Does anyone know what the difference is (in meaning) of these two icons?
They're both AM clouds, PM sun, but the one on the left was used more on the maps on TWC for Partly Cloudy, iirc.
I can't see any pictures, but I'm assuming he's referring to the two AM clouds icons, the one with the cloud on the upper left and one with the cloud on lower left part of the sun. The first one (upper left) was used on the XL, and the early days of the Weatherscan IntelliStar. The second one debuted sometime in late 2002 on the national broadcasts. As Jesse stated, it was also used as a Partly Cloudy icon on national broadcasts from 2002-2003, except without the AM indicator of course.
The newer AM clouds icon debuted on the local forecasts on the IntelliStar in 2004, and the Weatherscan IntelliStar started using it too several months later on the extended forecast (the older version was still used on the regional maps however, until the icons upgrade in late 2006). The domestic XL still uses the older AM clouds icon to this day.
There was also an even older version of the AM clouds icon on the national broadcasts that looked similar to the mostly cloudy icon. They stopped using that in 2001 or 2002.