The change in temperature is basically the "distance" between the two values. And the distance between two numbers is given by their difference.
In fact, if you subtract two identical number, you get something like
[tex] 13-13 = 0 [/tex]
which makes sense in our metaphor, since it means that the distance between 13 and 13 is zero. On the other hand, if you pick something like
[tex] 26- 5 = 21 [/tex]
it makes sense again, since we're saying that 26 and 5 are 21 units apart.
So, the change in temperature is the difference between the temperature:
[tex] 8 - 20 = -12 [/tex]
This means that the two values are 12 units apart, and the minus sign means that the temperature has decreased.
So, we can say that the change in temperature is a decrease of 12 degrees.
