I see people using percentage increases to talk about temperature; for example
"Two weather predictions were presented with somewhat conflicting data, with the Energy Information Administration (EIA) predicting the 2007-08 winter to be 4 percent colder than 2006-07, but still 2 percent warmer than the 30-year average."
Is there any meaningful way to interpret this? I don't know what it means for one year to be 4 percent colder than another.
On
A sensible reading is in degree-days, the heating requirement of the winter. You integrate the temperature difference between the outside and some desired temperature, like 60-65F (which is assumed to be an outside temperature at which buildings do not need to be heated), over the winter. You can then compare these values between years reasonably with percentages. If this is what they mean, it is not well expressed.
This kind of talk makes no sense except in degrees above absolute zero. $64^\circ$ degrees Fahrenheit is not twice as warm as $32^\circ$ Fahrenheit. If it were, then $17.777\ldots^\circ$ Celcius (i.e. $64^\circ$ Fahrenheit) would be twice as warm as $0^\circ$ Celcius (i.e. $32^\circ$ Fahrenheit).
$4\%$ cooler would mean around $18^\circ$ or $20^\circ$ cooler a the Fahrenheit scale. I hesitate believe that two winters differ by that much from each other in average temperature. More likely it's just someone who flunked math playing fast and loose with language.