Ever notice the weather maps they show you on TV? They show fronts as lines and they put them into motion sometimes. And darned if the line will be a state or two away in the morning and over you town in the afternoon.

Those lines actually represent atmospheric pressure. And that's one of the primary things that affect an engine's performance. And that pressure can change as fast as those lines on TV move.

And mornings can have the highest humidity of the day. And afternoons can have the highest temperatures of the day.

All three of those things have an effect on our engines. The effects all work to influence how many air molecules the atmospheric pressure can push into the carb at any given time.

Temperature, humidity, pressure. One can change fairly quickly, and the other two are very probably going to change for sure.