It's a cold foggy morning, you've just pulled away from the house, and my, doesn't the bike run fine! Neat thing that cold damp air. Just what the engine likes, right? But, why?
Popular opinion has it that it's the extra oxygen in the air due to its low temperature, which results in higher cylinder pressures and hence increased engine power. But in reality, that's not it. Sure, the cold morning's air has some added oxygen, merely from the fact that the air is cold. Less expansion of the air means a bit more density, thus more oxygen. However, fog is moisture, and moisture in the air displaces its oxygen, resulting in a net loss of oxygen density, not a gain. The result is richened air/fuel mixture coming from the carburetors. This, more than anything, makes that still-cold machine seem to run better the first few miles from the house.
And there is one more factor. A fog-banked morning has a canopy, or shield, if you will, around it. Your machine is running within a kind of corridor of air pressure. This creates a soundboard that reflects the sound of the engine back to the rider. The bike sounds different, and this difference, which is mostly made of crisp mechanical engine noise, is intuitively felt as heightened performance.