Last I checked, the record for 2.4GHz links was something like 100km. Why are you skeptical?

Again, heavy rain, smog and a couple of good sized trees

2.4GHz requires line of sight and has major problems with water and trees.

900 MHz is much more forgiving.

Yeah, but we're only talking about <4km here. Rain fade in the 2.4GHz band could produce up to 2dB of attenuation under flash-flood-thunderstorm conditions. 2dB is not a big deal! At 10km or 20km or 40km you might have a point.

So, 2.4GHz can transmit through trees?

It depends. It doesn't penetrate very far into the trunk, but it doesn't have that much trouble with leaves and twigs. (Until they get rained on.) So you can have a line of sight that's pretty much blocked by trees visually, but quite clear in 2.4GHz.

So, if it is raining and the tree gets wet, it will not transmit through the tree, correct?

It's not that simple. Attenuation increases when it rains and the tree gets wet, but whether the attenuation is enough to cut off your connection entirely depends on a lot of things. But sometimes, yes.

Well, this was some useful discourse.