Dangerous rule that, Once you've got it runing nicely at a medium ambient temp think about plugs rather than twidling screws. If you keep leaning out, you WILL blow the lump. Plugs are rated for fuels and ambient temps, better to keep the lubrication constant and just get a plug of the correct temp range, so if it's cold out, you need a plug below #5.

The general system is low number eg #3 = hot plug and high number eg #8 is a cold plug. It all relates to the heat the filament retains after ignition so that it can ignite the next ignition, so obviously on a cold day the filament needs to hold more heat to ignite the colder mixture. There is the denser air arguement but that is irrelevant.