32 ºF equals 0 ºC…or does it?

Posted by Dan Olmstead
April 28th, 2021

Fahrenheit is the standard for temperature measurement in the United States. We use a simple conversion formula for Celsius equivalent and we are done, right? Not if you rely on degree day models.

It is easy to use a simple formula for conversion to ºC from ºF.

We can add, subtract, multiply and divide ºC. Just like ºF. ºC is the standard for scientific research and is important for international trade and business.

NEWA uses degree day (DD) modeling for many insect pest and plant disease predictions. These DD models require a minimum temperature, usually called a “base” temperature. In short, plants and insects do not grow if temperature is below this threshold. DD models can be built using ºC or ºF base temperatures.

0 ºC equals 32 ºF. That means output from a base 32 ºF degree day model should be the same as output from a base 0 ºC model, right?


Degree days from a base 32 ºF model will always accumulate faster than those from a base 0 ºC model over the same period of time. Things go awry when accumulation values from one type of model are used to interpret research-based predictions generated from the other.

Let’s consider a fictional scenario where research is published showing that a valuable crop begins flowering at 250 base 0 ºC accumulated degree days from January 1. In my rush to get a protective spray on to prevent insect damage, I remember that NEWA has a base 32 ºF DD model, which is equal to 0 ºC. I run the numbers and discover I completely missing the period of maximum protection.

But did I miss my window? In reality, I did not. Things went sideways because I used output from the base 32 ºF DD model to interpret plant development rates that were tabulated using a base 0 ºC model from research.

Base 32 ºF over-predicted plant development in this scenario.

When it comes to using degree day models for agricultural predictions, always pay close attention to both base temperature and measurement unit.

Remember 32 ºF is not equal to 0 ºC…for degree day models at least.

NEWA is part of the New York State IPM Program and Cornell Cooperative Extension at Cornell University. NYSIPM partners closely with the Northeast Regional Climate Center to make NEWA available to growers in member states nationwide.