Climate Zone

3a equals -35 to -40 degrees F. or -.37.3 to 40.0  degrees Celsius

3b equals -30 to -35 degrees F. or -34.5 to -327.3 degrees Celsius

4a equals -25 to -30 degrees F. or -31.7 to -34.4 degrees Celsius

4b equals -20 to -25 degrees F. or -28.9 to -31.6 degrees Celsius

5a equals -15 to -20 degrees F. or -26.2 to -28.8 degrees Celsius

5b equals -10 to -15 degrees F. or -23.4 to -26.1 degrees Celsius

6a equals -5 to -10 degrees F. or -20.6 to -23.3 degrees Celsius

6b equals 0 to -5 degrees F. or -17.8 to 20.5 degrees Celsius

The above ranges of temperatures are the generally agreed upon temperatures where a plant will survive reliably through the winter.  Madison Wisconsin is a 5a but outside the city it is a 4b.