Question: I am confused, which is not unusual. Your weather team continues to state that to help fight ozone, we should mow, add fuel to our car, etc. late in the afternoon, when it is cooler. I have always thought that the coolest part of a day is at, or just after sunrise. This seems verified by looking at hourly temperatures. That means we should do the above chores during the early morning hours until maybe around 10:00-10:30 a.m., or a couple of hours after sundown. The radiant heat would still be pretty warm until then. So when should I fuel up?
Answer: Regarding your comments on "the coolest part of the day," you are right on the money -- for the vast majority of days, the daily minimum occurs sometime near sunrise.
But here's the hook when it comes to ozone: we say "fuel when its cool" hoping that folks will fuel after work (6PM or later) and not before. So by "cooler," we don't mean the coolest part of the day, but just not in the heart of the late morning through early afternoon.
Why? Because it is not really the temperature that produces ozone, it's the amount of sunshine. Ozone is a by-product of photo-chemical reaction of nitrous oxides, hydrocarbons and oxygen. Without getting into the details, these chemicals interact in the presence of sunshine to produce low-level ozone.
By 6-7PM or so, the amount of incoming solar radiation has fallen off sufficiently to effectively terminate any additional significant ozone formation for that day. So you're thinking, isn't the amount of sunshine equally low at 6-7AM? Well, yes ... but here is the difference.
By fueling in the evening, we have 12-18 hours of time for those chemical components to diffuse out of the environment. But at 6-7AM, there is less than 6 hours of time for the vapors to dissipate before the ozone process is running full tilt. Even with our frequent near-calm conditions at night in the summer, that additional 12 hours of time is a plus.
Hope this helps explain the logic!
WAFB Storm Team