Looks like I won't have to disable rain after all. I discovered that my laptop has two graphics controllers, a bog standard Intel HD and a more power Radeon HD card. TS3 has been using the first one up till now. Switching it to the other was simple enough, and now there is an all round improvement to the games appearance, and no more weird glowing areas whenever it rains at night.
The intent is to provide players with a sense of pride and accomplishment for getting past the loading screen.
I got a new computer in a Black Friday sale and now my game loads in about 5 minutes. It's been awful for my housework. It used to be that I'd set it up to load, do some cleaning, select a file, do the dishes, and then come back to a loaded game. Now I can't even finish a row of knitting!