ORIGINAL: gubbs3
For most small block engines (.12-.18) and hot or medium plug is usually the best choice. Both of those work very well with 20% nitro in most enignes. You don't need more nitro than that because the benefits are limited compared to a larger list of negatives. With 20%, a hot plug is usually a good choice.
On the cold weather thing:
Cold air is more dense than warm air, this requires a richer needle setting to maintain the correct fuel/air mixture. With that being said, in every stroke you are getting a more power packed charge in the cylinder. This does not warrant using a hotter plug. A hotter plug will increase your engine's timing which is already elevated due to the dense air. If any changes are to be made, a colder plug would be a better choice to retard the timing.
The opposite is true for hot and humid weather. Hot air is less dense than cold, and humid air is less dense than dry. This means leaner needle settings must be used to maintain the correct fuel/air mixture and with the light air, less compression would retard the timing, requiring a hotter plug.
absolutely true....
in general you want to run the hottest plug possible without causing the engine to detonate.
for.12-.18 engines you want to use a hot plug or medium plug for higher nitro.
However, I do use a hot plug and 30% nitro in my big block engines with no problems. This is because I've plenty of head clearance and do not have to worry as much about detonation.
When you run a hotter plug, you're actually advancing the timing like gubbs said.
If you run too cold of a plug, you will find that you'll have to tune the engine leaner in order to produce the same amount of power. This is why you want a hotter plug because as most people know, running an engine lean is not good for lubrication.