You will always want to run a hot glow plug it is easier starting on your engine. A hot medium or cold glow plug will not make your engine run any hotter or colder the cold plug will have a thicker element on the inside wich will not it heat as hotter therefore a hot glow plug has a thinner element on the inside which will allow the glow plug to get hotter which will make your car easier to start. So there will be no need to change the glow plug because the temp has changed outside.