Dr. Thomas Snow was the husband of Dr. Carla Tannhauser and the father of Caitlin Snow. He was believed to have died when Caitlin was young from ALS. Years later however, it was discovered by Ralph Dibny that Thomas' death certificate was a fake.
Thomas was very close to his daughter and they would watch old holiday movies together at Christmas on snowy nights. He furthered Caitlin's interest in science and the two partook in many educational activities and games together, including looking at the constellations through their homemade telescope and and studying bugs from the backyard. While teaching Caitlin the periodic table, Thomas and his daughter often played a game in which they created their own set of elements.
At some point when Caitlin was a child, Thomas took her out for a ride on her bicycle. However, Caitlin fell off her bike in front of an approaching car and Thomas rushed to her aid. Upon seeing his daughter's transformation into Killer Frost, he tried to prevent Caitlin from seeing her reflection.
Thomas eventually began suffering from multiple sclerosis, though he often tried to hide his symptoms from his family, not wanting them to worry. Carla tried to cure her husband as he fought the disease for years. However, he apparently died from ALS, much to Caitlin and Carla's heartache.
After Thomas' supposed "death", Carla buried herself in her work in an attempt to cope with the pain, neglecting Caitlin in the process. As a result, Caitlin and Carla became estranged from one another for years.
In May 2018, Ralph Dibny discovered that Cameron Makhent, the judge who signed Thomas's death certificate, didn't exist, casting doubts on the credibility of his death. Cisco used his vision powers on Caitlin and the death certificate to discover who forged it, and Caitlin saw her mother signing it.
After some investigating, Caitlin uncovered several mysterious documents detailing her father's work in non-existent elements and an apparent suicide note from her mother's company file room. However, after analyzing the molecular structures of the elements and comparing them with her childhood periodic table, Caitlin deciphered a message from her father which read "CAITLIN COME FIND ME".
Thomas was a very loving, supportive husband and father. He encouraged Caitlin's interest in science and always pushed her to be the best she could be.