Pizza dough texture and workability
Yes, the heat is the difference. You get a better gluten production with a colder dough. That's a pretty basic fact of baking, there are even recipes which insist on using ice cold water for making bread. 

If you want to have the same effect in summer, you should use cold water. As cold as possible, I even tried once slush from the ice cream machine just for kicks and it indeed produced very strong gluten (actually too strong for my preferences). 

You should use the cold water on the actual dough. The sponge should use room temperature water, because ice cold water is rather unpleasant to yeast. They can live with it after they've been reawakened from their dry state, but it's not good to use it for their hydration.