A simple average degree-day uses the daily MAX and MIN, a lower threshold TLOW (such as 50 degrees Fahrenheit), and the formula: ((MAX + MIN) / 2) - TLOW. So for a MAX and MIN of 100 and 40, and TLOW of 50, we get ((100 + 40) / 2) - 50 or 70 - 50 = 20 degree-days for one day. Degree-days are accumulated by adding over consecutive days.

A growing degree-day (as defined for corn development) is similar except that the lower threshold is substituted for the MIN (if the MIN is the lesser of the two), and an upper threshold is substituted for the MAX (if the MAX is the greater of the two). For the above MAX and MIN and TLOW, and upper threshold THI of 86 we get ((86 + 50) / 2) - 50 or 68 - 50 = 18 growing dds.

Other popular formulae include single and double triangulation, single and double sine curve, and real time degree-days. All of these require more complex calculations. Most degree-day models should use the method that they were derived (or validated) with, but it generally is OK to substitute the single for the double calculation methods. **Because degree-day calculations can be tedious and data are not always available, we have most methods online in the form of a calculator that are linked to public weather data from thousands of weather stations throughout the US and surrounding areas including much of Canada**.

You can use these maps to estimate for any location, even those without weather stations, degree-day accumulations for the current year, normal (historical average) degree-day accumulations, and the deviations from normal to date for the current year. You can also use the maps to locate stations that have similar weather, in order to best determine a station representative of your own location. An interactive degree-day mapmaker is a feature of the website, which allows end users to create custom DD maps and GIS data for particular regions, thresholds, and dates.

- A Google maps interface to weather stations and degree-day models
- Plant disease and other hourly-driven models relevant to many crops (2020: more than 22 models available)
- Graphs and summary tables of degree-day models
- Documentation and validation status of models (via links listed in model inputs)
- Ability to calculate degree-days for previous years and historical average data
- Forecasts - using 7-day NWS NDFD, 7-month NMME, and recent historical average data
- A file-upload version of the degree-day model that allows anyone to use their own weather data with the models, if formatted correctly

This project funded in part by grants from USDA-NIFA, USDA-APHIS-PPQ, and USDA-RMA-ipmPIPE.

*This page updated Feb. 1, 2021*

Len Coop, IPPC/Horticulture Dept, OSU, Cordley 2040, Corvallis OR 97331; email: coopl@oregonstate.edu phone: 541-737-5523