An abreviated answer...
In general spark plugs tips need to operate within a temp window, typically between 500-850 deg C. Within the window they self clean, burning any carbon and oil deposits which otherwise would lead to tracking and misfire. Below 500 deg a plug will foul, above 850 deg a plug pre-ignites and starts to melt.
A colder, harder, plug typically has a shorter insulator surrounding the centre electrode which conducts heat away from the plug tip more effectively than a hotter plug.
A number of factors influence the operating temp of an engine, in particular revs, load, whether lean or rich, cylinder cooling, etc. So more cooling (colder weather), lower revs, lower load, richer mixture needs a hotter plug. Hotter weather, higher revs, higher load, leaner mix go for the colder plug.
Hope this helps...
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