Difference between “diurnal” and “daily”?

Question

I want to describe some data which are measured every one hour (Fig. 1).

They fluctuate during "one day" (i.e. 24h). They also fluctuate everyday (e.g. magnitude on 1/17,1/18 differs largely).

And I was wondering, (1) when I describe the fluctuation of "1/17 0:00 ~ 1/17 24:00", should I use "diurnal fluctuation" ? (2) when I describe the fluctuation of "1/17,1/18,1/19,1/20...", should I use "daily fluctuation" ?

I compared the meanings I found in a dictionary - "diurnal" means "1. belonging to or active during the day; 2. having a daily cycle or occurring every day; ". I think meaning 1 is ruled out. However, meaning 2 contains the word "daily", which brings confusion to me.

Answer 1

Diurnal means "when the sun is out", it's the opposite of nocturnal and typically used to describe animal or other living-being behavior.

Daily means once a day. When saying daily X an implication is that a routine is intended, which makes daily X sound weird if you are talking about something that does not happen one at the same time each day or where happening at the same time each day is not significant. In any event daily doesn't work to describe an hourly event.

Try the phrase "over the course of a day" if you want to talk about how something changes over a day. If there is a consistent one time during the day that a trend emerges (e.g. at 3am each day the level falls), you can use daily to describe that.