Thermal, California. For the former community in Fresno County, see Thermal, Fresno County, California. Thermal is an unincorporated community within the Coachella Valley in Riverside County, California located approximately 25 miles (40 km) southeast of Palm Springs and about 9.5 miles (15.3 km) north of the Salton Sea. The community's elevation is 138 feet (42 m) below mean sea level.