Which formula is used to calculate degree days?

The calculation of degree days is an important concept in horticulture and entomology, as it helps predict insect development and plant growth in relation to temperature. The correct formula uses the average of the maximum and minimum temperatures, adjusted by a predetermined minimum threshold temperature. This approach helps estimate the accumulated heat units that are necessary for the biological processes in plants and insects.

Using the given formula, by calculating the average temperature (the sum of the maximum and minimum temperature divided by two) and then subtracting the minimum threshold, you get the degree days for that specific period. This measure works because it effectively captures the amount of heat available for development, only counting temperatures above a certain biological minimum that are conducive to growth or development.

The other options provided do not accurately reflect the correct method for calculating degree days. They either adjust temperature values improperly or include incorrect arithmetic operations that do not yield the measure needed for degree day calculations. Therefore, the first option firmly represents the established approach in the field.