Why do my fireplace ashes smell more on hot days?

We have a wood-burning fireplace. Had a fire in it a week ago on a rainy day, but haven’t cleaned it out yet. Still some ashes and remnants of a charcoalised log.

I’ve since noticed that on very hot days (37º Celsius today), I can really smell the ashes when I go by, and especially today - even when I’m in another room. When it was cooler outside, I couldn’t smell them except when I was right by the fireplace.

How come? Is it because when it’s really hot outside, the air in the house is cooler than outside, so there’s no draft going up the chimney? But when it’s cooler outside, there is a draft because the air in the house is warmer than outside?

That could be it. I notice the same thing back at the old house. Also, if you have AC as we do there, you typically keep your windows shut when it’s on, thus trapping pervasive airborne odors (and ain’t much more pervasive than old smoke).

I think it has something to do with the humidty.
Back when I lived at my parents house, they had a fire place with the same problem.

Anyways, down in the basement, under the fireplace is a little door in the cinderblock wall, I believe it is used to clean out the ashes when you brush them down under the fireplace. Well, I learned that I could open that door and smoke a cigarette right in front of it. It worked great, the smoke got sucked up and no one was the wiser (no one smoked in our house). But the only time it didn’t work was when it was hot and sticky out, for some reason, if I would try it on those days, periodically, there would be a ‘down draft’ so to speak and all the smoke would start coming back out at me. I have no idea why this happened, but I’m guessing it happens for the same reasons as what your asking about.

Oh, I also notice this at work. At work we have an attic with several vent fans in it. 99% of the time, I can go up the attic stairs/ladder and smoke and all the smoke goes right up into the attic. Except, when it’s hot and humid, when it’s like that,…wait…I have a thought
When it’s cold out, you have the furnace on, so as the hot air tries to rise, some of it will find it’s way to the fireplace chimney and escape that way, it will take the smell (or smoke) with it. However on these hot, humid days, when you have the AC on, it’s the opposite. The cold air is trying to move downward, and the hot air in the chimney is trying to move up, so there’s not going to be much movement (since the hot air is already up and the cold air is already down). If there’s not much movement, then the smell is just kinda ‘hanging around’ and free to drift into your living space.
Does that make sense?