Temperature increase and decrease

When the weather changes the temperature can go up or down. If a temperature goes up, gets warmer, it is a temperature increase. If a temperature goes down, gets colder, it is a temperature decrease.

Example:
During the summer when you wake up the morning temperature could be 3°C. By lunchtime it is up to 10°C.

In the early afternoon the temperature is 15°C. By early evening the temperature goes down to 12°C.

But what happens if we have negative temperatures?

Example:
During winter the early morning temperature could be -3°C. By lunchtime it is up to 6°C.
From -3°C to 0°C is 3°C.
From 0°C to 6°C is a further 6°C.
The total of 3°C + 6°C makes 9°C.

In the early afternoon the temperature is 3°C. By early evening the temperature goes down to -5°C.
From 3°C to 0°C is 3°C.
From 0°C to -5°C is a further 5°C.
The total of 3°C + 5°C makes 8°C.