The average 16-hr Leq noise exposure depicted by the daytime contours represents the average noise level that would be measured over a summer period, defined as the 92 days from 16 June to 15 September. So a noise monitor would need to collect data for the entire 92-day period in order to provide a result directly comparable with the contours.
A sound level meter can read a number of parameters. One common reading is the instantaneous noise level that corresponds to the sound pressure level at the actual time of a noise event. It should be noted that this level is different to an averaged level such as the 16-hour Leq in dBA, which is used in the daytime aircraft noise exposure contours. It is often the case that the maximum level of an instantaneous noise level measurement reading taken during an aircraft flyover is erroneously compared against Leq contour levels.