When shooting a panorama it is very important that the air is as clear as possible, so that the entire scene is lit evenly. But how can I predict in advance if the air is going to be particularly clear? Is that information available somewhere in weather reports?
You might want to look at weather forecasts for astronomy - some of these sites go into more details than generic weather forecasts, and have predictions for things of interest to astronomers, including seeing (how steady/turbulent the atmosphere is) and transparency (how clear the atmosphere is).
A web search for "astronomical weather forecasts" will find lots of sites (They do forecasts for the whole day, not just nighttime).
For example, this site https://in-the-sky.org/weather.php graphs predicted cloud cover, transparency, seeing, humidity and temperature.
Usual disclaimers apply - as that site says: "Use them with caution and at your own risk. They are always subject to a high degree of uncertainty and weather conditions may change at short notice."
But they can be useful - and there are a number of smartphone apps available that can access the same data, as well as the web sites.
I have no idea if this holds true, but in growing up in Phoenix, AZ...the city is massive and sits within a valley, so it usually has a constant smog bubble. That bubble usually gets pushed out during thunderstorms, when wind and rain can get rid of it.
From that, I can only pose that, the best and clearest skies will be available the day after a good storm.
Visibility is very important as well for sailing/boating and flying. This is a forecast of weather models among other better known such as cloud coverage, rain, winds, ... So when I want to check whether conditions for a panorama shooting or for any outdoor shooting, I am using websites/apps which display the results of global weather models like for instance https://www.windy.com and I select "visibility" (right panel for windy on desktop). I would also check "winds", and "clouds" (and rain of course) as they are also important for outdoor shooting but it is easier to find them, fair enough. There are many different whether models and you should use the weather model which is a good match for your location and/or cross checks different models (you will find more details at: https://www.meteoblue.com/en/weather/forecast/multimodel/). I would go for a fine grain model and short term model as transparency/visibility can depend from local effects.