“What are degree days?” is a common question with a simple answer: degree days measure how much the daily temperature differs from a certain baseline. This is helpful for understanding energy usage, agricultural conditions, and the local environment.
What Is a Degree Day?
The degree day is the difference between the daily mean temperature and a base temperature, usually 65°F (18°C). To find the mean temperature, add the daily high and low temperatures together, then divide them by two. You’ll then compare this to the baseline temperature, either by subtracting it from 65°F or the other way around.
This measurement can track heating and cooling needs, as 65°F is considered a comfortable temperature for most buildings. In a well-functioning heating system, a degree day will be proportional to the energy needed to heat or cool a building.
Types of Degree Days
There are five different types of degree days: heating degree days (HDD), cooling degree days (CDD), growing degree days (GDD), freezing degree days (FDD), and thawing degree days (TDD). The first two are used for energy, while growing and freezing degree days are used for agriculture. Thawing degree days are often used for climate research. You can use weather data from Visual Crossing to calculate any of these degree days.
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Heating Degree Days (HDD)
Heating degree days are helpful for tracking how much energy is needed to keep a building at 65°F (18°C) when it’s cold out. This can help energy companies, such as natural gas and electric companies, determine energy demand on a given day. Individual companies and homeowners can also use this to assess whether their buildings are energy-efficient.
To calculate heating degree days, you need the daily high and daily low temperatures for that location. 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. First, add them together and divide by two to get the average. Next, you’ll subtract this from 65 to see the number of heating degree days.
65 − [(52 + 32) / 2] = 23 Heating Degree Days
You can also calculate this in Celsius by converting the temperatures and replacing 65 with 16. The more heating degree days there are, the more energy you’ll need.
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Cooling Degree Days (CDD)
Cooling degree days measure how hot a day is relative to 65 degrees Fahrenheit, considered the most comfortable temperature. Unlike heating degree days, cooling degree days subtract 65°F from the average temperature, like so:
[(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
Companies can use this to optimize their cooling systems and manage energy usage when the outside temperature is very high.
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Growing Degree Days (GDD)
Growing degree days help farmers and gardeners estimate plant development, as plants grow faster in warm temperatures. It is used over a longer period to help with growing cycles. You can analyze gardening weather through Microsoft Excel by using growing degree days.
To calculate GDD, you will need to know the minimum threshold temperature at which a certain species will grow. The calculation looks almost the same as degree cooling days, just switching 65°F for the species’ baseline temperature. Negative values are ignored, as they mean that the outside temperature isn’t right for the plant’s needs.
[(Daily High + Daily Low) / 2] − Plant Base Temperature = GDD
Let’s look at an example. Assume you have a chosen base temperature of 60°F. The daily high is 85°F, and the daily low is 75°F.
[(85 + 75) / 2] − 60 = 20 GDD
The next day, the high is 60°F and the low is 50°F, so that we would get this.
[(60 + 50) / 2] − 60 = −5 GDD
Since we ignore negative values, we have 20 total degree days. This tells us that your garden has had optimum temperature for 20 days, regardless of the calendar date. GDD helps farmers predict harvest times, improve crop yield, and enhance plant care.
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Freezing Degree Days (FDD)
Freezing degree days measure how cold it has been relative to freezing, which tracks potential ice growth. Like growing degree days, FDD is taken over a long period of time. However, it can include both positive and negative values. The formula is the same as heating degree days, but uses 32°F as a baseline temperature instead.
32 − [(Daily High + Daily Low) / 2] = Freezing Degree Days
Let’s take a daily high of 32°F and a daily low of 20°F to determine how many degree days we would have in a typical January week.
32 − [(32 + 20) / 2] = 6 Freezing Degree Days
The next day, there is a daily high of 56°F and a daily low of 46°F.
32 − [(43 + 33) / 2] = −6 Freezing Degree Days
Because we can use both positive and negative numbers, we get a sum of 0 FDD, meaning there is no ice growth.
FDD is mostly used to track ice formation in cold areas and to conduct scientific studies about climate change.
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Thawing Degree Days (TDD)
Thawing degrees are used in the summer to determine how warm the weather has been. They use the same formula as cooling degree days, but the base temperature is 32°F instead. They are used over an entire season rather than day-to-day. Like freezing degree days, you can use negative values, which is helpful if you have a sudden cold snap during the spring.
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
Climatologists can use this measurement to understand seasonal temperature trends and track climate change.
What Is Degree Day Temperature Data Used For?
Degree day data can be useful for many applications, including energy planning, agricultural decisions, and climate research.
The energy sector uses heating degree days and cooling degree days to plan for increased fuel demands, such as during a winter storm or a heatwave. Individual companies can also use them to identify whether their buildings are energy efficient.
Farmers and gardeners can use growing degree days to protect their plants and identify when it’s the right time to harvest. They can also use it to plan what species to add to their garden, as some fruits and vegetables may not thrive in their climate.
Lastly, climate scientists use this weather data to track climate change and protect local species. Each of these measurements can be invaluable to specific sectors, as they all function similarly.
Who Uses Degree Days?
Energy companies, governments, agricultural workers, and scientists all use degree days to make actionable decisions.
The US Department of Energy and local power companies use heating and cooling degree days to track energy consumption. This helps them prepare for a service area’s needs or identify ways to reduce energy bills. Engineers can also use this to identify a building’s energy consumption.
Those in the agricultural field use degree days to protect crops, identify the best time to plant certain species, and estimate harvest periods. This ensures that they aren’t wasting valuable space on plants that can’t grow in their area and that they can allocate resources effectively.
Finally, researchers and scientists use this data to measure environmental changes, like ice sheet thickness, methane levels, and ocean temperatures. This helps them take the right steps to protect species.
Professionals from all industries can use Visual Crossing’s Timeline Weather API to find the average daily temperature. We use high-quality data from places like the National Weather Service and NOAA, and our data is constantly updated throughout the day.
Celsius-Based Degree Days and Fahrenheit-Based Degree Days
Degree days can be used globally, though you will need to convert between the two temperature scales. The conversion ratio between the two is a constant 9/5, making it easy to switch between the systems.
As an example, let’s assume you have 15 degree days in Celsius and want Fahrenheit. The calculation would be:
15 Degree Days C x (9/5) = 27 Degree Days F
If you have degree days in Fahrenheit but want Celsius, you’ll reverse the ratio to 5/9:
27 Degree Days F x (5/9) = 15 Degree Days C
What Are Population-Weighted Degree Days?
These degree days are used at a regional or national scale to understand how low or high temperatures will impact a population, especially energy consumption.
The US has nine different Census regions, which are roughly proportional in population. Degree day values are multiplied by this population weight to understand the area’s needs. You can also add them all together to find a national average to see how much energy the country is using.
Calculating this helps ensure the electric systems can handle demand. It also shows where governments need to invest in better infrastructure.
Where Can I Find Degree Day Data?
Businesses, scientists, and agricultural planners can access granular degree data through the Visual Crossing Timeline Weather API by signing up for a Corporate plan, which includes solar energy weather data, agriculture weather data, and maritime data. Our extensive data can help you make the best weather-related decisions for your operations.
FAQs About Degree Days
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What are degree days used for?
Typically, degree day information is used to understand how to maintain a comfortable temperature for different applications, such as heating buildings or growing crops. It can also be used for climate research.
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What is the base temperature for degree days?
The base temperature will vary based on the type of degree day you are using. Heating and cooling degree days use 65°F (18°C). Growing degree days generally use a crop’s specific base temperature. Lastly, freezing and thawing degree days use 32°F.
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How do I calculate degree days?
To find the degree day, you will need the average daily temperature and the base temperature for the specific degree type. You will then either subtract it from the base or subtract the base from the average temperature.
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What are population-weighted degree days?
Population-weighted degree days will weight a cooling or heating degree day by the ratio of a certain region’s population. This helps governments plan for infrastructure upgrades or prepare for sudden surges in heating and cooling needs.
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Where can I find reliable degree day data?
Visual Crossing’s Timeline Weather API provides reliable degree day data. You can sign up for a free account and access your API to start finding the weather information you need for your projects.