Answer:

-13 is the warmer temperature.
Explanation:
When comparing negative numbers, the number closer to zero on the number line is greater.
(OR, the negative number that, if it was positive, would be less).
In this case, that number is -13.
We now know that -13 is greater than -21, so we can write this inequality:

In regards to temperature, greater numbers usually mean hotter temperature, so -13 is the warmer temperature.