We're interested in diurnal, I mean daily, variation.

我们对昼夜变化感兴趣，我的意思是每天变化。

When we think about variation, and this is even true when we thought about variance, which was the mean variation in y.

当我们考虑变异时，甚至当我们考虑方差时也是如此，方差是 y 的平均变异。