As the temperature drops, the cranking power required by the car increases. However, as more cranking power is used, the amount of battery power available decreases.
Cold Cranking Amps (CCA) is critical for good cranking ability. It refers to the number of amps a battery can support for 30 seconds at 0°F until the battery voltage drops to unusable levels. For example, a 12 volt battery with 600 CCAs means the battery will provide 600 amps for 30 seconds at 0°F before the voltage falls to 7.20 volts (six cells). The higher the CCA, the more powerful the cranking ability.
If you live in a cold climate, you should consider the CCA rating when choosing a battery. If you live in a very hot climate, you don't need as much CCA.