The resistance of a typical conducting wire is low when temperature is low and high when temperature is high. The increase in resistance alongside temperature is due to an increase in energy of the wire atoms, which cause them to vibrate more and impede the path of the electrons flowing through.
In other words as temperature decreases then so does the ohm reading (decreasing resistance). That said, if you are preparing your car in in the comfort of your 80 degree home in the northeast and then go to The Fray where the temperature can be anywhere from 60 to 70 degrees, you ohm reading will be less. You might say big deal... however if your armature ohmed out at 16.0 ohms at 80 degrees it will ohm out lower at 60 or 70 degrees and will therefore be illegal according to the rules.