Answer
$65^{\circ}C$.
Work Step by Step
Adiabatic compression heats the air in the bag by about 10 Celsius degrees for each kilometer decrease in altitude. The 10 km decrease in altitude means the $-35^{\circ}C$ air is heated by $100 C^{\circ}$ to $65^{\circ}C$.
This is discussed on page 341 and in Figure 18.6.