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I am not sure how well you know about cars. We all have very limited knowledge of them. So don't be offended if I offer my limited knowledge :).
You didn't explain "cold, or warm", did you mean the weather? or the engine temperature? There is a big a mount of coolant inside your radiator. When then engine is hot, the coolant circles, to let the heat out. When the engine is cold ( when you first start the car), then big amount of coolant is cut off--no circlation. So the engine temperature stays the same...therefore the needle stays at the middle all the time. Hope this helps.