Running

Is there a rule of thumb for adding more clothes to a basic shorts and t-shirt as the weather gets cooler?

The general consensus I heard is "dress as if it was 10°C warmer than it actually is".
Or (pretty much to the same effect) "dress so you will be a bit cold the first five minutes of your run".

Of course it's a personal thing, but for me this rule of thumb worked quite well.
There's also this website, http://www.dressmyrun.com where you can fine-tune the results to your preferences ("I like to be warmer / colder") in the settings menu (upper right of the page),

Regarding knees - I pretty much run in shorts for most of the year (with gore windstopper briefs underneath liner-less shorts if it gets chilly) - but I do want a double layer shirt, or longsleeve rather soon - and also (thin) gloves! Those thin icebreaker merino gloves are pricey but really fucking good, should have bought those sooner..