Rare Cooling Degree Days in December

Cooling Degree Days? Perhaps you have heard of it. Heating Degree Days and Cooling Degree Days are useful in helping to determine energy demands along with other factors when moving to another location.

It is based on a standard numerical value of 65 degrees. A high temperature and low temperature are added together and divided by two for the day's average temperature. If the average temperature is < 65, then a heating degree day is computed based on the difference between the standard and the actual average.

For example, if the day's high temperature is 60 and the day's low temperature is 40, then the average is 50 (60 + 40 = 100; divide by 2 = 50). Since the average temperature of 50 is less than 65, heating degree days are applied, in this case, 15 Heating Degree Days.

But, in Louisville, the day's high temperature was 73 degrees. Let's assume that the low temperature for today will be 65 degrees, since today has not officially ended until midnight. It was actually 66 early this morning but may push slightly lower just before midnight soon.

If we add 73 and 65, this equals 138; divide by 2 and we get a 69 degree average. Since the average value is higher than 65, cooling degree days are applied, in this case, 4 Cooling Degree Days.

It is very rare to obtain an average temperature of greater than 65 degrees in Kentucky during the month of December. Louisville typically averages 1 Cooling Degree Day for the month. But most years, we simply record zero.

The last December we recorded Cooling Degree Days was in 2012, also 4 CDD's. Since 2000, including the year 2012, during the past 15 years, we in Louisville have recorded a total of 5 CDD's, not counting any CDD's that will be computed by tomorrow morning.

Sunday, it is possible we may record a very rare second consecutive day of Cooling Degree Days for the month of December.

The record for CDD's in Louisville for December is 8, set in 1982, yes another El Nino year. Two consecutive days of average temperatures greater than 65 degrees were achieved.

I do not know if Louisville will be able to tie or break the mark set in 1982, but it will be close.

Another record that may be in jeopardy is the all-time high temperature for December here in Louisville, which is 76. It would be quite coincidental if Louisville should tie the all-time high temperature and the all-time Cooling Degree Days records on the same day in December.

MS