You may think that this is the first time you have encountered directed numbers but really, when you think about it, they are not new. Think of a thermometer. In Moscow the temperature is -20^o Celsius while in Melbourne it is 17^o Celsius. Which city has the highest temperature?

Look at the animation above. The temperature starts at 0 then drops 4 degrees to -4. It drops again by another 4 degrees to -8. So therefore -8 is lower than -4. The temperature then rises by 10 degrees to +2.

We can say this mathematically by the equation below (0 - 4 - 4 + 10 = +2). For a fall in temperature we subtract and for a rise we add.

Try these exercises.
The temperature in the morning is 0 degrees but quickly falls by 8 degrees. Just before lunch, the temperature rises by 6 degrees due to a warm wind but falls by 2 degrees before sunset.
Weat is the temperature at night?
Is it higher than 0 degrees?
Write a mathematical expression for this.
Solution

The temperature outside is 12 degrees. A cold wind causes the temperature to fall by 14 degrees. As the clouds part and the sun starts to shine the temperature rises 2 degrees and remains constant until night time.
What is the temperature at night?
Is it higher than 0 degrees?
Write a mathematical expression for this.
Solution