cable likely to get at 100 degrees F ambient
temperature?

You might start by reading

http://en.wikipedia.org/wiki/Heat_transfer_coefficient

, but they don't give usable coefficient values.