In your head C - F conversion double it (the C temp) subtract 10% add 32 and you get the F temp. -40 C X2 -> -80 - 10% -> -80 -(-8) -> -72 + 32 -72 + 32 = -40 F
You should have NO problems with them. Sandisk rates their Ultra II cards at operating temps of 0 to 70° C (32 to 158° F) and non-operating temps of -25 to 85° C (-13 to +185°F). I have never had any problems with mine, and I am sure their temp ratings are very conservative.