From the Apple Support Article linked below, is the following:
Recommended conditions for using and storing Apple Watch Ultra
Your Apple Watch Ultra works best in the following conditions:
- Ambient temperature while you're wearing Apple Watch Ultra: -4° to 130° F (-20° to 55° C)
- Ambient temperature while Apple Watch Ultra is charging: 32° to 95° F (0° to 35° C)
- Ambient temperature when Apple Watch Ultra is being stored: 4° to 113° F (-20° to 45° C)
- Water temperature during underwater activities with Apple Watch Ultra: 32° to 104° F (0° to 40° C)
- Relative humidity: 5% to 90%, noncondensing
Ambient temperature and other conditions can affect some features on Apple Watch Ultra and other Apple Watch models. For example, your battery might not last as long in cold temperatures. The ECG app is accurate only when used in ambient temperatures from 32° to 95° F (0° to 35° C). Heart rate measurements can be less accurate in low ambient temperatures. And in very low ambient temperatures, or in conditions where the temperature changes rapidly, the touch screen on your Apple Watch might not respond as expected.
Keep Apple Watch within acceptable operating temperatures - Apple Support