As I've mentioned before, ozone is a double agent in the atmosphere. Way up high, ozone is good - it blocks much of the sun's harmful ultraviolet rays. On the other hand, ozone down near the ground is a pollutant, especially prevalent in summer. With the warm months behind us, we can look back on the ozone pollution season and see how we fared.

You need hot weather for ground-level ozone to form, so in general the warmer the summer, the more unhealthy ozone days you get. This summer was the fifth warmest on record, so it's no surprise that we had a lot of unhealthy ozone days - 21 in all. That's the most since the record hot summer of 1993.

There is an ozone forecasting and alert system in place - the Ozone Action Program - and thanks to that program, 13 of those 21 unhealthy days were predicted a day in advance. Of the other eight, seven were forecast to be just below the unhealthy range. Given the infancy of ozone prediction science, that's actually a pretty good track record, one that has been steadily improving over the years.