noun
diurnal variation; daily variation; diel cycle
Scientific term for changes that occur over a 24-hour cycle, often used in meteorology, biology, and environmental science.
気温の日変化をグラフに示した。
I showed the diurnal variation of temperature on a graph.
この湖では、溶存酸素の日変化が大きい。
In this lake, the diel cycle of dissolved oxygen is large.
Similar meaning, but 日内変動 is more common in medical or physiological contexts (e.g., hormone levels).
Compound of 日 (にち, 'day') and 変化 (へんか, 'change, variation'). The reading follows standard on'yomi.