[Polar Bear]

There may be a scientific or meteorological answer, but a simple multiplication of a temperature in degrees (Fahrenheit or Celsius) is not it. These temperature scales are just a numeric tool invented by man to help describe heat and cold.

You could possibly use the Kelvin scale, where 0 degrees is absolute zero...total absence of motion. There are no negative Kelvin temperatures. Temperature on the Kelvin scale is sort of a measure of energy (I'm no expert), so halving the Kelvin temperature could possibly be construed as twice as cold.