I wish I could provide a better answer for you, however... it really depends on the bus that you get.
All the new Nova busses (4815-4861, and soon 4862-4886) do not leak... (the only exception is some condensation from the A/C... but that's only onto the driver... me... lol)
I know that 4806 and 4807 leaked pretty bad this past June. We wrote it up, and Im hoping it was taken care of.
Some of our old 27xx series that use Lexan windows don't have proper sealing around them... during turrential downpours, they *might* leak a little... however, those busses (2701-2744) are usually used as cast shuttles, and during peak busy times... They're 20+ years old, and although they often times run better than the brand new busses we have... they dont get used a lot.
2745-2780 were rebuilt in 1998, and got brand new windows and sealants... so they really shouldn't leak. It also should be noted that those busses typically have the best A/C units.
As for the rest of the fleet.. *shrug* it's hit or miss. I'm hoping that Maintenance has fixed all the leaks that we reported this past June when it rained every day for the entire month. As for recently, I havent noticed any leaks when it's been raining. I'm going to be meeting with some folks, and I will bring this topic up, and see if I can get a better answer for you.
On a side note... the top rear vent in the back is the A/C unit... Im wondering if that might have been condensation dripping from it? *shrug*