Degree Days: Application of Surface Temperature

I wrote the following blog post last week in the American Meteorological Society community forum, explaining one of the terms in the official AMS Glossary. The post went over a general overview of degree days, and also put it into context with the recent Cherry Blossom blooms in Washington, D.C. I have added more information near the bottom regarding latest heating degree day statistics for the 2018-2019 season.


According to the AMS Glossary of Meteorology, a degree-day is defined as:

Generally, a measure of the departure of the mean daily temperature from a given standard: one degree-day for each degree (°C or °F) of departure above (or below) the standard during one day.

Let's say for example the average temperature for a given day was reported as 70 degrees Fahrenheit. If we say the standard temperature (or reference temperature) for that day is 65°F, then this corresponds to 5 total degree-days above the standard. This also works the other way around, as a report of 60°F would also have 5 total degree-days below the standard.

Individual degree-days are good to assess how "warm" or "cold" it has been according to some reference, but they are more effective if they are aggregated over a particular season. By doing this, users of the data are able to assess how a particular season is going so far, or can even be compared against other seasons. Numerous sectors of the weather, water, and climate enterprise have used seasonal degree-days in some fashion, including agriculture, energy, and infrastructure.

Typically, there are three types of degree-days: heatingcooling, and growing. Each comprise of the same mathematical concept (departure from a given reference), but are applied slightly different. A heating degree-day is defined as the number of degrees below a temperature reference (which is typically set at 65°F). When the average temperature for a day is below 65, a degree-day is calculated. If the temperature is at or above 65, it is given a value of 0. A cooling degree-day is the opposite, as days above 65 degrees are aggregated, and anything below is 0.

Energy companies are really interested in heating and cooling degree-days, as they can relate to the amount of power that is consumed by their customers. For really cold and warm days, the public are more inclined to run their heat and air conditioner systems, respectively. This can lead to a higher demand of electricity, and in the end, higher costs for both the energy companies, as well as the consumers. Below are examples of winter heating degree-days (blues) and summer cooling degree-days (reds), using a reference temperature of 65°F. Here, we see that during the summer of 2018, there were noticeably more cooling degree-days in Southern Texas, as well as parts of California, Nevada, and Arizona. During the 2018-2019 winter, parts of North Dakota and Northern Minnesota saw a high amount of heating degree-days. Energy companies can use this information to look for hot spots (or cold spots) across the country, and be better prepared for the demand.

While heating and cooling degree-days are important for energy consumption, growing degree-days are related to the development of of plants, insects, and disease organisms. Members of the agricultural and health sectors are interested in this metric. While similar to heating degree-days (number of degrees above a standard), growing degree-days usually has a standard reference temperature different than 65°F. In fact, numerous reference temperatures can be used, depending on the type of organism being analyzed. For example, cool season plants (canning pea, spring wheat, etc.) have a base temperature of 40°F, and warm season plants (sweet corn, green bean, etc.) have a base temperature of 50°F. Therefore, it is important to understand the reference temperature used before a growing degree-day is calculated.

A relevant example is the Prunus serrulata, also known as the Japanese Flowering Cherry. According to a report from the University of New Hampshire, this flower begins blooming at 50 growing degree-days (using a reference temperature of 50°F), and reaches full bloom at 100 growing degree-days. Washington D.C is known for its Cherry Blossom Festival and spectacular views of cherry blossoms in Late March and Early April. Using temperature data from the nearby station at Reagan Airport, we can take a look at the growing season so far, and compare it to all the other years in its period of record. At the time of writing, data was available up to the 1st of April, 2019, and while the growing season has just begun, there were already 75 growing degree-days for the year 2019. This indicates the cherry blossom season has just begun in the area, and is nearly reaching a full blooming period.

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Winter 2018-2019 Heating Degree Day Statistics

Instead of providing the top five heating degree day locations, we will instead show the stations that have the highest degree days percentage from normal, meaning that percent values had colder than normal season.

  • 136.8% - Chadron, Nebraska

  • 129.2% - Farson, Wyoming

  • 123.8 %- Great Bend, Kansas

  • 122.5% - Spearfish, South Dakota

  • 122.2% - Edgemont, South Dakota

The leading station, Chadron, Nebraska, has had 6259 heating degree days since October 1st, putting it at 136.8% above normal. According to the chart below, it might mean that the season isn’t done yet!

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