A vocabulary of landscape — a landscape of words
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thermal sum
The accumulated total of heat units — typically degree-days above a baseline temperature — required for a biological event to occur. A cherry tree doesn’t bloom on a date; it blooms when enough warmth has accumulated. Thermal sum is the calendar plants actually use — not days, but degrees.
Etymology
“Thermal” from Greek “thermē” (heat); “sum” from Latin “summa” (total, highest amount). Compound term from agricultural and ecological science, describing accumulated heat units (also called growing degree days) used to predict plant and insect development stages.