The last few days here have been quite warm with temps in the 80s approaching 90 degrees.
I've noticed when it started getting warmer. My station has a much noisier signal in places where there hasn't been any. Higher temps cause the air to become more denser causing some signal degradation to AM radio signals. Anyone else notice this?
Warmer air is less dense(lighter) and colder air is more dense(heavier), the reverse of what you said. The difference in signal or whether it's noisier if caused by warmer weather is because of less dense air.
When we were kids AM DXing was always better in winter.
With warmer weather there is more intense sunlight and longer days as the sun is higher in the sky and it's less light and longer nights that are best for AM. This is why you are experiencing the changes. Also maybe more noise interference in the summer season.
Warmer air contains more moisture, and I believe that's the cause.
My FM signal is severely affected by the weather. On those cooler, crisp mornings, I can listen up to 50% further than on warmer/humid days. I find the difference quite amazing.