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# Def of "Degree Day" , please

#1
02-28-14, 02:28 PM
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Join Date: Feb 2002
Location: port chester n y
Posts: 2,117
Def of "Degree Day" , please

A clear definition of the meaning of "Degree Day" is much appreciated. The NYT's daily weather report gives this value which is 65 degrees minus the mean temperature,

The value of yesterday's " Degree Days" was 39 which equals 65 - 26 , 26 = the mean temperature , a "hi" of 31 and a "lo" of 20 .

The total value of Degree-Days to date ( Feb 28 ) for Feb. = 874 / 28 = 31 , 31, which , as best I know, tells me for Feb, the mean daily temperature was 31 degrees lower than 65 .

#2
02-28-14, 03:52 PM
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Join Date: Oct 2006
Location: USA
Posts: 23,539
It's kinda confusing actually... but let me try.

One can select any 'base temperature' they wish, but 65F has been used for many years and still used by most. This represents the temperature above which a typical home will need no added heat input. A trend is moving toward a lower base temperature due to improvements in building technology that mean a typical home might not need added heat below say 60F.

We'll still use 65F as base temperature.

The way to arrive at DD is to take the HIGH and the LOW temp for the day and average it (divide by 2), and then SUBTRACT that result from the BASE TEMPERATURE.

Say the high is 40F and the low is 20F. That averages to 30F. Subtracted from base of 65 one comes up with 35 Degree Days for that particular day.

Degree days are cumulative over the course of a season.

Keep in mind that we are talking about HEATING degree days.

There are also COOLING degree days, and GROWING degree days. Basically these use different base temperatures and the base is subtracted from the average as opposed to the average subtracted from the base.

Heating & Cooling Degree Days - Free Worldwide Data Calculation is a neat site.

Uses data from Weather Forecast & Reports - Long Range & Local | Wunderground | Weather Underground and will collate the data into an .XLS spreadsheet form that you can download. Check it out.

Oil delivery companies (probably propane too) use Degree Days to time deliveries to their 'automatic delivery' customers.

They collect usage data over a period of time and correlate it with DD data that they collect and develop what they call a " K Factor ", which is in the form of " Degree Days per Gallon ".

Many companies use a "Johnson..." to collect the DD data and it automatically uploads to the delivery software program that prints daily delivery schedules.

The Daywatcher Degree-Day Logging System, Made by Johnson Degree Day

Over time they learn that customer X's home has a K factor of say 8.5 and that they want to deliver 150 Gallons at a time. They know that when 1275 Degree Days have occurred since X's last delivery, it's time to send out the truck. It's a remarkably consistent method! More accurate than one would imagine.

#3
02-28-14, 11:34 PM
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Join Date: Jan 2006
Location: San Jose
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