What Are Degree Days & How to Use Them?

Outside air temperature is helpful, but it does not always help corporations and other organizations know how to utilize their resources effectively, especially as a simple high or low temperature does not automatically translate to energy usage or agricultural output. In these circumstances, it’s possible to use degree days to translate these averages into applicable, evidence-based metrics.

A degree day is a vital measurement of outside air temperature with numerous applications, including identifying energy consumption, creating agricultural plans, or tracking environmental changes. 

By subtracting the outside temperature from a given base temperature or vice versa, a degree day creates a rough estimate of heating and cooling needs to identify energy demand. There are other options based on the specific needs of the organization, whether that is to assess crop growth or identify changes in ice production.

What Is a Degree Day?

A degree day is the difference between the daily mean temperature and the base temperature, measuring how hot or cold the weather has been over a 24-hour period. 

The mean temperature is calculated by creating an average temperature — you will add the daily high and daily low together, then divide them by two. This number is then compared to a baseline temperature to get the degree day. Either the base temperature is subtracted from the average or the average is subtracted from the base, depending on which measurement you are using. 

For most purposes, this measurement is used to determine energy needs, and the degree day is considered roughly proportional to the temperature at which a building does not need heating or cooling. 

Degree day information should be directly proportional to the energy needed to heat or cool buildings. If it is not, then this represents excessive energy consumption and the need to improve energy efficiency. Degree days are one of the approximation methods used to supplement detailed temperature records, helping to give an accurate idea of the local climate and its variations throughout the year. 

Types of Degree Days

There are five different types of degree days:

  • Heating degree days
  • Cooling degree days
  • Growing degree days
  • Freezing degree days
  • Thawing degree days

Heating and cooling degree days are those most closely related to energy consumption, while the three other calculations have specialized uses for agriculture and climatology. 

Degree days differ in what base temperature they use. Degree days calculations for energy usage will apply a “human” baseline of 65°F (18°C), which is generally considered to be a comfortable temperature for a climate-controlled building. Degree day calculations for climatological or agricultural purposes use the freezing point as the base.

All these degrees days are calculated using weather data from Visual Crossing.

Heating Degree Days (HDD)

Heating degree days identify low temperatures, helping ascertain how much energy is needed to keep a building heated to an ideal temperature of 65 degrees Fahrenheit or 18 degrees Celsius. This can help energy companies such as natural gas companies or electric companies assess demand on a given day, as a larger number of heating degree days represents that more energy will have to be outputted.

Individual companies and homeowners can also use degree day information to scrutinize their energy consumption. If there have been only a few heating degree days in a month but their energy bill is high, this means they may need more insulation to keep the internal air temperature comfortable.

To calculate heating degree days, you need the daily high and daily low for the location you want to assess. The calculation is as follows:

65 − [(Daily High + Daily Low) / 2] = Heating Degree Day

As an example, assume the daily high is 52°F and the daily low is 32°F. The calculation would then be:

65 − [(52 + 32) / 2] = 23 Heating Degree Days

The higher the degree day, the more energy is needed to counteract the outside temperature.

To calculate in Celsius, you can replace the value 65 with 18.

Cooling Degree Days (CDD)

Cooling degree days do the opposite of heating degree days — they tell you how much energy is needed to cool buildings to a comfortable 65 Fahrenheit. However, the calculation uses the same base temperature and also utilizes the mean temperature. 

How to Calculate CDD

The calculation for cooling degree days subtracts 65°F from the average temperature, unlike heating degree days. It is as follows:

[(Daily High + Daily Low) / 2] − 65 = Cooling Degree Day

To look at this, assume the daily high is 95°F and the daily low is 82°F. The cooling degree day would be:

[(92 + 80) / 2] − 65 = 21 Cooling Degree Days

As with HDD, if you have a very high CDD, this indicates that you need far more energy than usual to keep the building comfortable for residents. 

Growing Degree Days (GDD)

Growing degree days represent a more sophisticated way to measure the factors influencing a plant’s growth than just a note about how long it has been since the seedling was planted. As plants generally grow faster in warm temperatures, you can analyze gardening weather through Microsoft Excel by inputting growing degree days. 

GDD is meant to be used over a longer period of time than HDD or CDD, as the degree day identifies how much benefit certain temperatures have provided to a plant over the course of its lifecycle.

The more detailed a record you can create of your plant’s progress, the better able you are to identify when it’s right to harvest. 

How to Calculate GDD

Instead of using 65°Fahrenheit as a baseline, GDD uses the minimum threshold temperature at which a certain species will grow, which will vary depending on the country of origin and the hardiness of the plant. 

The calculation looks like this:

[(Daily High + Daily Low) / 2] − Plant Base Temperature = GDD

Let’s look at an example. Assume you’re planting a tomato plant whose base temperature is 60 degrees. The daily high is 85°F, and the daily low is 75°F. 

[(85 + 75) / 2] − 60 = 20 GDD

If we have a wild temperature swing the next day, with the high being 60°F and the low being 50°F, our calculation would look like this:

[(60 + 50) / 2] − 60 = −5 GDD

In most situations, negative GDD are not counted and any negative value is treated as zero. You would then add these together to have a total of 20 GDD. Essentially, your plant has enjoyed 20 days of optimum temperature for its needs, regardless of how many calendar days have passed. If you know the typical growing cycle for this particular crop, you can estimate when it will bear fruit. These values can also be used for predicting when fertilizer, insect control, and irrigation may be required.

Freezing Degree Days (FDD)

Freezing degree days is a measurement designed to track how cold it is and how long it has been, using the freezing point as a baseline. This is an average of the days where average temperatures were below freezing, and it helps to measure the growth of ice sheets during winter weather. 

Same as GDD, FDD is usually taken over a greater period of time, and it can include both positive and negative values in order to identify how much ice formation is expected during the winter season. 

FDD, along with other statistics such as average snowfall, windchill, and water temperature, allows researchers to keep track of ice formation, which is a crucial element of temperature moderation in colder climates. 

How to Calculate FDD

Similar to heating degree days, you will subtract the average temperature from a base temperature, but in this case, that temperature is 32°Fahrenheit. The calculation is as follows:

32 − [(Daily High + Daily Low) / 2] = Freezing Degree Days

Let’s assume we have a daily high of 32°F and a daily low of 20°F. Our example would look like this:

32 − [(32 + 20) / 2] = 6 Freezing Degree Days

For the next example, assume that the daily average is higher than freezing, such as a daily high of 56°F and a daily low of 46°F:

32 − [(43 + 33) / 2] = −6 Freezing Degree Days

These two days together would make a sum of 0 FDD, suggesting that there would be no ice growth over this period. 

Thawing Degree Days (TDD)

While freezing degree days are used in the winter, thawing degree days are used during summer to assess how hot a given year has been, helping to estimate changes in the climate and weather throughout the warmer seasons.

Like cooling degree days, it subtracts a base temperature from the average temperature, but it uses the freezing point instead of a “human” temperature. TDD, similar to FDD, is typically used over an entire season rather than just day-to-day. 

How to Calculate TDD

To identify thawing degree days, you can use this calculation:

[(Daily High + Daily Low) / 2] − 32 = Thawing Degree Days

As an example, let’s say that, on a nice May day, the daily high is 76°F and the daily low is 66°F. The thawing degree days would then be:

[(76 + 66) / 2] − 32 = 39 Thawing Degree Days

It’s unlikely that it would be below freezing during the summer, but it does occasionally happen during late spring. If this were to happen, then similar to FDD, the negative value would be added to the positive value in order to show how many degree days were present over the measured period. 

Climatologists can assess the number of thawing degree days in a given season to see if they equal the average of the last few years. If they don’t, it means that this year was warmer or cooler than normal.

What Is Degree Day Temperature Data Used For?

Degree days are useful in many applications, including energy consumption, agriculture, weather tracking, and climate research. Each of the five types mentioned here is generally used by different sectors to estimate particular needs, whether that is how much energy is needed to heat or cool a building on a given day or how long a plant needs to reach maturity.

Heating degree days and cooling degree days are proportional to energy demand for a certain building during a period of time, usually 24 hours, which is useful for identifying the costs of keeping buildings at a comfortable temperature for workers or residents.

A growing degree day measures the temperatures that a certain plant species needs to thrive. If the GDD shows that it has been too cold for a plant to grow, then a farmer or gardener can take steps to protect their plant. 

Freezing and thawing degree days help to track how cold or warm a year has been in a certain state or country, which can help scientists assess changes to average temperatures and protect local species. 

Who Uses Degree Days?

A variety of professions use different types of degree days to assess their work and ensure that they are ready for temperature fluctuations. The US Department of Energy, as well as local power companies, will use heating and cooling degree days to track energy consumption and prepare for the emerging needs of a service area based on the air temperature. 

They may also be used by engineers to measure how much insulation buildings need to counteract outside air temperature and reduce energy costs. 

Farmers, vintners, and others in the agricultural field use degree days to protect their crops, assess the best time to plant certain species, and predict when plants in a certain location will mature. It’s crucial to plant a crop at exactly the right time, as each species has a base temperature in which they flourish. They can also identify whether it is too hot or cold for plants in a specific location to grow, allowing them to make alternate arrangements if they still wish to grow that crop.

The government utilizes these measures in order to identify energy usage and take steps to protect citizens during abnormally hot or cold times. For example, a city may determine that when the degree day is at a certain limit, they will open heating or cooling centers. This helps municipalities manage expenditures without putting residents at risk. 

Finally, researchers and scientists also use degree day calculations to understand how weather changes over time. This temperature data can be combined with things like ice sheet thickness, methane levels, and ocean temperatures to develop plans to protect species. 

Celsius-Based Degree Days and Fahrenheit-Based Degree Days

Celsius and Fahrenheit degree days are proportionate to one another, but their base temperatures are measured on a different scale. A Fahrenheit degree day is 9/5th of a Celsius degree day. In other words, it takes 9 Fahrenheit-degree days to make 5 Celsius-degree days. 

Unlike when converting between the measurements when measuring temperature directly, you do not need to subtract 32 because the degrees are essentially zeroed out. All you need to do is multiply the Celsius measurement by the ratio 9/5, and you will have the Fahrenheit version.

As an example, let’s assume you have 15 degree days in Celsius. The calculation would be:

15 Degree Days C x (9/5) = 27 Degree Days F

If you want degree days in Celsius, you’ll reverse the ratio like so:

27 Degree Days F x (5/9) = 15 Degree Days C

What Are Population-Weighted Degree Days?

Whereas cooling or heating degree days are generally used on a smaller scale, for particular buildings, population-weighted degree days allow engineers and government officials to measure energy use across an entire region by its population. 

The US Energy Information Administration utilizes this to understand how much energy is utilized in a certain period divided by the entire population, which can help them understand how much energy is consumed by a given region and, accordingly, the country as a whole.

America has been separated into nine different Census regions, which constitute between three and eight states depending on the population. These are weighted by the ratio of the total population in the area to the total population of the United States. The degree day values are then multiplied by the population weight in the given region, which is then added to the eight other regions to create the national average. 

Not only does the sum calculated help ensure that electric systems can handle the given heating or cooling demand during a period, but it also serves to show where more investments are needed to ensure that people remain safe during extreme weather. 

Research shows that cooling degree days are rising faster than heating degree days when weighted by population, both in the United States and the world as a whole. This is useful information for governments to know, as it can help them devise new ways to cool regions based on the population, as well as the region’s given resources. 

Where Can I Find Degree Day Data?

Degree day information can be found along with temperature data and other metrics in the Visual Crossing Weather API. This data subset is available for a Corporate plan, which includes additional options such as solar energy weather data, agriculture weather data, and maritime data. It’s perfect for companies, nonprofits, and other organizations that need more granular information in order to make weather-related decisions for their operations.