What is the data availability rule in the ranking?

AirVisual included cities which have the following criteria:

1) a>50% “Calendar Availability” for 2018
2) >41% “Average Daily Availability” (equivalent to >10h of readings within 24h) per day that data is available.

“Calendar Availability” indicates the percentage of days of the year which have at least one hour’s measurement from at least 1 station.
“Average Daily Availability” indicates the mean proportion of 24h for which those calendar days have hourly readings (from at least 1 station)

Source:
https://www.iqair.com/newsroom/what-is-the-data-availability-rule-in-the-ranking