Diurnal refers to something that occurs or is active during the day. It can also describe a cycle or pattern that repeats itself daily, such as the rising and setting of the sun or the behavior patterns of certain animals.
Another issue is urban heat island (UHI) effects: higher temperatures occur in urban areas than in outlying rural areas because of diurnal cycles of absorption and later re-radiation of solar energy and (to a much lesser extent) heat generation from built / paved physical structures. (ipcc.ch)
To reduce these uncertainties, we have made enhancements to the currently operational TOMS surface UV irradiance algorithm (Version 1) by including the effects of diurnal variations of cloudiness, an improved treatment of snow / ice, and a preliminary aerosol correction. (pubs.giss.nasa.gov)
Investigation of diurnal variation in sustained exercise performance. (blog.bridgeathletic.com)