The car temperature is the measure of heat the engine is making. Overheating means a huge issue, and if the gauge points towards cold, there is not usually as much of an issue, as it occurs mostly when the engine starts in a cold environment. However, the weather should not really make the engine heat up, as the measure of heat mostly comes from the combustion and working of the engine. If the gauge is shifting very noticeably to hot when driving you may need to have it checked out. This is mostly just when the gauge moves half or all the way to hot, at which point you can feel your engine sputter or such. No need to worry if it shifts a couple millimeters in super hot weather, but halfway or more is a serious problem.