The given table shows the maximum and minimum temperatures in centigrades over five consecutive days in a certain city. On which day did the greatest temperature variation occur?

| Day | Max. $\left({ }^{\circ} \mathrm{C}\right)$ | Min. $\left({ }^{\circ} \mathrm{C}\right)$ |
| :---: | :---: | :---: |
| $2^{\underline{\underline{n}}}$ Tuesday | 7 | -12 |
| $3^{\underline{\underline{n}}}$ Wednesday | 0 | -11 |
| $4^{\underline{\underline{n}}}$ Thursday | -2 | -15 |
| $5^{\underline{\underline{n}}}$ Friday | 9 | -8 |
| $6^{\underline{\underline{n}}}$ Saturday | 13 | -7 |
The variation in temperature is the difference between the maximum and the minimum. We have completed the given table with the variations as follows.

| Day | Maximum temperature, in ${ }^{\circ} \mathrm{C}$ | Minimum temperature, in ${ }^{\circ} \mathrm{C}$ | Temperature variation, in ${ }^{\circ} \mathrm{C}$ |
| :---: | :---: | :---: | :---: |
| Monday | 7 | -12 | $7-(-12)=7+12=19$ |
| Tuesday | 0 | -11 | $0-(-11)=0+11=11$ |
| Wednesday | -2 | -15 | $-2-(-15)=15-2=13$ |
| Thursday | 9 | -8 | $9-(-8)=9+8=17$ |
| Friday | 13 | -7 | $13-(-7)=13+7=20$ |

Therefore, the greatest temperature variation occurred on Friday.