Greg Harvey, age 16, of Hattiesburg, Miss., for his question:

WHAT IS A DEGREE DAY?

A degree day is a unit of measure that is used in  estimating heat requirements in buildings and fuel costs. The degree day is based on the Fahrenheit temperature scale and is essentially a measurement of the difference between a standard indoor temperature and the lower outside temperature.

One degree day corresponds to a difference of one degree Fahrenheit between the outside mean temperature and a given reference temperature, which is taken as 65 degrees Fahrenheit for heating calculations.

Hence, the winter day with a minimum temperature of 30 degrees Fahrenheit and a maximum temperature of 50 degrees Fahrenheit, or a mean temperature of 40 degrees Fahrenheit, would count as 25 degree days.

For a given building, annual heating costs are directly proportional to the total number of degree days in the year.  New York City has about 5,300 degree days during the total heating season, Chicago has about 6,300 and New Orleans only 1,200.

Degree days are also used to evaluate air conditioning costs, with a reference temperature of 75 degrees Fahrenheit used to determine cooling loads. Temperatures above rather than below are used.