AirCompare - How to Interpret the Results
The height of each bar equals the total number of unhealthy days (averaged over the last 5 years) for each month. Three categories of unhealthy days from the Air Quality Index are possible in this graph: orange days (unhealthy for sensitive groups), red days (unhealthy for everyone), and purple days (very unhealthy). If a bar is made up of more than one color, you can see the proportion of orange days, red days, and/or purple days that add up to the total number of unhealthy days for each month.
In general, a month with a shorter bar is a better time to travel than a month with a taller bar, though some people might also want to take into consideration how much red and/or purple a bar contains.