I often use the word “diurnal” when talking about daily cycles. This is somewhat of a hot topic in atmospheric science, because model’s generally do a very bad job of producing rain at the right time (Lee et al. 2007, Pritchard and Somerville, 2009). This isn’t a problem for the general problem of understanding anthropogenic climate change, but it becomes more of an issue when we want to zoom in and understand region specific effects of climate change. Below is an interesting figure from Pritchard and Somerville (2009) that uses an HSV color coordinate to describe the phasing (left) and amplitude (right) of the peak daily precipitation during northern hemisphere summer from TRMM data. There’s a lot of interesting patterns to look at here, but the great plains of the united states has a particularly interesting signal.
Using “diurnal” to describe an 24-hour cycle is not completely wrong. Most definitions of “diurnal” indicate that it can be used this way (see examples here, here and here). However, I want to argue that there is a better word, and that word is “diel“. This term is mostly used in biological research circles, but I don’t see why we can’t use it in atmospheric science.
The distinction is that “diurnal” and “nocturnal” describe when something with a daily cycle preferentially happens during the day or night hours, respectively. Diel describes something that has a 24-hour cycle, but is ambiguous as to when the “peak” of the cycle occurs.
Disclaimer: I realize this is a stupid semantic issue that no one really cares about, but I thought it was interesting.