In a blog I wrote last year, I explained what degree days were and how they're used. Recall that heating degree days accrue on days when the mean daily temperature is below 65 degrees Fahrenheit (F), while cooling degree days accrue on days when the mean daily temperature is above 65 F. Remember that degree days are used by energy and fuel companies to help predict energy demand and plan for the appropriate allocation of energy resources to meet that demand. However, there is more than one way to calculate the mean daily temperature, and depending on what method you use, you could end up with significantly different results.
The first method, which is perhaps the most widely accepted method for calculating the mean daily temperature is simply the average of the high and low temperature for the day. For example, on a given day at a given location, the high was 94 F and the low was 58 F. That would result in a daily mean temperature of 76 F. Subtracting 65 from 76, we get 11 cooling degree days.
A second method for calculating the mean daily temperature would involve averaging the hourly temperature readings throughout the 24-hour period. So on the same day and location, let's assume high pressure is dominating the weather conditions, resulting in a normal diurnal temperature curve (Figure 1). Using the hourly temperature data in Table 1, we end up with a mean daily temperature of 75.3 F, resulting in 10.3 cooling degree days. So there ends up being a slight difference in the daily mean temperature between the two methods assuming a normal diurnal temperature curve.
Now let's look at a day when a late afternoon cold front passes the location (Figure 2). In this scenario, the daily high and low are still 94 F and 58 F, respectively, but the temperature drops sharply following frontal passage late in the day. We would still end up with a mean daily temperature of 76 F and 11 cooling degree days by averaging the high and low temperature. Using the hourly data in Table 2 however, we end up with a mean daily temperature of 69.9 F, and 4.9 cooling degree days, which is a significant difference from the latter method for calculating the mean daily temperature.
So which one is right? It depends on who you ask. For official record-keeping, the National Weather Service uses the method of averaging the daily high and low temperature to calculate the daily mean temperature and resulting degree day values. However, most automated weather stations calculate the daily mean temperature by averaging the hourly temperature, or temperature readings at more frequent intervals based on the station's data logging configuration. I would argue that calculating the mean daily temperature by averaging the hourly temperature data will provide a more accurate result, given the finer granularity of the data.
But as evidenced in the above examples, how the daily mean is calculated is not a "six one way, half-dozen the other" kind of choice. What method you select to calculate the mean daily temperature could give you vastly different results that impact degree day calculations, and the decisions made by the users of that information.