In degree-day calculations, growth occurs only when daily mean temperatures exceed what?

• A. A base threshold temperature
• B. The average yearly temperature
• C. The maximum daily temperature
• D. The current season's average

Answer: (A)

Degree-day growth is driven by heat above a minimum level. Growth units are earned only when the day’s mean temperature rises above a base threshold temperature, which is the minimum heat required for development. If the daily mean exceeds this base, you count the difference (daily mean minus base) toward accumulation; if it does not exceed the base, no growth is recorded for that day. This base value is specific to the organism or process being modeled. Over many days, these daily increments sum to predict development stages. The other options don’t fit because degree-day calculations rely on heat above that minimum threshold, not on yearly averages, maximums, or seasonal averages.