Find the change in temperature or elevation.

$\text{From}-120\text{feet to}-90\text{feet}$.

To add two numbers with similar signs, add their absolute values and keep the similar sign with the resulting number.

To add two numbers with opposite signs, subtract the absolute value of smaller number from absolute value of larger number and keep the sign of larger number with the resulting number.

To determine the change from first number to the second number, subtract the first number from second number.

To find the change in elevation from $-120 \text{feet}$to $-90 \text{feet}$, subtract the initial height $-120 \text{feet}$from the final height $-90\text{\hspace{0.17em}feet}$and solve further as shown below

$\text{Change in elevation}=-90-\left(-120\right)$

To subtract $-120$, add its opposite $120$and simplify further as shown below

$\begin{array}{c}-90-\left(-120\right)=-90+120\\ =30\end{array}$

Thus, the change in elevation is localid="1648826417756" $30 \text{feet}$.