It is not uncommon for hourly measurements to be missing from sources that are published in real-time. This can happen for a few different reasons:
1) data may be published late, for example, a number of hours late or even a day late or longer. For example, Italy collects hourly data which is published the following day, which makes this difficult to aggregate and report in real-time.
2) Sources may sometimes publish data with intermittent gaps in hourly readings (e.g. one reading once every 3 hours), the reason for this is not always clear.
3) Stations may also temporarily undergo maintenance, which will cause a temporary outage of readings.
Missing data in this ranking has been addressed through the data availability criteria, as described above. Cities with multiple stations have an advantage in that, if one station is missing some hourly data, the city can refer to measurements from the other stations. The availability criteria aim to ensure that each included city is supported by a reasonable data frequency to be considered representative for 2018.