How can temperatures be calculated with each other?
If I pour half a glass of hot water (50°C) into a cold one (10°C) will the new temperature of the water be around 30°C?

Furthermore, if outside temperatures are 19°C and they'd change to 20°C how much warmer did it really become? Can we say, that it became one nineteenieth warmer? That doesn't make much sense to me, since -100°C is still technically warm and maybe you get what I am trying to say.