From “Supercool: Water Doesn’t Have to Freeze Until -48 C (-55 F)” (ScienceDaily, Nov. 23, 2011), we learn: 48 degrees below zero Celsius (minus 55 Fahrenheit):
Supercooled liquid water must become ice at minus 48 C (minus 55 F) not just because of the extreme cold, but because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others, according to the new study by chemists Valeria Molinero and Emily Moore.
… in the strange and wacky world of water, tiny amounts of liquid water theoretically still might be present even as temperatures plunge below minus 48 C (minus 55 F) and almost all the water has turned solid — either into crystalline ice or amorphous water “glass,” Molinero says. But any remaining liquid water can survive only an incredibly short time — too short for the liquid’s properties to be detected or measured.
Interesting discussion of supercooling and the strange properties of water.