Ruuvi sensors are recommended to be used within -20°C to +65°C. Maximum operating temperature for electronics and enclosure must stay within -40°C to +85°C temperature range.
The standard battery shipped with the device works from -20°C to +65°C but you can shop for an extended range battery.
Ruuvi has to be used with the included waterproof enclosure to prevent water or any other conductive element from getting in contact with the sensitive electronic components.
Important notice: do not place the Ruuvi in direct contact with water.